Download Review of techniques and practices in controlling tilapia populations

ISSN 1683-7568
AQUACULTURE SECTION
Review of techniques and
practices in controlling tilapia
populations and identification of
methods that may have practical
applications in Nauru including a
national tilapia plan
by
Romeo D. Fortes, Ph. D.
Secretariat of the pacific Community
Noumea, New Caledonia
REVIEW OF TECHNIQUES AND PRACTICES IN CONTROLLING TILAPIA POPULATIONS
AND IDENTIFICATION OF METHODS
THAT MAY HAVE PRACTICAL APPLICATIONS
IN NAURU INCLUDING A NATIONAL
TILAPIA PLAN
Submitted to:
TIM ADAMS
Director, Marine Resources Division
Secretariat of the Pacific Community (SPC), Marine Resources Division, BPD5 98848, Noumea
Cedex, New Caledonia
Prepared by:
Romeo D. Fortes, Ph. D.
Consultant, Secretariat of the Pacific Community (SPC), Marine Resources
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Original text: English
Secretariat of the Pacific Community Cataloguing-in-publication data
Fortes, R. D. (Romeo)
Review of techniques and practices in controlling tilapia populations and identification of methods that may have practical applications in Nauru including a national
tilapia plan / prepared by Romeo D. Fortes
(Aquaculture Technical Paper / Secretariat of the Pacific Community)
1. Mozambique tilapia – Government policy – Nauru. 2. Introduced organisms
– Environmental aspects – Nauru. 3. Aquaculture – Environmental aspects – Nauru. 4. Nile tilapia – Control – Environmental aspects – Nauru.
I. Title. II. Secretariat of the Pacific Community.
LC 639.8968 5
Agdex Pacific Islands 492/679
ISSN 1683-7568
ISBN 982-00-0123-4
AACR2
Executive summary
The Mozambique tilapia (Tilapia mossambica Peters=Oreochromis mossambicus
Peters=Sarotherodon mossamibics Peters) was introduced in Nauru in 1960 primarily to feed
on mosquito larvae (Ranoemihardjo, 1981) and for food (Ben Ponia, Aquaculture Adviser, SPC,
New Caledonia, personal communication). Instead, it rapidly became abundant in what are considered in Nauru as lagoons (sunken pools surrounded by mangroves) and ponds (depressions
created by bomb craters). It also became an aggressive competitor with milkfish (Chanos chanos
Forsskal) leading to the collapse of milkfish aquaculture. The Republic of Nauru then implemented a Tilapia eradication program through FAO in 1979 and 1980 where rotenone was used
as the toxicant.
When introductions of tilapia in many parts of the (1950 to 1970) similar problems experienced in Nauru were also felt in many countries. However, most countries such as the Philippines continued to regard it as source of food. In the early 1970’s techniques to control their
reproduction were developed in the country then applied in aquaculture production. Now, most
of the species of tilapia are used in aquaculture. The experiences in many countries that lived
through similar problems may be given a second look and may be considered as a means of controlling tilapia in Nauru.
The most important tilapias in aquaculture are the Nile tilapia (T .nilotica); Mozambique
tilapia (T. mossambica); and the blue tilapia (T. aurea) plus a number of mouth brooding tilapia hybrids used in aquaculture (red T. mossambica hybrids) with T. aurea, T. nilotica, and T.
urolepis hornorum including T. galilea and T. melanotheron. These species account for 99.5% of
global tilapia production. Nile tilapia now dominates global tilapia aquaculture, accounting for
72% or 474,000 tons in 1995. Total world tilapia landings from capture and culture has been estimated at 1.16 million tons with cultured tilapia accounting for 57% of the total (659,000 tons).
Cuba is the world’s largest producer of blue tilapia. The largest tilapia producing nations are in
Asia. China is the world’s largest tilapia producer (315,000 tons), accounting for 48% of global
production, followed by the Philippines, Thailand, Indonesia and Egypt. The USA is the world’s
largest tilapia consumer. In some countries like Australia, Nauru, Fiji and Palau complete eradication of tilapia were undertaken.
Failure of tilapia culture in the past, has often been due to uncontrolled spawning, thus
control measures were developed. There are basically seven (7) methods of controlling tilapia
populations that have been carried out in some countries for aquaculture and/or plain eradication purposes. Theses are (1) periodic harvesting of fry and fingerlings; (2) monosex culture of
which single-sex fish are obtained through: manual separation of sexes, hybridization, hormone
augmentation and genetic manipulation methods such as androgenesis, gynogenesis, polyploidy
and transgenesis; (3) culture in cages; (4) high density culture; (5) biological control; (6) sterilization and (7) eradication by means of fish toxicants.
While tilapia has tremendous potential as an economic commodity, it has also a significant role to play in environmental biodiversity and impact to a number of economic activity
in a number of countries such as Nauru, Australia, Fiji and Palau among others. Thus, general
courses of action have been offered to prevent the infestation of undesirable species including
tilapia into a country. In general, the principles behind controlling/managing of pests and nuisance species may be considered in controlling tilapia population. Preventive measures, careful
surveillance and monitoring coupled with a regular procedure to prevent entry into other water bodies is the preferred approach. The general methods of control are: physical/mechanical,
chemical and biological control; genetic engineering, and environmental management and cultural control.
Applicability of identified techniques to Nauru situation. Before any recommendation
of the applicability of the identified methods can be made, it is important to know the situation
of the island country in terms of geography, topography, climate, educational status of the citizens, aquatic environment, governance and economy. The Republic of Nauru was described by
Wetland International (1990) as comprising a single raised coral limestone island with a total
land area of about 2,130 hectares) and a population of 9,100 in 1990.The Republic of Nauru is
one of the three great phosphatic-rock islands of the Pacific. The climate is tropical with an annual rainfall averages of 2,000 mm. Streams are non-existent. Agriculture is very limited. There
is very little surface water on the Nauru’s highly permeable terrain, much the largest permanent
water body being Buada Lagoon. Many of the ponds and the two lagoons were used for the rearing of milkfish.
Identified processes that may be used in controlling tilapia in the waters of Nauru.
Among the seven (7) methods in use for the control of tilapia populations in aquaculture the
following techniques may be appropriate and could be applied as soon as possible in controlling the Mozambique tilapia in the natural bodies of water in Nauru. (1) periodic harvesting of
fry and fingerlings including the parents; (2) biological control; and (3) eradication of tilapia
using organic toxicants and/or other chemicals. The other methods such as monosex culture
through manual separation of sexes, hormone augmentation, cage culture, high density culture
and probably sterilization may be performed on trial runs using the fry/fingerlings collected in
activity number 1. Exercises on manual separation of sexes, hormone augmentation, may be
performed.
These trial runs may run up to at least one year depending upon the resources availability (manpower, financial and commitment).
Implementing strategy for a national tilapia program in Nauru. Sustained control of
the Mozambique tilapia from the significant waters of Nauru may be difficult, tedious and could
take some time, but may not be too late yet. As suggested earlier, controlling the Mozambique
tilapia in the waters of Nauru would need a thorough planning strategy that should at least consider the 5 stages in planning as recommended (Finlayson et al.2000) as follows:
a. preliminary planning, where the project concept and alternatives are developed, public input
is invited, and acceptance is encouraged;
b. intermediate planning, incorporating an environmental analysis where the project is refined
and public acceptance is encouraged;
c. final planning and project implementation, involving management through the development
of project-specific work plans;
d. performing the treatment; and
e. summation and critique of the project
This planning strategy recognizes that responsibility for tilapia control lies with government, community and industry. The plan must be put together after extensive community consultation and should consider opinions from the stakeholders. The goals, priorities and activities
in the plan should be identified in meetings with stakeholders using the information available.
Also, some additional key issues should be considered during the process, such as:
• prevention of tilapia infestation is feasible if there is cooperation among the various
stakeholders;
• sustained tilapia control, would be more realistic than complete eradication;
• It is very important for the participants in the program to be familiar with the ecology
and biology of tilapia, and that sustained research into other possible control methods be
initiated
• Tilapia as an economic resource
National tilapia plan for the Republic of Nauru. While it is important to eradicate/control
the undesirable tilapia species in Nauru, it is also important to consider the desirable hybrid tilapia as an economic commodity in South Pacific. More recent advances in selective breeding and
sex control technologies registered the greatest impact. As tilapia has gained in importance as
an international aquaculture commodity, so has there been a considerable increase in research
effort to improve tilapia stocks. Among these are hybridization, chromosome set manipulations
and application of transgenesis. From the SPC Aquaculture Action Plan 2003-2005 milkfish
(Chanos chanos) and tilapia (Tilapia spp.) have been identified for aquaculture.
Table 1. Issues and needs in formulating national tilapia plan for nauru
Category Institutional, legal and
administrative aspects Issues Constraints •
Demise of milkfish industry
•
•
commitment by
authorities
•
•
Human resources Proliferation
of Mozambique
tilapia
Ownership of
the areas for fish
farming
• Lack of skilled/technical manpower
•
declining supply
of fry
•
High cost and
poor quality of
feeds
•
Need for water
recycling technologies
•
poor site selection and poorly
managed farms;
Technical aspects •
impact on the
natural ecosystem;
•
tilapia introduced
other than for
farming
•
subsistence
farming has had
variable success.
Required actions •
Eradication/control
of Mozambique
tilapia
•
Review of policies
•
Programs to upgrade skills of technical and extension
officers
• Organization training, seminars and
workshops •
Consider other species of aquaculture
commodities (e.g.
hybrid hybrid species, etc.)
•
Poor quality of
seeds
•
Promotion of polyculture system
•
Poor water supply
•
Research waste
management and
treatment
•
Undertake research
on water recycling
technologies
Table 2. Action plans and objectives of a national tilapia plan for Nauru
Action plans Objectives •
Key areas of concern
•
•
Key development
areas in lagoon/pond
management To eradicate/control Mozambique
tilapia
To develop utilization techniques
for Mozambique
tilapia in aquaculture
To formulate an
effective management plan for
the utilization,
protection and
conservation
lagoons and
aquatic ecosystems
Target beneficiaries •
Private sector
•
Fish farmers
•
Government
•
Stakeholders
•
Government
•
Private sector
•
Stakeholders
Expected results •
Enable the government and
other stakeholders to move
on to other
aquaculture
species
•
Formulated
utilization
and management plan for
the aquatic
ecosystems for
Sustainable
resource use
production
Immediate Plan. In order to commence a project as a strategy to initiate the program of fisheries activities in the bodies of water of Nauru, an action plan for tilapia may be in order. This
action plan is divided into three phases as follows: (a). eradication and control of Mozambique
tilapia; (b). stocking of lagoons/ponds with carnivorous species to serve as biological control for
feral tilapia; and (c). enhancing the productivity of the lagoons/ponds for tilapia aquaculture
Trial Runs.Trial runs for the following monosex method in controlling tilapia reproduction
may be carried out using the fry/fingerlings collected in activity number 1. Exercises on manual separation of sexes, hormone augmentation, cage culture, high density culture and probably
sterilization, may be performed.
These trial runs may run up to at least one year depending upon the resources
availability (manpower, financial and commitment).
REVIEW OF TECHNIQUES AND PRACTICES IN CONTROLLING TILAPIA* POPULATIONS
AND IDENTIFICATION OF METHODS THAT MAY HAVE PRACTICAL
APPLICATIONS IN NAURU INCLUDING A NATIONAL TILAPIA PLAN
Romeo D. Fortes, Ph. D.
College of Fisheries and Ocean Sciences
University of the Philippines in the Visayas
5023 Miagao, Iloilo, Philippines
e-mail address: [email protected]
1.0. Introduction
The Mozambique tilapia (Tilapia mossambica Peters=Oreochromis mossambicus
Peters=Sarotherodon mossamibics Peters) was introduced in Nauru in 1960 primarily to feed
on mosquito larvae (Ranoemihardjo, 1981) and for food (Ben Ponia, Aquaculture Adviser, SPC,
New Caledonia, personal communication). Instead, it rapidly became abundant in what are considered in Nauru as lagoons (sunken pools surrounded by mangroves) and ponds (depressions
created by bomb craters and/or outcome of phosphate extraction). (Note: According to UNEP/
IUCN (1988) there is no true lagoon in Nauru). The Mozambique tilapia then evolved into an
ecological pest and unattractive food fish.. It also became an aggressive competitor with milkfish (Chanos chanos Forsskal) in ponds leading to the collapse of milkfish aquaculture, a traditionally important fish culture activity in this island country the fry of which are just collected
from the reef at low tide, acclimatized for 2-3 weeks to lower salinity, then released into these
lagoons and ponds. Growth of the milkfish however, was observed to have slowed down, which
was attributed on the following as the major raison d’être: overcrowding due to the fast rate of
spawning of tilapia resulting to massive competition with milkfish and consequently resulting
insufficient availability of natural food. Concerned by this, the Republic of Nauru, implemented
a Tilapia Eradication Programme through FAO in 1979 and 1980 in which the organic toxicant
rotenone, an extract of the roots of tropical plants (the jewel vine or Derris spp. and the lacepod or Lonchocarpus spp. belonging to the bean family Leguminosae) that is commercially
available in powder or liquid form, was applied in the aforementioned ponds and lagoons.
The introductions of tilapia in many parts of the world happened within the period 1950
to 1970. Problems similar to what were experienced in Nauru were suffered by many countries
where tilapia was introduced. In the Philippines, when the Mozambique tilapia was introduced
in May, 1950, it became a burden to the milkfish industry, it was an ecological pest, nuisance and
competitor with milkfish in ponds. However, the Philippine government continued to regard it as
source of food because they are easily reared in backyard ponds. In the early 1970’s techniques
to control their reproduction were developed in the country then applied in aquaculture production. Now, most of the species of tilapia are used in aquaculture and tilapia production has been
increasing each year and becoming more and more a significant part of the aquaculture industry.
1
Consultant, Secretariat of the Pacific Community (SPC), Marine Resources Division, BPD5 98848, Noumea Cedex, New Caledonia
2
For the purpose of this report, the genus Tilapia shall be used in all species of tilapia following the idea that all of the tilapias belong to the
single genus of Tilapia (Robins et al. 1991).
∗
After almost half century of its introduction into Nauru, the Mozambique tilapia in this country may have undergone considerable genetic
The experiences in the countries that lived through parallel predicament may be reviewed.
It is worth knowing what the rest of the world has implemented for tilapia. In the Americas, the Mozambique tilapia was introduced in the majority of the tropical and semi-tropical
countries in 1950s and 1960s. In the 1970s, various management practices for the control of
unwanted reproduction were applied in North, Central and South America. These practices included use of predators (biological control), monosex culture (through hand-sexing, genetics
and hybridization, hormone augmentation (sex reversal), high density culture and sterilization.
The Nile tilapia (Tilapia .nilotica) became the dominant culture species. Production of tilapia
has grown from near zero to more than 200,000 metric tons annually in the past 30 years. The
advent of feeds has encouraged the intensive commercial production of tilapia. The introduction
of hybrid tilapia (e.g. red tilapia) in the 1980s enhanced the domestic market value of the species
particularly in Jamaica and Colombia. At present, nearly all commercial producers of farmed
tilapia in the Americas rear sex-reversed fish. Grow-out techniques vary with the countries and
include culture in static water ponds, with partial water exchange in raceways, cages, and indoor
re-circulating systems. The leading producers of farmed tilapia in the Americas are Brazil and
Colombia, which have industries of small, medium and large-scale producers. Most of the production from seven out of 10 countries producing tilapia in the Americas is consumed domestically (Popma, 2002).
Total world tilapia landings from capture and culture has been estimated at 1.16 million tons (FAO 1997), with cultured tilapia accounting for 57% of the total (659,000 tons). The
most important tilapias in aquaculture are the maternal mouth brooders (Schoenen 1982; Pullin
1985): the Nile tilapia (T .nilotica); Mozambique tilapia (T. mossambica); and the blue tilapia (T.
aurea); plus a number of mouth brooding tilapia hybrids used in aquaculture (especially red T.
mossambica hybrids) withT. aurea, T. nilotica, and T. urolepis hornorum. These species account
for 99.5% of global tilapia production (FAO 1997). Of secondary importance are T. galilea and T.
melanotheron (principally in West African lagoons). Since the mid-1980s, there has been a shift
in producer preferences away from the Mozambique tilapia towards growing Nile tilapia. Nile
tilapia now dominates global tilapia aquaculture, accounting for 72% or 474,000 tons in 1995
(FAO 1997). Cuba is the world’s largest producer of blue tilapia, which are grown in an enhanced
reservoir fishery supplemented by hatcheries (Fonticiella and Sonesten 2000). The largest tilapia producing nations are in Asia. China is the world’s largest tilapia producer (315,000 tons),
accounting for 48% of global production, followed by the Philippines, Thailand, Indonesia and
Egypt (FAO 1997). The USA is the world’s largest tilapia consumer. US tilapia consumption is
estimated at 51,645 tons of live weight equivalent (Engle 1997). The USA imports over 3 times
the amount of tilapia it grows, with the major importers (in order by value): China, Thailand,
Costa Rica, Indonesia, and Columbia. Tilapia imports contribute a measurable share of the large
US trade deficit in seafood products. It is the third largest imported aquaculture product to the
US after shrimp and salmon and has received rapid consumer acceptance in US seafood circles
as the ‘new white fish’(ATA 2000). Tilapia are the most frequently requested fish in the US restaurant trade, and new markets carrying the fish for the first time report rapid acceptance (ATA
1995). The culinary characteristics of the fish match almost perfectly the desires of the US consumer, for example, a white flesh, boneless, relatively odorless, with a very mild flavor. Tilapia
are increasingly being seen as a replacement for cod and hake which are in short supply. Tilapia
sales have exceeded those of trout in the US each year since 1995 (ATA 2000). As a result, tilapia production in the Americas is expected to exceed 500,000 t by 2010 (Fitzsimmons,2000).
However, in some countries like Australia (Baron/Mitchell Tilapia Management Group, QFS,
2000) including the earlier efforts in Nauru (FAO 1979;1980), tilapia are not seen as an economic
resource but an ecological pest, that eradication programs are being undertaken through several
means (use of organic toxicants, construction of outfalls and others were undertaken unfortunately, but have not succeeded.
10
The various techniques that were developed and practices that were applied in controlling tilapia in many countries, mostly for aquaculture purposes, may also be used for the its prevention and control particularly in the identified water bodies of Nauru where the Mozambique
tilapia was introduced forty-five years ago. It is therefore the objectives of this undertaking to:
1.1. review the past attempts to control tilapia populations elsewhere in the world and identify
various techniques that may be applied in Nauru for tilapia population control;
1.2. determine the applicability of identified techniques taking note of the special environmental
conditions and the level of institutional capacity in Nauru; and
1.3. develop a broad strategy to implement a national tilapia program in Nauru.
2.0. Review of various techniques used in controlling
tilapia population in aquaculture
Failure of tilapia culture in the past, has often been due to uncontrolled spawning, resulting to the production of large numbers of fry and stunted populations. Development of various
techniques and practices and their appropriate applications have overcome this problem. Now,
the growth of fingerlings are enhanced and can attain a commercially acceptable size in a reasonable period of time. In order to sustain and insure the success of this system, it is necessary
to have a selection of strains with fast growth rate, late maturing and use of single-sex stock. It is
also necessary to create an environment that is conducive in allowing the fish to carry out its activity normally. Thus it is essential to prepare the ponds by first completely draining them after
each cropping then treat with environment-friendly toxicant to eradicate nuisance and harmful
species. It is also important to use healthy fingerlings of one age class for stocking. Feeding with
a suitable protein-rich diet is necessary.
There are basically seven (7) methods of controlling tilapia populations that have been
carried out in some countries for aquaculture and to some extent, eradication purposes. In a
number of these countries, tilapia is considered an economic resource and an important food
source for the people. The various methods and/or techniques that follow are: (1) periodic harvesting of fry and fingerlings; (2) monosex culture of which single-sex fish are obtained through:
manual separation of sexes, hybridization, hormone augmentation and genetic manipulation
methods (e.g. androgenesis, gynogenesis, polyploidy and transgenesis); (3) culture in cages; (4)
high density culture; (5) biological control; (6) sterilization and (7) eradication using organic
toxicants and/or other chemicals. Application of these methods in specific countries has been
done with certain degree of success.
2.1.
Periodic harvesting of tilapia fry and fingerlings.
Pond culture is still the most popular method of growing tilapia. A mixed-sex population of
tilapia is stocked in ponds and the normal pond culture procedures are carried out. However,
instead of waiting to obtain the total yield at one harvest, collection of fry and fingerlings is done
periodically every one to three weeks. One advantage of this method is that the fish are able to
utilize natural foods. However, there is a need for a management scheme in order to maximize
tilapia production - the extensive systems (using only organic or inorganic fertilizers) and the
semi-intensive systems (using high-protein feed, aeration and water exchange). The systems are
described by Mair (2002) in more details as follows: In the extensive system, the sexually
11
mature fish are stocked in earthen ponds. Ponds are then fertilized and/or feeds are introduced
to support broodfish and early growth of fry and fingerlings. Ponds are seined regularly then
finally drained and harvested completely after a set period of three weeks to six months depending on desired efficiency. During seining broodstocks are usually captured first, in coarse mesh
nets, and the pond then seined with finer mesh nets to removed fry and fingerlings of varying
sizes. Brood stocks are then released back into the pond. With regular seining, ponds may not
be drained completely for up to a year. Management of such ponds is quite simple and capital
and labor requirements are low. Also, as a high proportion of the harvested seeds are already
fingerlings, they can be sold immediately without requiring nursing. Productivity for such systems varies ranging from 400-4000 fry/kg female/month (Little and Hulata 2000) but is often
quite as a result of fingerling mortality due to predators that might be in the pond and the effects
of cannibalism both by broodstock and by the omnivorous juvenile tilapia which result from
the earliest spawnings. The mixed size and age of fry collected from these systems, render them
unsuitable for hormonal sex reversal and thus these systems are almost exclusively used for production of mixed sex fry and fingerlings. The collected fry and fingerlings are then graded prior
to sale or further nursing.
On the other hand, in the semi-intensive system, broodstocks are stocked into small,
shallow ponds, typically 200-500 m² in area and 0.5-1m deep. Ponds are prepared and broodstocks are stocked for a cycle of normally 3-5 weeks. Fry collection strategies, which commence
about 15 days after stocking, rely on the fact that very young fry shoal together in groups, usually
close to the surface of the pond. Fry can then be scooped up using long handled nets from the
dyke of the pond or large scoop nets can be used to sweep the whole upper layer of water. The
disadvantage of entering the water to regularly collect fry is that pond sediments are disturbed
increasing turbidity and thus decreasing primary productivity, and this should be avoided as
much as possible.
Mixed-sex populations of fry are cultured together and harvested before or soon after
they reach sexual maturity, thereby eliminating or minimizing recruitment and overcrowding. A
restricted culture period limits the size of fish that can be harvested. In mixed-sex culture, tilapia are usually stocked at low rates to reduce competition for food and promote rapid growth. On
the other hand, in order to allow the tilapia to approach its maximum size at a reasonable culture
period, the density must be reduced by harvesting the fry and fingerlings to reduce competition
for food. This method is effective in small ponds but it is labor intensive and requires little skill.
In order to take advantage of this situation, the fry and fingerlings collected may be graded then
sold or nursed for use later as seeds.
The major drawback of pond culture is the high level of uncontrolled reproduction that
may occur in grow-out ponds. Tilapia recruitment, the production of fry and fingerlings, may
be so great that offspring compete for food with the adults. The original stock becomes stunted,
yielding only a small percentage of marketable fish weighing 1 pound (454 grams) or more. In
mixed-sex populations, the weight of recruits may constitute up to 70 percent of the total harvest
weight.
2.2. Monosex culture.
Single-sex tilapia are used as the stocks under this method of farming. Because the major problem of pond-cultured tilapia is excessive reproduction, and subsequent stunting of fish due to
overcrowding use of all-male tilapia as the stocks may be able to combat this problem. Males are
preferred because they grow almost twice as fast as females. This technique is called monosex
culture and is used when large fish are required by the market. Monosex culture may resolve the
12
problem of uncontrolled reproduction in rearing ponds. Rakocy and McGinty (1989) reported
the following account relevant to the all-male tilapia culture as follows: Male monosex culture
permits the use of longer culture periods, higher stocking rates and fingerlings of any age. Expected survival for all-male culture is 90 percent or greater. A disadvantage of male monosex
culture is that female fingerlings are discarded. The percentage of females mistakenly included
in a population of mostly male tilapia affects the maximum attainable size of the original stock
in grow-out phase. The stocking rate for male monosex culture varies from 4,000 to 20,000/acre
(≈10,000 to 50,000/ha.) or more. At proper feeding rates, densities around 10,000/ha. allow the
fish to grow rapidly without the need for supplemental aeration. About 6 months are required to
produce 500-g fish from 50-g fingerlings, with a growth rate of 2.5 grams/day. Total production
approaches 5.4 tons/ha. A stocking rate of 20,000/ha. is frequently used to achieve yields as high
as 11 tons/ha. At this stocking rate the daily weight gain will range from 1.5 to 2.0 grams. Culture
periods of 200 days or more are needed to produce large fish that weigh close to 500 grams. To
produce a 500-gram fish in temperate regions, over wintered fingerlings should weigh roughly
70 to 100 grams and be started as early as possible in the growing season. A stocking rate of
20,000/ha does require night time emergency aeration when the standing crop is high. Stocking rates of 30,000 to close to 50,000/ha. have been used in 0.5 to 1-ha. ponds but this requires
the continuous use of two to four, one-horse power paddlewheel aerators per pond. Yields for a
single crop range from 15 to 25 tons/ha. With optimal temperatures, feeding rates depend on
fish size and density. Optimal daily feeding rates for fish of 30, 50, 100, 175 and 450 grams are
3.5, 3.0, 2.5, 2.0 and 1.5 percent of body weight, respectively. If densities are high, sub-optimal
feeding rates may have to be used to maintain suitable water quality, thereby increasing culture
duration.
One technique that allows reliable control of the population is that of stocking the pond
with fingerlings and raising of a single sex. Some species of the genus Tilapia can be easily sorted
into males and females. Either the colors are sufficiently differentiated to serve as reliable sex
indicators or the structure of the anal papilla is used - the opening of the oviduct being distinguishable in the female and not present in the male. With experience it is possible to sex even
small immature fish with speed and confidence. A second check is made when the fish have
grown somewhat larger and distinctive sex-coloration’s are more discernible. Since this technique fails if there is a single female present in the raising pond, care must be taken to ensure
that there are no females left over from a previous stocking. The sexing of small tilapias although
feasible is tedious. In addition to being not entirely reliable. (Hickling, 1963). Single-sex population maybe obtained utilizing the following methods/techniques: rear fry to fingerling size in a
nursery tank/pond and then separate the male fingerlings from the females. The male fingerlings
are then prepared for final grow-out. Other than manual sexing, all-male fingerlings can also be
obtained by other methods such as: genetics and hybridization and hormone augmentation (sexreversal). None of these methods is consistently 100 percent effective, and thus a combination of
methods is suggested.
2.2.1. Manual separation of male from female fish.
This is the process of separating males from females by visual inspection of the external urogenital pores, often with the aid of dye applied to the papillae. Secondary sex characteristics may
also be used to help distinguish sex. This requires skilled personnel and trained labor. It necessitates relatively large-size fingerlings (50-80 g) but errors can easily be made even by skilled
workers and only about 90% efficient. The sexing of small tilapias although feasible is tedious,
time-consuming and often results in a loss of 40-50 percent of the fingerling production due to
the discarding of females. This is difficult for large ponds inasmuch as large numbers of fish are
needed and the process is slow. (Hickling, 1963). Manual sexing is commonly used by producers.
13
Reliability of sexing depends on the skill of the workers, the species to be sorted and its size. Experienced workers can reliably sex 15-gram fingerling T. hornorum and T. mossambica, 30-gram
T. nilotica, and 50-gram T. aurea.
The International Center for Aquaculture and Environment of Auburn University in Alabama, U.S.A. developed a procedure for manual separation of sexes in tilapia
in graphical form and is being reproduced here (Source: Bocek ,A. Ed. Undated)
A farmer can readily distinguish male and female tilapia with practice. When tilapia
reaches about 10 cm in length (about 20 g) the sexes are distinguished by inspecting the genital
papillae on the fish’s underside (Figure 1). Experienced worker can manually separate by sex
about 2000 fish per day with an accuracy of 80 to 90%. Therefore, some reproduction will always occur. The method is tedious, stresses fish and is not 100% effective. However, production
of manually sexed tilapia fingerlings for grow-out to market size can be accomplished by farmers with few financial resources and little fish culture experience. The procedure is illustrated
in Figures 2 and 3.
Procedure For Manual Separation Of Sexes
Figure 1: Ventral view of a tilapia .showing the main parts.
Figure 2: A farmer examining and sorting tilapia by sex.
14
Figure 3: Small fish may be held in one hand and examined. Large fish are held with two hands.
♀
♂
Figure 4: This close-up shows a female (top) and male (bottom) tilapia together. Note that the female
has two openings in the papilla for passage of urine and eggs, while the male has only one opening for
urine and sperm passage.
2.2.2. Hybridization.
Hybridization in fishes is crossing two closely related but distinct subspecies of fish. As a tilapia
control measure, hybridization maybe used to produce a high percentage of male fish so that
reproduction is lower therefore overcrowding and stunting is avoided and bigger tilapia are produced. Among the major constraints in producing hybrids are: maintaining the purity of brood
stocks, limited fecundity of parent fish which restricts fry production, difficulty in producing
sufficient number of hybrid fry due to spawning incompatibilities between the parent species
15
and at times an impasse in preventing the entry of wild tilapia into culture ponds. Inasmuch as
not all crosses produce 100 percent males, the hybrids may still be subjected to manual separation of sexes or hormone augmentation. While hybridization saves time, space and feeds it is not
without problems.
In 1958 a cross between T. hornorum males and T. nilotica females was reported to
have produced all-male offspring by crossing one purebred local variety of T. mossambica that
was descended from the original tilapias first found in Java in 1939 with one that is indigenous
to brackish swamps on the island of Zanzibar off the coast of Africa. However, it requires pure
strains otherwise it is likely that some females may be produced if the broodstocks are not of
pure line. The F1 generation, being fertile, may backcross with the female parent fish in rearing
ponds and produce fry of both sexes (Hickling, 1963). This was corroborated by Chevassus-auLouis (2002) who stated that hybridization as a method to control tilapia population by producing single-sex fish, obliges fish farmers to either produce or buy F1 hybrids or maintain and
control “pure” tilapia parental stocks. However, it does not allow the combination of the characteristics of parental species in any other proportion than a 50/50 ratio. This is not always optimal
and it always gives very heterogeneous populations. Production of F1 hybrids that are essentially
related to the production of monosex varieties has been the passion in tilapia. Considerable efforts have been made to use hybridization to control overpopulation in tilapia pond culture systems however, this is hardly sustainable. It must be kept in mind that the method of producing
a population of monosex tilapia through hybridization sometimes results in failures and there
now exist other and more efficient methods based on sexual inversion or population control. In
order to improve the future performances of F1 hybrids, two parental species must be selected,
which is a heavy and complex process. Performing it otherwise, may only obtain half of the possible genetic progress (Chevassus-au-Louis, 2002). It was suggested that using hybridization in
building “founder” gene pools between interesting tilapia species presents several advantages.
Due to the need to maintain pure line stocks to satisfy these requirements, special hatchery facilities and skilled labor are required. Thus, production of hybrid fingerlings expensive. Due care
must be exercised to avoid contamination by other tilapia to insure purity among the crossbred
fingerlings (Hickling, 1963).
Hybridization can also be illustrated by the Molobicus Program now being conducted in
the Philippines by BFAR, PCAMRD and CIRAD (Westly del Rosario, Head, National Fisheries
Training and Research Center, Bureau of Fisheries and Aquatic Resources, Department of Agriculture, Bonoan, Pangasinan, Philippines, personal communication).The idea of the program
is to produce a hybrid population that is tolerant to salinity through successive production of
F1 hybrids between mossambicus and niloticus and successive backcrosses with mossambicus
then the hybrid population is selected for growth. The evolution of salinity tolerance between
pure niloticus and pure mossambicus and of the F1 and first backcross has illustrated that the
progress through successive generations is regular. The strategies using F1 are not completely the
simple use of F1 for or direct use of F1 in aquaculture. It is complementary to the new strategies
of combining selection and hybridization and so the founding pool of genes can be associated
with the F1 and a selected fish obtained from a strain or pure species. This program while not a
method for tilapia control, aims at selecting a fast-growing tilapia resisting high salinity. A set of
genes in proportions between 1/8 and 7/8 of genes of one species is obtained and pooled. From
this pool of genes, it is possible to conduct a genetic selection within a hybrid population in the
same way one would conduct selection with pure species. That genetic selection, when achieved,
can easily be extended to the farmers in a sustainable way since they can breed and grow the
selected strain as they would for the pure species.
There was a suggestion however, that for a genetic and hybridization programs to be
credible, an evaluation process must be created by a common structure (Mr. Pierre Morissens
16
(c/o PCAMRD-CIRAD Project Los Baños, Laguna). This is necessary in order to know exactly
what the benefits of the different approaches are. With all the speculations developed abroad,
it is hard to assess that any one strategy or approach is automatically better than another. Furthermore, what is true for a given generation may not be true after a few successive generations.
It therefore becomes a necessity to set up a system to check objectively and independently the
performances of any strain produced. These tests should cover the use of the different strains
under various farming systems especially in places where many farmers are producing tilapias
intensively. The intensive and the very extensive production of tilapia with the framework of
polyculture is very important also so that the testing should cover the different farming systems.
The results of these tests should be accessible to any user.
2.2.3. Hormone augmentation.
This is a technique which has been adopted with some degree of success. The principle behind
this method lies on the fact that at the stage when the tilapia larvae are said to be sexually undifferentiated (right after hatching up to about 2 weeks or up to the swim-up stage) the extent of
the androgen (male hormone) and the estrogen (female hormone) present in a fish is equal thus,
augmenting one of the hormones that is originally present in the fish will direct the fish to either
male or female depending upon the hormone introduced. Accordingly, if the tilapia larvae are
fed with feeds that are incorporated with male hormone (e.g. 17α-methyltestosterone), the fish
will develop into phenotypic male (physically appears and functions as male but possesses the
female genotype (XX); in the same way, if a female hormone is mixed with the feed that is taken
by the fish, then the fish will be directed to phenotypic female (physically appears and functions
as female but possesses the male genotype (XY). This is commonly referred to as “sex reversal”
wherein the sex of the fish that is produced after having been fed with hormone-treated diet is
either male or female depending upon what hormone (androgen or estrogen) was taken in by the
fish. To insure that the hormones shall function effectively, it is very important that the fry or
larvae must eat only the specially prepared artificial feeds containing the hormone with the exclusion of any other food sources. This means that the larval rearing facilities must be absolutely
clean to prevent the fry from eating other food sources (e.g. algae) that may develop inside the
rearing container. Feeding the larvae with hormone-treated diet, e.g., 17α-methyltestosterone or
estrogen between the second and sixth week after hatching has been observed to have produced
high percentage of males and females, respectively.
Some constraints in the use of hormones in fish is that the presence of hormone residue
in adult fish has not yet been studied thus its effect on the consumers is not yet known. Hormones may also be difficult to obtain in some countries and hatchery facilities and skilled labor
are required.
2.2.4. Genetic manipulation methods.
(Androgenesis, gynogenesis, polyploidy and transgenesis). Genetic improvement in tilapia has
been achieved through the exploitation of qualitative variances (e.g., selective breeding, hybridization and crossbreeding and genetic manipulation (e.g., sex control chromosome manipulation and transgenesis). Two examples of successful selection programs in tilapia are the
ICLARM-coordinated Genetically Improved Farmed Tilapia (GIFT) Project and the Freshwater
Aquaculture Center (FAC)–Central Luzon State University (CLSU) Project which produced the
YY Super males. The GIFT and the YY Super Males produced through YY male technology are
not considered genetically modified organisms. The GIFT Project reported a 70% increase in
growth rate over 7 generations of combined selection in a synthetic Nile tilapia strain developed
17
from newly introduced African and domesticated Asian stocks. The FAC/CLSU Project reported
a cumulative response of 18.4% after 16 generations of within family selection using a Philippine
strain. Recently, traditional selective breeding methods have been enhanced by advanced DNAbased technologies. Molecular markers have been used to identify quantitative trait loci (QTLs)
that can enable selection for desirable traits in marker-assisted selection (MAS) programs. Successful genetic manipulation methods, (i.e., gynogenesis, androgenesis, polyploidy and transgenesis) their potential as well as risks to commercial aquaculture have been reported. Future
prospects in the applications of traditional selection methods and new molecular technologies in
the genetic improvement of tilapias are now available. The continuation of selection and sex control programs for growth and environmental tolerance traits possible using indigenous species
or strains from Africa is foreseen. The importance of markers in bioinformatics (e.g., pedigree
analyses in farm based trials and breeding programs) washad been stressed. To avoid the deterioration in quality of tilapia culture stocks, farmers are advised to choose their cultured strains
carefully. The following basic management guidelines should be adopted:
1.
Keep high effective population size,
2.
Avoid unconscious selection
3.
Transfer stocks between hatceries, and
4.
Maintain stock/strain purity.
Adopting viable traditional selection methods can improve commercially desirable
strains in tilapia. Applying novel DNA-based techniques (e.g., MAS) in genetic stock improvement schemes may consequently give faster results in enhancing tilapia production (Dr. Graham
Mair during the International Forum on Tilapia Farming in the 21st Century held at Los Baños,
Laguna, Philippines on February 25-27, 2002).
The realization of the need to improve the tilapia stocks brought about the creation of the
GIFT (Genetically Improved Farmed Tilapia). This started in 1988 when direct transfers of Nile
tilapia to the Philippines from Egypt, Ghana, .Kenya and Senegal were made. The tilapias were
held at the National Freshwater Fishery Training and Research Center/Bureau of Fisheries and
Aquatic Resources (NFFTRC/BFAR), Muñoz, Nueva Ecija. These served as the founder stock.
From these stocks, production of mixed base population (synthetic breed) was initiated and run
for at least one generation of selection. The tilapia that were produced from these are called the
GIFT tilapia and it is claimed that they grow at an average of 60% faster than the other breeds.
In order to sustain the quality of the stocks, the genetic pool is being maintained by continuous
broodstock selection. The beneficial role of genetics in aquaculture has been demonstrated in
this specific project by showing that growth of a fish can be improved and its quality enhanced
through the application of the science of genetics. This is consistent with its objective to increase
the quantity and quality of fish protein from aquaculture and to obtain continuous long term
progress in productivity.The GIFT fish was a result of the selection program designed to improve the breeds of tilapia for low-cost sustainable aquaculture in developing countries. It was
evaluated by DEGITA (A Project on the Dissemination and Evaluation of Genetically Improved
Tilapia) before the GIFT was released to the farmers. Today, the GIFT fish has been disseminated
to the farmers through the six accredited operators and one pilot hatchery in the original site in
Central Luzon State University, (Abella, Tereso, CLSU, Nueva Ecija,Philippines personal communication).
Androgenesis is a mechanism that can directly produce YY-males, at least in theory, to
be used as broodstock to breed all-male progeny. It is the diploidization of the paternal genome
18
(androgenote) which is accomplished by interference with first mitosis for eggs that have been
genome-neutralized before activation (Shelton, 2001). It is also the selection of individuals for
sex inheritance characteristics that will be used in monosex production. Intraspecific breeding
programs have been developed to exploit the sex inheritance mechanism in the tilapia T. nilotica
to produce male populations. These programs are built on the premise that the mechanism of sex
inheritance must conform closely to a mono-factorial sex determination with a heterogametic
male. Sex inheritance in tilapia, however, appears to be more complicated. The sex ratio of an
individual spawn often does not conform to the expected 1:1 ratio. A better understanding of sex
inheritance in tilapia and the identification of tilapia populations with a minimum variation in
progeny sex ratios from individual spawns is needed for a successful intra-specific breeding program to produce male tilapia. In a study by Phelps and Carpenter (2001) sex ratios in tilapia did
not appear to be passed on from one generation to another. A realized heritability for sex ratio of
–0.09 was calculated. No family with skewed sex ratios produced progeny from sibling matings
with similarly skewed sex ratios.
The androgenetic approach to developing YY males simplifies the identification of YY
males as all males produced should be of the YY genotype. This is based on the theory that interand intraspecific breeding programs can result in populations with highly skewed sex ratios and
often give inconsistent results. Interspecific crosses have not proven to be practical due to difficulties in maintaining the parent species integrity. Intraspecific breeding programs have been
developed to exploit the sex inheritance mechanism in Nile tilapia, T. nilotica.
YY-Male Technology for production of monosex tilapia. It has been shown that all-male tilapia
population can be obtained from a cross between the offsprings of ♀ T. mossambica x ♂ T. hornorum. Furthermore, a cross between ♀ T. hornorum and ♂ T. mossambica could yield 75%
males and 25% females as follows:
Knowing that chromosomal mechanism of sex determination in tilapias is possible, super male
tilapias were thought of because of its desirable characteristics in aquaculture. The YY-male
technology is nothing but genetic manipulation of sex in tilapia. In this method, sex reversal to
female is done initially. This is carried out by hormone augmentation. The female hormone (estrogen) is mixed into the feeds then fed to one week old tilapia fry which at this stage are still
♀ T. hornorum
X
♂ T. mossambica
WZ
X
XY
WX
WY
XZ
ZY
♀♀
♂♂
♂♂
♂♂
Figure 5. Illustration of a cross between ♀ T.hornorum x ♂ T. mossambica showing a yield of 75%
males and 25% females.
sexually undifferentiated and the amount of the male and female hormones in their systems are
equal. Feeding the fish with the treated diet is done up to 2 weeks and the sexually undifferentiated tilapia fry would develop into females. These “sexually reversed” females are actually phenotypic females (physically appears and functions as female but possesses the male genotype
(XY). When these phenotypic females are crossed with normal males (XY) the resulting progeny contains approximately ¼ of the novel genotype YY. Further crosses of YY-males with ¦YY
19
(sex reversed females) followed by a second generation of sex reversal could produce YY-females
which could then be crossed with YY-males for the production of all-male producing YY-male
broodstock. The success of this technology has been tried and the large scale production of allmale tilapia is now possible. The super male YY tilapia has shown to have produced genetically
male tilapia with reports from studies that the fish grew better than the mixed sex tilapia by
about 50% and 23% better than the sex-reversed tilapia. Today, there are about 40 accredited
hatchery operators supplying the industry with the needed all-male quality fingerlings (Abella,
Tereso, CLSU, Nueva Ecija,Philippines).
The methods described above may be practical to use in countries such as the United
States of America where technical requirements are easily accessible. Work on the development
of tilapia breeding and genetic improvement program is a continuing process in the Philippines
to produce improved breeds and strains of tilapia for culture. Pond and cage culture of saline
tilapia for brackish and sea water environment is being pursued vigorously. The GIFT-PNTBP
(Genetically Improved Farmed Tilapia-Philippine National Tilapia Breeding Program) is a major
program of the Philippine government in support of tilapia aquaculture. The GIFT tilapia strain
is now called Genomar Supreme Tilapia.
Dr. Graham Mair (International Forum on Tilapia Farming in the 21st Century held at
Los Baños, Laguna, Philippines on February 25-27, 2002) further reported the following: The
small founding population of tilapias (i.e.,Tilapia mossambica and T .nilotica) introduced in the
region has resulted in low genetic variability and poor performance of the cultured Asian stocks
compared to the indigenous African stocks. Poor broodstock management has led to the genetic
deterioration of domestic tilapia populations. The breeding of tilapias is fairly easy and can be
done extensively, semi-intensively and intensively in tank, cage and pond-based hatchery systems. Productivity in these systems varies widely but is often sub-optimal. Breeding efficiency
can be optimized through proper stock management, broodstocks nutrition and improved nursery techniques.
In the same forum (Tilapia Farming in the 21st Century) Dr. Hans Magnus Gjoen of Biosoft (Philippines) remarked that the continuation of the GIFT strain which is now called Genomar Supreme Tilapia is a conventional set-up for a selection program for additive genetic effects.
The main strategy is to keep the families separate using a conventional tagging system placed in
a communal rearing facility to avoid the common environment effect. There are several drawbacks to this: (1) the common environment introduced by this separate rearing facilities becomes
very difficult to separate from genetic effects later on when one does the genetic evaluation; (2)
there will be low numbers within each compartment because of the high costs linked to the tagging. The whole system is quite costly both in investment and in operation.
2.3. Culture of fish in cages.
The rearing of fish and other organisms in a structure that is suspended in water above the pond
bottom but anchored and where the water maintains free circulation that may come from a natural or man-made lake or the water body is referred to a cage culture or the practice of rearing
fish/aquatic organisms in cages. It can be applied in other existing bodies open water such as
large reservoirs, farm ponds, rivers, cooling water discharge canals, estuaries and coastal embayments. In the southern U. S., tilapia are among the most suitable fishes for cage culture. Cage
culture of tilapia is considered a method of controlling tilapia population because spawned eggs
fall through the cage mesh and die. Furthermore, the breeding cycle of tilapia is disrupted in
cages, and therefore mixed-sex populations can be reared without the problems of recruitment
and stunting, which are major constraints in pond culture. Also, eggs fall through the cage bottom and do not develop even if they are fertilized. However, reproduction could occur in cages
20
with 1/10-inch mesh or less, being small enough to retain eggs (McGinty and Rakocy, Undated).
It was observed that male tilapia that are loose in open waters could still “mate” with female tilapia inside the cage (mating “behind bars”). The observation can be described as follows: Female
tilapia releases eggs in one corner of the cage then picks them up with its mouth. The eggs stay
in the mouth until a male outside the cage gets near it. The male then allows the female to suck
its genital papillae until it releases the sperm which fertilizes the eggs inside the mouth of the
female. The eggs after fertilization hatch into fry and swims inside the cage when they reach the
swim-up stage (Fortes, R. D., College of Fisheries and Ocean Sciences, University of the Philippines in the Visayas, Miagao, Iloilo, Philippines).
McGinty and Rakocy (Undated) considered Tilapia nilotica (Nile Tilapia), T. aurea (blue
Tilapia), Florida red tilapia, Taiwan red tilapia and hybrids between these species and strains
as the most appropriate species or strains of tilapia for cage culture. The choice of a species for
culture in the United States of America depends mainly on availability, legal status, growth rate
and cold tolerance. Many states prohibit the culture of certain species. Unfortunately, T. nilotica,
which has the fastest growth rate, is frequently restricted. The ranking for growth rate of the remaining species or strains are Florida red tilapia > Taiwan red tilapia > T. aurea.. Hybrids of T.
nilotica x Taiwan red tilapia grow as fast as T. nilotica. Hybrids of T. aurea x Florida red tilapia
grow at an intermediate rate between Florida and Taiwan red tilapia. Cold tolerance, which is
important in northerly latitudes, is greatest in T. aurea..
Other advantages of cage culture are: (1) ease and low cost of harvesting; (2) close observation of fish feeding response and health; (3) ease and economical treatment of parasites and
diseases; and (4) relatively low capital investment compared to ponds and raceways. On the other hand there are also some disadvantages as follows: (1) risk of 1oss from poaching or damage
to cages from predators or storms; (2) less tolerance of fish to poor water quality; (3) dependence
on nutritionally-complete diets; and (4) greater risk of disease outbreaks. Cage culture offers
several important advantages. Culture of fish in cages however, requires intensive feeding with
high quality ration and cage materials may also be expensive.
2.4. Culture at very high densities.
High density culture in ponds, cages, pens or raceways is considered a control measure for tilapia. It works on the principle that crowding reduces the urge to reproduce. When carried out in
mesh cages that maintain free circulation of water, high density culture could attain significantly
higher fish production. However, it requires intensive feeding with a high quality ration, availability of good water supply, needs electric, gas or diesel aeration devices and skilled management.
2.5. Biological control.
This is a means of controlling certain fish population by stocking predacious fish as fingerlings or
adults in a pond. This concept and method was developed by Dr. Homer W. Swingle, recognized
as the Father of Aquaculture in the United States of America who hailed from Auburn University
in Alabama. It needs an effective predatory fish that can control excessive reproduction. This
method produces at least two different kinds of fish – the predatory fish and the prey species.
The successful stocking of the large new lakes created by dams in Southeastern United States
was based on the predation of the black bass (Micropterus salmoides) on the bluegill sunfish
(Lepomis sp). In the case of tilapia, large fish must be stocked initially or they will be eaten by
the predators. The predators will not only crop down the surplus fry and thereby permit better
growth in the Tilapia population; they will also contribute to the variety and value of the total fish
crop. It has been shown that excessive production of tilapia fry can be overcome by the addition
of a predatory species to control the population by feeding on fry and fingerlings. Predator species
such as Lates niloticus, Hemichromis fasciatus and Clarias lazera have been used for this purpose (FAO/UNDP Task force onAquaculture Development and Coordination Programme,1980)
21
Using the predator-prey method, one reservoir in Malaca had produced more than
6000 pounds a year of good-sized tilapias weighing between three-quarters of a pound and two
pounds each (Hickling, 1963). In order to keep the predator-prey balance at an advantageous
level, there must be frequent stocking. Several species of predatory fishes have also been used
for this purpose in other parts of the world. In East Africa the Nile perch (Lates niloticus), a fine
sporting fish, has been tried; in the Cameron’s the black bass and a fish related to the Tilapia
called Hemichromis fasciatus have been stocked in tilapia ponds; in Jamaica the local tarpon
(Megalops cyprinoids) has served as an effective predator.
At the College of Fisheries and Ocean Sciences, University of the Philippines in the Visayas, Leganes, Iloilo, Philippines, three species of fish were used to control the reproduction of
tilapia in pond culture. These are: tenpounder (Elops hawaiiensis) (Fortes, 1979;1980); tarpon
(Megalops cyprinoides) (Fortes, 1979; 1980) and sea bass (Lates calcarifer) (Fortes, 1986).A
predator-prey ratios of 1:20, 1:10 and 1:10 were found to be effective in controlling tilapia fry and
fingerlings, respectively.
In Nigeria, Fagbenro (2000) conducted a quantitative evaluation of the predation efficiency of two hybrid clariid catfishes (Heterobranchus longifilis x Clarias gariepinus) and (H.
bidorsalis x C. gariepinus) in controlling recruitment of Nile tilapia (Tilapia niloticus) while producing market-size fish. This was conducted in small earthen ponds (200 m2) using three tilapia
: hybrid catfish stocking combinations (5:1, 10:1, 20:1). After 180 days the treatments 5:1 and 10:1
showed better production.
In the United States, largemouth bass (Micropterus salmides) has been used as the predator for tilapia (T. aurea). A forage/predator ratio of 0.7 and 1.4 are suitable for controlling spawn
and producing large tilapia and large bass, respectively The technique of culturing predatory
fish with a prolific forage species has potential for other game fish, particularly species of high
economic value and whose food requirements are not readily satisfied with commercial feeds.
Maintaining tilapia: bass ratio seems feasible for one production cycle in a temperate climate.
Both the tilapia and bass could be marketed as a food fish crop. The carnivore (bass) harvested
from such a system could be used for management of sport and recreational fisheries (Wurts et
al., 2001).
At the Asian Institute of Technology, Thailand, Yi et al. (2002) tested the efficiency of
snakehead (Channa striata) in controlling recruitment of mixed-sex Nile tilapia (T.nilotica) in
ponds and to assess growth and production characteristics of Nile tilapia in monoculture and
polyculture with snakehead in fertilized earthen ponds from March through October 2000.
There were six treatments: A) monoculture of sex-reversed all-male tilapia; B) monoculture of
mixed-sex tilapia; C) polyculture of snakehead and mixed-sex tilapia at 1:80 ratio; D) polyculture
of snakehead and mixed-sex tilapia at 1:40 ratio; E) polyculture of snakehead and mixed-sex
tilapia at 1:20 ratio; F) polyculture of snakehead and mixed-sex tilapia at 1:10 ratio. Sex-reversed
and mixed-sex Nile tilapia were stocked at 2 fish m-2 at sizes of 10.5 to 11.6 g and 7.2 to 8.1 g,
respectively. Results show that snakehead were able to completely control Nile tilapia recruitment at all tested predator:stocked-prey ratios, and the best predator:stocked-prey ratio was
1:80. The addition of snakehead into Nile tilapia ponds did not result in significantly greater
tilapia growth, but it significantly lowered total net and gross yields of adult plus recruited tilapia. Snakehead growth was density-dependent, decreasing significantly with increasing stocking densities. While snakehead biomass gain was not significantly different at stocking densities
from 0.025 to 0.1 fish m-2, the gain was significantly lower at a stocking density of 0.2 fish m-2.
The present experiment demonstrates that snakehead are able to control Nile tilapia recruitment
completely and provide an alternative technique for Nile tilapia culture.
22
Various predatory fish species have been used with varying success in combination with
different tilapia species depending on their availability. These species include snakehead (Channa striata or Ophiocephalus striatus) (Pongsuwana, 1956; Chimits, 1957; Tongsanga, 1962;
Chen, 1976; Cruz and Shehadeh, 1980; Hopkins et al., 1982; Wee, 1982; Balasuriya, 1988); Ophiocephalus obscuris (de Graaf et al., 1996); Micropterus salmoides (Swingle, 1960; Meschkat,
1967; McGinty, 1985); Lates niloticus (Meschkat, 1967; Planquette, 1974; Lazard, 1980; Bedawi,
1985; El Gamal, 1992); Hemichromisfasciatus (Bardach et al., 1972; Lazard, 1980); Cichla ocellaris (Lovshin, 1977; McGinty, 1983; Verani et al., 1983); Clarias sp. (Meecham, 1975; Bard et
al., 1976; Lazard, 1980; Janssen, 1985; de Graaf et al., 1996); Cichlasoma managuense (Dunseth
and Bayne, 1978); Elops hawaiiensis (Fortes, 1980); and Megalops cyprinoides (Fortes, 1980).
However, the difficulty in breeding or obtaining predators of the correct size often resulted in
limited application of this population control method (Balarin and Hatton, 1979; Penman and
McAndrew, 2000).
Biological method to control the forage species requires draining of the pond, sorting out
the fish and restocking with the right proportion of predator and prey. The technique is a sophisticated one, with plenty of room for error. Another drawback of this strategy is that often, it is
difficult to get adequate numbers of predator fingerlings (Hickling, 1963).
2.6. Sterilization.
In Nigeria, Ekanem and Okoronkwo (2003) tried pawpaw (Carica papaya) seed as fertility control agent on male Nile tilapia. Mature male tilapia with mean weight 40 g were treated for
30 days with 4.9 g/kg/day (low dose) and a 9.8 g/kg/day (high dose) of ground pawpaw seeds
incorporated into their feed. In order to determine the effect of the treatments, a control treatment was added using fish of similar sizes fed with feed that did not contain pawpaw seed. No
spawning occurred in any of the replicates in the high dose treatments during the 30-day treatment period. Fish in the control experiment spawned two weeks and five weeks after while fish
in the low dose treatment spawned three weeks after the treatment was discontinued. When
sections of the testes were examined histologically, swollen nuclei in the low dose treatment and
disintegrated cells in the high dose treatment were observed. This indicated the positive effect
of pawpaw seeds in inducing sterility in Nile tilapia. The application of this method of controlling reproduction in tilapia is straightforward and can easily be adopted by poor fish farmers
inasmuch as pawpaw seeds are available all year round in the tropics and subtropical regions.
Pawpaw seeds contain active ingredients such as caricacin, an enzyme carpasemine, a plant
growth inhibitor, and oleanolic glycoside, the last of which had been found to cause sterility in
male rats (Das 1980). Other means to carry out sterilization program is the use of irradiation to
cause sterility to the target fish.
2.7. Eradication with organic toxicants and other means.
The desire to get rid of noxious species that was introduced into a country, legally or illegally,
necessitated the use of toxicants that could eradicate the deleterious species. Toxicants such as
rotenone has been used for this purpose. Substances such as potassium chloride, hydrated
lime (CaOH), ROCCAL™ (Benzalkonium chloride) and other molluscicides/toxic compounds
that are effective and highly toxic to most fish species have also been used in eradicating noxious
species. The rapid proliferation of tilapia in an area when introduced can be demonstrated by the
experience in the Mexican State of Quintana Roo. Tilapia was introduced into this state in 1974
by direct releases into water bodies and later in 1982, raising tilapia in cages. After 18 to 22 years,
surveys were conducted (1992 to 1996) to determine the distribution of tilapia and its relative
abundance. Escape from floating cages was the major cause of tilapia proliferation. Tilapia was
found in 50% of the intensive culture sites visited and T. mossambica and hybrids, probably with
23
T. nilotica, were collected in four sites where it had not been introduced. Others were believed
to be by invasions from nearby lakes and unauthorized introductions and became dominant
(>20% of the total number of individuals) in most localities where it appeared ( Schmitter-Soto
and Caro, 1997).
As mentioned earlier, similar occurrence occurred in the Republic of Nauru when T. mossambica or the Mozambique tilapia was introduced. Because of the widespread distribution of
tilapia in the Republic of Nauru, the government implemented a Tilapia eradication program
through FAO in 1979 and 1980.
The import of tilapia to Queensland was prohibited in 1963 but tilapia was able to enter the
country and became widespread in Queensland. The proliferation of tilapia was attributed to:
• Movement through the irrigation channel into the Walsh River (major Mitchell River
tributary);
• Movement through water releases or overtopping of the spillway and into a catchment via a
flooded low lying area and by irrigation pumps and drains;
• Direct translocation (i.e. by humans)
• Movement directly into a river via irrigation channel and into the wetlands
The problem of tilapia in the Barron and tributary streams was first reported in 1986.
They were all well established in the southern parts of the dam. Spot eradication of T. mossambica was performed in Townsville in small, contained water bodies (e.g. ponds, dams) with the
use rotenone. Unfortunately eradication of tilapia by this method was not successful. Prevention
of further spread of the Mozambique tilapia into the river systems became a priority and one
attempt was to place screen on the outfall of some dams (e.g. Tinaroo Dam). (Baron/Mitchell
Tilapia Management Group, QFS, 2000).
Based on the information described earlier, the infestation of undesirable species may be
prevented if protocols shall be strictly followed. The following courses of action may not apply to
all but there are provisions that could be applied to noxious aquatic animals including tilapia.
(See Attachment A).
24
3.0. Application of indentified techniques to Nauru situation
3.1. The Republic of Nauru
(a)
(b)
Figures 6a and 6b. A photograph of the island of Nauru (a) and a (b) sketch showing the Buada Lagoon and othe important structures. (Photo source: Department of Economic Development, National
Tourism Office, Republic of Nauru through Jane’s Nauru Home Page)
Before any recommendation of the applicability of the identified methods can be made, it is important to know the situation of the island country in terms of geography, topography, climate,
educational status of the citizens, aquatic environment, governance and economy. The Republic
of Nauru as described by Wetland International (Scot, 1993) was the major reference material
(See Attachment B).
25
3.2. Identified techniques that may be used in controlling tilapia
in the waters of Nauru
The important aquatic resources of Nauru which have been utilized for fish culture and
to certain extent, fishing are: the Buada Lagoon, Anabar Lagoon and the 28 small ponds created
out of bomb craters (assuming that the latter are still existing up to this time) (Scot, 1993; Ranoemihardjo, 1981) and depressions out of phosphate extraction. These bodies of water especially Buada Lagoon, were among the first recipients of Mozambique tilapia (Wetland International,
1990; Ranoemihardjo, 1981). It is also possible that the introduced Mozambique tilapia into Nauru did not come directly from Africa because it was reported that outside of Asia exotic tilapiine
fishes were not imported directly as native genetic resources from Africa but arrived as transits
from third or fourth party sources (Trewavas 1983). As a result, feral tilapias have hybridized
and introgressed in aquaculture settings before escaping to the wild (Costa-Pierce 2003). The
most widely dispersed tilapia species has been the Mozambique tilapia which was once known
as the ‘Java’ tilapia because most introductions of this fish originated from West Java, Indonesia,
its first established locale outside Africa (Hickling 1960). By in the mid-1970s the Mozambique
tilapia deteriorated in many recipient environments, and the small sized, poor quality fish lost
consumer acceptance (Pullin 1988). After 45 years in Nauru, Mozambique tilapia may have undergone tremendous transformation and consequently, introgression as a result of the genetic
depression through continuous in-breeding. Based on existing information, the attempts to rid
Nauru of this “poor quality and small sized tilapia that has lost consumer acceptance” have not
succeeded thus, attempts to apply the methods used of controlling tilapia in aquaculture shall be
attempted.
The following techniques and practices that are used for the control of tilapia populations
in aquaculture may be appropriate and could be applied as soon as possible to control the Mozambique tilapia in the natural bodies of water in Nauru as follows: (1) periodic harvesting
of fry and fingerlings including the parents; (2) biological control; and (3) eradication of tilapia using organic toxicants and/or other chemicals.
3.2.1. Periodic harvesting of fry, fingerlings including parents.
This can be carried out in the ponds (bomb craters) that are infested with the Mozambique
tilapia. (Note: There is need to identify which of the 28 ponds, if still existing, are infested with
Mozambique tilapia). Harvesting may be done by the following methods.
3.2.1.1. Seining.
Requirements:
a. Fine mesh seine net with holding pole on its side (seine is 6 x 4 meters, Length x Width; pole
is 6 meters in length)
b. Buckets (at least 2 buckets to keep the catch)
c. Two (2) persons
Procedure. The seine is held at both ends by each person then hauled across and/or around the
pond. The catch is then emptied into the bucket. This procedure is repeated several times in one
day until no more fish is caught. After 2 weeks and every other 2 weeks (if still necessary), the
method is done again until no more tilapia is caught.
26
3.2.1.2. Complete draining of ponds.
When no more tilapia is caught by seining every 2 weeks, the pond is completely drained, if this is
possible, the remaining fish picked up individually then held/kept in containers (buckets, etc).
3.2.1.3. Poisoning.
This should be done only after no more Mozambique tilapia is caught by draining and/or seining. It is highly recommended that an organic toxicant be used (e.g. rotenone). After 24 hours the
pond is allowed to be filled with water then neutralized with potassium permanganate. If refilling of water is not possible, the pond may be left alone until water gets in.
3.2.2. Biological control.
This method may be applied in bigger water bodies (e.g. Buada and Anabar lagoons). However,
before doing so, estimation of fish population from catch statistics or other means should be done
and the predatory fish identified. The estimate of fish population shall determine the number of
predatory fish to be released in the lagoons. It is highly recommended that the predatory fish should be chosen from among those which have already been used successfully in other places and the species is available within the waters (fresh or
marine waters) of Nauru and neighboring islands. However, native species may
be tried on an experimental basis to determine the predator– prey dynamics
between the species.
Based on geographical distribution, the following predatory fishes are candidate species
for the needed carnivore in the tilapia-carnivore relationship. All these fishes marine fishes are
found in the areas of the Pacific Ocean and can easily be acclimatized to low saline waters (e.g. 2
to 10 ppt).
a. Lates calcarifer (sea bass or barramundi)
b. Elops hawaiiensis (lady fish or tenpounder)
c. Megalops cprinoides (tarpon)
The following freshwater fishes may be considered, if they are already established in
Nauru:
a. Channa striata or Ophiocephalus striatus and Ophiocephalus obscuris
(snakehead)
b. Lates niloticus (Nile perch);
c. Hemichromis fasciatus
d. Cichla ocellaris
e. Clarias sp.
f. Cichlasoma managuense
Biological control as a means to control undesirable fish must also be done with care.
That is why, the carnivore to be used as the biological agent must have already been successfully used. For example, it is often thought that stocking northern pike, muskellunge or walleye
(predators) should result in reducing the excessive population of sunfish. It turned out that the
stunted sunfish are too large to be consumed by predators in the sizes normally stocked thus,
more predation on bass than on sunfish was observed. In carrying out biological control method
to control the proliferation of tilapia or other forage species, the following should be considered:
27
a. Depending upon the forage/carnivore (f/c) ratio that have been established for the selected
forage and carnivorous fishes, the number of predators to be used shall be determined based
on the assessment of the stocks of tilapia in the lagoons.
b. The above list of potential predatory fish (specifically the freshwater species) that could
serve as control agents for tilapia may not be endemic to Nauru and therefore a period for
acclimatization may be necessary. Slow process of acclimation should be done (from zero
parts per thousand of salinity to the salinity of water in the lagoon).
c. In the case of sea bass or barramundi, tenpounder and tarpon, which are all marine species,
can be acclimatized to waters with low salinity (a slow acclimation process will also be
necessary – from sea water salinity to the salinity of the water in the lagoon).
d. The marine species (sea bass, tenpounder, tarpon, etc.) do not reproduce in low saline waters
hence, they can not multiply. Only the number released less the mortality therefore, remains
and grows in the given body of water (lagoon, pond, etc.) to control the nuisance species.
Also, the size of forage fish that can be eaten by some predatory fishes is limited according
to the size of the predator because they prey only on the certain size range of the forage
species.
Procedure:
a. Estimate the fish population in the body of water by catch statistics or by other means. The
number and kind of species other than tilapia must also be known. This is important because
the role of the other species must also be considered.
b. Choose the predatory species to serve as the biological control agent for tilapia then obtain
the f/c ratio from the procedures that are available.
c. Based on the information obtained in the population estimation and assessment, determine
the number of predators required.
d. Based on the status of the tilapia population and reproduction checks, the stocking size of
the predator and the time they will be released can now be determined.
e. On the designated date and time, the carnivorous fish are released into the pond and the
initial observation recorded.
f. After 2 weeks, a population check by fishing in the lagoon with ordinary fishing gears and
reproduction checks by making several hauls along the shoals of the lagoon by means of fine
mesh net are performed. The catch from fishing and reproduction checks are then examined
and analyzed. This is done regularly every two weeks within a period of 6 months.
g. Using the data from the above, evaluate the condition of the lagoon in terms of the presence
of tilapia.
h. OPTIONAL: If necessary, use of organic toxicants may be resorted to.
3.2.3. Eradication by means of toxicants and/or other chemicals.
As have already been mentioned, it is not a good practice to rely solely on chemicals, such as the
special pesticide or fish toxicant technically known as piscicide, as a means of controlling undesirable fish. Only if necessary, should the use of piscicides be resorted to but it is important that it
should form part of an integrated pest management strategy. The chemical (organic or inorganic)
must complement rather than obstruct other elements in the design. It should also limit the neg-
28
ative effects on the environment. It is therefore advised that use of registered products must be
observed in using chemicals as control agents. In aquaculture, the use of most chemicals can be
regarded as wholly beneficial with no attendant adverse environmental effects or increased risks
to the health of aquacultural workers, if carried out properly. However, its over-use and misuse
could bring about negative effects on the aquatic environment and on the physical condition of
persons using them in fish farming (GESAMP 1997).
There are several piscicides which are extensively used in fisheries management in the
United States of America, Canada and Australia but only four are currently registered for general
or selective fish control or sampling. These products include the general piscicides, Antimycin
(Fintrol), rotenone (Noxfish), lampricides (Lamprecid) and Bayluscide. In a number of
countries, commercial use of piscicides is a regulated activity. For example, in Michigan, U.S.A,
it is classified under pesticide applicator certification category 5, aquatic pest management, according to the Michigan Pesticide Control Act 171.
In developed countries, the primary reasons for use of piscicide were mainly to control
undesirable fish populations so that sport fish could be stocked and managed for recreational
purposes in lakes, ponds, and streams without competition, predation, or other interference by
the undesirable fish (Lennon et al. 1970 cited by Finlayson et al. 2000). Today, the most frequently reported uses of piscicide (in order of the amount of active ingredient used) are (1) control of
undesirable fish to support recreational fisheries, (2) eradication of exotic fish, (3) eradication of
competing fish species in rearing facilities or ponds, (4) quantification of populations of aquatic
organisms, (5) treatment of drainages before initial reservoir impoundment, (6) eradication of
fish to control disease, and (7) restoration of threatened or endangered species (Finlayson, et al.
2000).
The rapid proliferation of tilapia in an area when introduced can be demonstrated also
by the experience in the Mexican State of Quintana Roo. As pointed out earlier, tilapia was introduced into this state in 1974 and about a score later these were already found in abundance even
in areas where they were not introduced (Schmitter-Soto and Caro, 1997). Similarly, when the
Mozambique tilapia was introduced into Queensland, Australia, they proliferated the Barron and
tributary streams which was first reported only in 1986. Attempt to eradicate tilapia in Queensland using the rotenone, an organic toxicant derived from the root extract of the plant known as
Derris sp. was not successful. The same was true where rotenone was applied in spot eradication
of T. mossambica in Townsville in small, in contained water bodies (e.g. ponds, dams). (Baron/
Mitchell Tilapia Management Group, QFS, 2000). In the case of Nauru earlier attempts (1979 and
1980) to eradicate Mozambique tilapia using rotenone was not also successful (Ranoemihardjo,
1981). (See Attachment C, Principles and Procedure)
29
4.0. Strategy for a national tilapia program in Nauru
4.1. The Mozambique tilapia must be controlled.
There is a significant concern that the Mozambique tilapia (Tilapia mossambica) which
have not yet been completely eradicated in the important bodies of water of Nauru (Buada Lagoon
and other smaller water bodies) shall continue to be a threat to the aquaculture initiatives of the
Nauruans and on the native aquatic communities. They are very effective competitors and can
quickly form self-maintaining populations because of their efficient reproductive strategy, flexible habitat preferences and simple food requirements. Invoking the precautionary principle, it is
essential that effective action be undertaken now to control the spread of the undesirable tilapia.
Experiences in Nauru and Australia have shown that it is very expensive and almost impossible
to cope with tilapia once they have established in an area and therefore from both an economic
and conservation perspectives their spread should be prevented. Earlier attempts of eradicating
the Mozambique tilapia in Nauru did not succeed for whatever reason. As a consequence of the
unsuccessful attempts to rid Nauru of Mozambique tilapia, it is highly possible that after about
45 years since its introduction to Nauru, they may already be well established. Sustained control
of the Mozambique tilapia from the significant waters of Nauru may be difficult, tedious and
would take some time, but may not be too late yet. As suggested earlier, controlling the Mozambique tilapia in the important water bodies of Nauru would need a thorough planning strategy
that should at least consider the 5 stages in planning as recommended (Finlayson et al. 2000) as
follows:
a. preliminary planning, where the project concept and alternatives are developed, public input
is invited, and acceptance is encouraged;
b. intermediate planning, incorporating an environmental analysis where the project is refined
and public acceptance is encouraged;
c. final planning and project implementation, involving management through the development
of project-specific work plans;
d. performing the treatment; and
e. summation and critique of the project
This planning strategy recognizes that responsibility for tilapia control lies with government, community and industry. The plan must be put together after extensive community consultation and should consider opinions from the stakeholder group who may have some ideas on
how to approach the problem.
The goals, priorities and activities in the plan should be identified in meetings with stakeholders using the information available. Also, some additional key issues should be considered
during the process, such as:
• prevention of tilapia infestation is feasible if there is cooperation among the various
stakeholders;
• sustained tilapia control, would be more realistic than complete eradication;
30
• It is very important for the participants in the program to be familiar with the ecology
and biology of tilapia, and that sustained research into other possible control methods be
initiated
•
Tilapia as an economic commodity
Vision. The infestation of tilapia in the waters of Nauru is controlled, proper management of
their impacts developed and new and desirable species and tilapia hybrids accepted (including
hybrid tilapia) as an economic commodity.
Goals
• Involve stakeholders in planning and implementing on-ground actions and control
measures;
• Prevent the spread of the Mozambique tilapia to other water bodies and restrict its further
spread within the island;
• Ensure that the Mozambique tilapia, within its present distribution within the island, is
maintained at a level that does not impact on biodiversity and economic values
• Increase community understanding of the ‘pros’ and ‘cons’ of the various tilapia species;
• In the long term maintain and sustain Mozambique tilapia control programs in Nauru but
without forgetting that tilapias in general are established economic resource;
Constraints
•
Ownership of the lagoons and ponds. (As the Buada Lagoon is owned by several groups
living on the embankment of the lake, all members must agree in any utilization efforts to
be implemented in the lagoon)
• Many of the ponds have been abandoned
• Limited knowledge of behaviour and ecology and impacts of tilapia and no research being
done to redress this situation.
• Limited resources and technical know-how of potential community participants
• Lack of public knowledge about tilapia species
• Perception by some members of the public that tilapia having lost its acceptance as food,
should be considered as an economic resource
Stakeholders
- Department of Island Development and Industry
- Nauru Marine Resources and Fisheries Authority
- Nauru community/Owners of the lagoons and ponds
- Industry
31
Key Group (To be identified)
The Problem. Tilapia was first introduced into Nauru in 1960 primarily for mosquito control
and later for food security. However, it soon proliferated into the significant waters of the island
which became out of control leading to the demise of milkfish farming (a very important economic activity) and became an ecological pest. Eradication programs were implemented but to
no avail.
Present status of the water bodies. Important bodies of water are believe to be proliferated by tilapia which caused the demise of milkfish farming. The lagoons and ponds are used as
dumping places for rubbish.
Potential impacts of tilapia on Nauru
•
There is the possibility that tilapia will establish in the estuaries and coastal areas which
may impact on the nursery grounds for many important commercial species including
prawns.
•
It is probable that Mozambique tilapia populations has already built up and established in
the lagoons
Risk Assessment. The degree of risk maybe determined by qualitatively assessing the extent
of of infestation of the Mozambique tilapia in significant water bodies of Nauru and the impact
of the infestation on ecological, social and commercial factors such as biodiversity, tourism and
the commercial fisheries.
Strategies for Implementation
1. Organization of the Key Group. Identify representatives from the various stakeholders who
shall compose the Key Group.
2. Formulation of Programs/Activities
a. Meeting(s) of the Key Group to formulate programs/activities (Understanding the
situation/Awareness Programs, etc.)
b. Consultations with the stakeholders
c. Information campaign (dissemination of information about the situation, possible
programs/activities, etc.)
3. Management of fish populations and other aquatic organisms in the existing bodies of water
in Nauru (possible transfer of tilapia from one body of water to another e.g. movement by
persons deliberately/unwittingly translocating fish directly into the the bodies of water,
Priority Actions
• Increase public awareness of the issue
• Instigate research into biological control of tilapia
• Monitor all sites of infestation and to determine the rate of population increase and the
success of any control measures
32
Education and Extension
•
Need for technical manppower
• Community Service announcements
• Training of volunteers on Mozambique tilapia and other pest fish issues
Research into general biology of tilapia and biological control options
• Link in with universities to instigate research into the biology/ecology of tilapia
• Implement relevant research projects
Monitoring
•
Utilize existing baseline information/data gathered through various projects (if any)
•
Data collection of fish species present in the bodies of water
Long term Actions
• Development of an Aquatic Resources Management Program in Nauru
33
5.0 . National tilapia plan for the Republic of Nauru
The increasing importance of tilapia in the world is demonstrated by its increasing
production, demand and attention. Considerable information are available demonstrating the
role positive role of tilapia in the economy of many nations. As pointed out earlier, the total
world tilapia landings from capture and culture has been increasing and many countries are
convinced that tilapia has attained the status of a world fish. The increasing production of tilapia
in the world can be seen in Figure 7.
Figure 7. Worldwide tilapia production according to species demonstrating that expansion in
production in recent years has come from the expansion in culture of Tilapia nilotica
(Data source: FAO, 2002 cited by Mair, 2001).
Taking into account the increasing importance of tilapia in the world while respecting the
action of other countries to get rid of the obnoxious tilapia species, the following plan regards
both as critical because of tremendous development in tilapia aquaculture while attempting
to preserve environmental biodiversity. Furthermore, there are advantages of integrating
fisheries into the development plan of the Small Island Developing States (SIDS) in view of the
suggestion that among the specific national measures deemed essential to ensure the sustainable
development of the SIDS are establishment/strengthening of institutional, administrative and
legislative arrangements in evolving fisheries development strategies including its integration
into national development plans as well as a design of comprehensive monitoring programs for
coastal and marine resources (Thorpe et al. 2004). Thus, the following specific plan integrates
fisheries management as a strategy in controlling the population of undesirable species of tilapia
while opening the door to the new and desirable breeds for future deliberation. This National
Tilapia Plan may be discussed in more details in the future.
It was learned from the SPC Aquaculture Action Plan 2003-2005 that here are only
two species of fin fishes that are listed as aquaculture commodities to be addressed within the
said period, namely: milkfish (Chanos chanos) and tilapia (Tilapia spp.). Several issues were
identified as among the lessons learnt which are enumerated in the following table (Table 1).
While the Republic of Nauru is not a “focus country”, it is assumed that tilapia, particularly the
hybrids is being considered as an economic resource like the other SIDS.
34
The major thrust of the national tilapia plan is to control the feral tilapia in
the different water bodies of Nauru and at the same time manage its population
in these environments. The use of hybrid tilapia will be introduced
5.1.
Policy needs.
Policies that are essential for sustained management and protection of lagoons/ponds must be
formulated, among these are:
•
Protection and conservation of fisheries and aquatic resources
•
Review ownership policy of aquatic resources
•
Conservation of spawners and/on fry/fingerlings
•
Review of policies
5.1.1. Strategy
The major strategy in increasing production of tilapia while controlling its population is
through:
a. increasing the yield per hectare of inland waters;
b. concentrate resources on targeted tilapia species
35
Table 1. Issues and needs in formulating national tilapia plan for Nauru
Category Institutional, legal and
administrative aspects Human resources Issues Constraints •
Demise of
milkfish
industry
•
•
commitment by
authorities
•
Proliferation of
Mozambique
tilapia
Ownership of
the areas for
fish farming
Required actions •
Eradication/
control of
Mozambique
tilapia
•
Review of
policies
•
Programs to
upgrade skills
of technical
and extension
officers
• Lack of skilled/
technical manpower
• Organization
training, seminars
and workshops •
declining supply
of fry
•
High cost and
poor quality of
feeds
•
Need for water
recycling
technologies
•
poor site
selection and
poorly managed
farms;
Technical aspects •
•
•
36
impact on
the natural
ecosystem;
tilapia
introduced
other than for
farming
subsistence
farming has
had variable
success.
•
•
Poor quality of
seeds
•
Consider other
species of
aquaculture
commodities
(e.g. hybrid
hybrid tilapia,
& other species,
etc.)
•
Promotion of
polyculture
system
•
Research waste
management
and treatment
•
Undertake
research on
water recycling
technologies
Poor water
supply
With these identified issues and needs as the take off point, a national tilapia action plan and
objectives for Nauru is hereby prepared (See Table 2).
Table 2. Action plans and objectives of a national tilapia plan for Nauru
Action plans Objectives •
Key areas of concern
•
•
Key development
areas in lagoon/pond
management To eradicate/
control
Mozambique
tilapia
To develop
utilization
techniques for
Mozambique
tilapia in
aquaculture
To formulate
an effective
management plan
for the utilization,
protection and
conservation
lagoons and
aquatic ecosystems
Target
beneficiaries •
Private
sector
•
Fish farmers
•
Government
•
Stakeholders
•
Government
•
Private
sector
•
Stakeholders
Expected results •
Enable the
government
and other
stakeholders
to move
on to other
aquaculture
species
•
Formulated
utilization and
management
plan for
the aquatic
ecosystems for
Sustainable
resource use
production
5.1.2. Research and extension needs
•
Training for technical manpower needs of the government
•
Non-formal
•
Formal
•
Programs to upgrade skills of technical and extension officers
•
Organization training, seminars and workshops
5.1.2.1. Research Needs
•
Water recycling technologies
•
Consider other species of aquaculture commodities (e.g. hybrid tilapia and other species, etc.)
•
Promotion of polyculture system
•
Research on waste management and treatment
•
Undertake research on water recycling technologies
•
Fertilization methods
37
Feeds and feeding (Identification and screening of potential feedstuffs; studies on locally available
supplementary feeds such as utilization of Mozambique tilapia as a protein source of fish feeds
5.1.2.2. Other Needs
•
Fingerling production
•
New areas for aquaculture activities
•
Production targets. Define production target and come up with programs in the pursuit of
this target.
•
Marketing
5.2. Immediate Plan.
In order to commence a project as a strategy to initiate the program of fisheries activities in the
bodies of water of Nauru, an action plan for tilapia may be in order. This action plan in divided
into three phases as follows: (a). eradication and control of Mozambique tilapia; (b). stocking of
lagoons/ponds with carnivorous species to serve as biological control for feral tilapia; and (c).
enhancing the productivity of the lagoons/ponds for tilapia aquaculture
Phase 1. Eradication/Control Of Mozambique Tilapia
As suggested earlier, from among the seven (7) methods in use for the control of tilapia
populations in aquaculture only the following techniques may be appropriate to apply in Nauru
at this time and as soon as possible, in controlling the Mozambique tilapia. (1) periodic
harvesting of fry and fingerlings including the parents; (2) biological control;
and (3) eradication of tilapia using organic toxicants and/or other chemicals.
These shall be implemented wherever and whenever appropriate.
Potential Bodies of Water for Aquaculture
As described in Section 3.1 above the Republic of Nauru has bodies of water that are
potential for aquaculture. Foremost of this is the Buada Lagoon and the other bodies of water
such as Anabar Lagoon and several other small ponds. These two lagoons and the small ponds
are the areas that are being considered where the national plan for tilapia in Nuaru shall be
carried out.
Figure 8. Buada Lagoon: See description on Section 3.1. (Source of photograph: SPC Fisheries
Newsletter #111 – October/December 2004)
38
a. Complete draining of lagoons/ponds. It is highly recommended that the lagoons/
ponds be completely drained, at least initially, in order to clean and clear them with garbage. It
is very important that the lagoons/ponds are cleared of the rubbish and other polluting wastes
because no fish may survive and/or grow normally in a polluted water. However, the Mozambique
tilapia may be able to withstand such condition and stay alive. Other fishes such as the walking
catfish (Clarias bathracus) and the Philippine catfish (C. macrocephalus) could tolerate the
identical situation. It should be recalled that polluted waters are: low dissolved oxygen, high
carbon dioxide, ammonia, organic materials and dissolved solids, among others. Under this
condition, no aquatic organism may endure and live normally).
Note: The biggest body of water, the Buada Lagoon covers an area of approximately 4
hectares and with an assumed average depth of 1.5 meters. On this basis, the volume of water
in the lagoon is approximately 60,000 cubic meters or 60 tons of water. (Note: A pump
with a capacity of 25 c.c./sec. can drain the whole lagoon in approximately 667 hours or 56 days
running at 12 hours/day. If the desire is to reduce the number of days, using 4 pumps with the
same capacity can completely drain the lagoon in 13.88 days ≈ 14 days). A bigger capacity pump
can tremendously reduce the time of draining.
b. Eradication by means of toxicants and/or other chemicals. The recommended
fish toxicants (rotenone) may now be applied under this phase while there still remains little
water (average depth of 5 cm) in the lagoons/ponds (at this point, draining is discontinued).
Please see Principles and Procedures in Section 2.7. If possible, the bottoms of the lagoons/
ponds must be exposed to the sun up to two weeks before filling.
c. Filling the lagoons/ponds with water. As soon as the lagoons/ponds are
neutralized with potassium permanganate, the lagoons/ponds are now ready to be filled up with
water. There might be some difficulties in carrying this out but it is essential to have at least the
lagoons filled up by constructing some simple structures that can systematically collect/lead
rainwater into the lagoons/ponds.
d. Sampling of bottom soils. It is desirable that soil samples be taken so that the
different parameters can be determined and their effects on the productivity of the water of the
lagoon/ponds established. This way, the essential nutrients that could enhance productivity of
water shall be known and appropriate amelioration is performed.
Phase 2.
Stocking Of Lagoons/Ponds With Carnivorous Species
To Serve As Biological Control For Feral Tilapia
Following the SPC Aquaculture Plan prepared in 2002 (for the period 2003 to 2005)
the following fin fishes were considered as priority species for aquaculture: milkfish, GIFT
tilapia, mullet and Mozambique tilapia. The presence of Mozambique tilapia in Buada Lagoon
calls for its complete eradication/control before any species is stocked in the lagoon. However,
complete eradication/control with piscicides has been shown to be unsuccessful and may even
be impossible without the cooperation and commitment of the various stakeholders. The use of a
carnivorous fish to control the population of Mozambique tilapia with milkfish shall be attempted.
The activities under this phase are dependent on the outcome of Phase 1.
Assumptions: The stakeholders, particularly the owners of the ponds and lagoons have agreed
to implement the project and that the major utilization of the lagoons/ponds is for farming/
fishing.
39
Activities: a. Collection and acclimation of fry/fingerlings of the primary (milkfish)
and the carnivorous (barramundi) species. While the ponds are being filled with water
(may take up to 2 months), fry and fingerlings of the target species may be obtained/collected
and acclimatized into low saline water. The acclimatized fish are then held in a container until
the lagoons/ponds are ready for stocking. Note: The fry/fingerlings of barramundi can easily be
acclimatized into low saline waters, the same is true for milkfish.
b. Reproduction checks. After the initial draining, clearing and poisoning of the fish
in the lagoons/ponds, there is no assurance that the Mozambique tilapia has been controlled.
Chances are, after filling the lagoons/ponds with water, fry/fingerling of Mozambique tilapia
may appear. It is therefore important that reproduction checks be undertaken to get a picture of
the sizes of fry/fingerlings and a good estimate of the population. It is only after the information
from reproduction checks has been obtained that re-stocking the lagoons/ponds with the
desirable fish should be performed.
c. Stocking the fish. (1) Primary fish. Before the fingerlings of the primary fish (e.g.
milkfish) are released into the lagoons/ponds, temperature and salinity of the source water
(where the fish were held) and the lagoon/pond water must be measured. The values of the
temperature and salinity of the water from the source and the lagoon/pond must be the same or
at least close enough (not more than 1OC and 5 ‰, respectively). The milkfish fingerlings must
be bigger than the tilapia fingerlings. Rule of the thumb dictates that the average size (in terms
of total length) ratio of tilapia fingerling-to-milkfish fingerling is at least 1:1.20, that is, milkfish
fingerlings should be larger than tilapia by at least 20%.
(2) The carnivorous fish. As in the primary fish, the temperature and salinity of the source
and lagoon/pond water must be determined. The desired values of these parameters are the
same as that of the primary fish. The size of the carnivore however, should be at least 54%, 30%
and 25% larger than the forage species (e.g. tilapia) for barramundi, tenpounder and tarpon,
respectively (Fortes, 1986; 1985; 1980; 1979). Stocking density will depend on the population of
the forage fish.
(3) Stocking rate. The rate of stocking the lagoons/ponds ideally depends on their
carrying capacity. However, at this point this may still be unknown for the target bodies of water.
Inasmuch as these water bodies are small, their carrying capacity can be determined according
to the inputs. Thus, the target carrying capacity may initially be a minimum of 600 kg/ha. Using
Buada Lagoon as an example, the carrying capacity is 2,000 kg. The primary species being
milkfish may be stocked at the rate of 3,000/hectare or 12,000 for the whole lagoon. The basis
for this is: target size of milkfish = 200 g; initial size = 2 g; mortality = 25%; harvest = 6 months
after stocking.
Target yield (kg)
Stocking rate =
+% mortality
Target size (kg)
=
2,000 kg/0.20 kg+25% of 10,000
= 10,00 + 2,500 = 12,500 fingerlings
40
The carnivore (barramundi) and the forage fish (tilapia) can add up significantly to the density
of fish in the pond. Again, the density of the forage fish can be estimated through reproduction/
population check.
Phase 3. Enhancing The Productivity Of The Lagoons/Ponds For Tilapia
Aquaculture
Maintenance, Operations and Monitoring. After stocking, there is a need to maintain
the lagoons/ponds and to monitor the fish populations as well as its productivity. The water
depth, mortality, water quality and relevant information must be gathered and monitored. It
is at this point that fish management must be integrated with the management of the lagoon/
ponds. The techniques and practices in controlling the population of Mozambique tilapia may
now be carried out in appropriate environments. The different parameters on water quality must
be obtained so that proper amelioration of needed nutrients can be made (e.g. kind and amount
of fertilizer, organic matter content, salinity and others. If need be, liming may be resorted to
depending upon the results of the physico-chemical and biological analyses of the water and soil
from the lagoon.
Activities:1. Periodic harvesting of fry, fingerlings including parents. This can be
carried out in the ponds (bomb craters/those resulting from phosphate extraction) that are
infested with the Mozambique tilapia, if still existing. Harvesting may be done by the following
method described in Section 2.1. The fry/fingerlings collected may be held in tanks/hapas for
later use.
2. Biological control. This method may be applied in bigger water bodies (e.g. Buada
Lagoon). The procedures are describe in Section 2.5 (Assumption: That a carnivorous fish
has been identified as the biocontrol agent. If Lates calcarifer (barramundi) is available, this is
highly recommended).Biological control and mechanical means of control for the aquatic plants
(submerged and emergent) may be resorted to
3. Other control methods. Monosex culture (Section 2.2) through manual
separation of sexes (2.2.1), hybridization (2.2.2), hormone augmentation (2.2.3) and genetic
manipulation (2.2.4); culture in cages (Section 2.3), high density culture (Section 2.4) and
sterilization (Section 2.6) may be done as soon as trained personnel are available. Training
may be done on-site by inviting experts as trainor-consultants.
Trial Runs. Trial runs for the following monosex method in controlling tilapia reproduction
may be carried out using the fry/fingerlings collected in activity number 1. Exercises on manual
separation of sexes, hormone augmentation, cage culture, high density culture and probably
sterilization, may be performed.
These trial runs may run up to at least one year depending upon the
resources availability (manpower, financial and commitment).
o000o
41
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45
Attachment A
Principles and Procedure:
(Source:Texas Parks and Wildlife Department, 4200 Smith School Road, Austin,
Texas 78744 PWD BR T3200-77 (7/02)
Rotenone kills fish not by removing oxygen from the water, but by inhibiting oxygen transfer and
cellular respiration. All fishes are sensitive to rotenone, but some species are more easily killed
than others.
a. The volume of water to be treated must first determine. Both the surface area and the
average depth must be determined. (Note: Surface acreage multiplied by average depth, in feet,
equals the volume in acre-feet.
b. If the pond is rectangular in shape, the length in feet would be multiplied by the
width in feet. The total number of square feet should be divided by the number of square feet in
one acre (43,560).
Example:
Pond length = 408 ft.
Pond width = 260 ft.
Surface acres = Length (in feet) x Width (in feet) 43,560
Surface acres = 408 X 260 = 106,080 = 2.44 43,560 43,560
c. If the pond is circular, the distance around the shoreline (perimeter or circumference)
of the pond should be measured in feet. That number should be multiplied by itself and then
divided by 547,390.
Example:
Shoreline distance = 800 ft.
Surface acres = Shoreline (ft) x Shoreline (ft) 547,390
Surface acres = 800 x 800 = 1.17 547,390
d. If the pond is triangular, the base and the height should be measured.
Example:
Base = 300 ft.
Height = 500 ft.
Surface acres = ½ (Base x Height) 43,560
Surface acres = ½ x 300 x 500 = 1.72 43,560
e. Average depth is calculated by making a series of depth soundings throughout the
pond. Soundings should be made every 15 or 20 feet apart in straight lines across the pond.
Each row of soundings should begin and end with a zero. The depth measurements should be
added and then divided by the number of soundings made (including zeros) to obtain the average
depth.
f. The amount of rotenone to use depends upon its strength, the type or types of fish
present, and the volume of water to be treated. Most commercial formulations contain either 2.5
or 5 percent rotenone, in either liquid or powder form. The liquid is preferred by many workers
because it is easier to apply, but the powder is usually more available.
46
g. Powdered rotenone is found in most farm and ranch supply stores; liquid rotenone
may have to be purchased from a fish farming supply store or chemical distributor. Care should
be taken to read and follow label directions carefully.
h. If 5% liquid rotenone is used, an application rate of 1 gallon per acre-foot of water is
recommended. Liquid rotenone should be diluted with water (5 parts water to 1 part rotenone)
before applying. The rotenone mixture can be applied by spraying, gravity flow from a barrel,
or merely pouring from a bucket.
i. If 5% powdered rotenone is used, an application rate of 10 pounds per acre-foot of water
is recommended. The powder should be diluted with enough water to make a slurry. The slurry
can then be distributed throughout the pond. Dry powdered rotenone should not be poured
directly into the pond.
j. Generally, the surface water should be at least 70 degrees for best results. If water is
more than 15 feet deep, the rotenone mixture should be applied to lower depths with a weighted
hose. It usually takes less than 30 minutes for rotenone to affect small fishes, but may take
several hours to kill larger fish. Water treated with rotenone is usually nontoxic to fish within
two weeks.
k. Detoxification rates depend largely on amount of sunlight and water temperature. If
there is any doubt about whether the water is detoxified, a few live fish can be placed in a minnow
bucket and suspended in the pond. If the fish are still alive after 24 hours, the pond is ready for
restocking.
Attachment A, continued
3.3. Recommendations. In controlling the population of a noxious fish (e.g. Mozambique
tilapia) in a body of water, it would require not only treatment with the most effective piscicide
but would also necessitate efficient implementation of the principles of fisheries management
- a continuous process that requires frequent monitoring, in-depth training and experience
working with complex aquatic ecosystems. Under certain conditions, fisheries management may
entail use of piscicide, if necessary thus, it is further strongly recommended that rotenone
should be used as the piscicide inasmuch as this is available in Oceania, Australia, southern Asia
and South America as a naturally-occurring substance derived from the roots of tropical plants,
the jewel vine (Derris spp.) and the lacepod (Lonchocarpus spp belonging to the bean family
Leguminosae.
In using rotenone as the piscicide in a project, there will be a need for a thorough planning
strategy considering the five stages in planning as recommended by Finlayson et al. (2000) of
the American Fisheries Society as follows:
a. preliminary planning, where the project concept and alternatives are developed, public input
is invited, and acceptance is encouraged;
b. intermediate planning, incorporating an environmental analysis where the project is refined
and public acceptance is encouraged;
c. final planning and project implementation, involving management through the development
of project-specific work plans;
d. performing the treatment; and
e. summation and critique of the project
47
Note: A small treatment performed on private land or a government owned hatchery may require
little planning before implementation, while a large project involving a public water supply may
require two or more years of extensive planning. The rotenone treatment should be consistent
with and supported by the current Fisheries Management Plan (FMP) when applicable, which is
either species specific or water body specific. The complexity of a rotenone project depends upon
social, biological, political, and physical characteristics and will dictate the degree of planning
required. For example, extensive planning may not be needed for rotenone use in sampling
except where downstream waters are potentially affected.
48
Attachment B
Nauru
(Source: Scott, D.A. (ed.) 1993. A Directory of Wetlands in Oceania. IWRB,
Slimbridge, U.K. and AWB, Kuala Lumpur, Malaysia0
“The Republic of Nauru which comprises a single raised coral limestone island is
located in the west central Pacific Ocean at 0°31’S, 166°56’E, some 42 km south of the Equator.
Its nearest neighbour is Banaba (Ocean) Island, 306 km to the east in the Republic of Kiribati.
It has a total land area 2,070 to 2,130 hectares) and in 1990, had a population of 9,100”.
“ The Republic of Nauru is one of the three great phosphatic-rock islands of the Pacific
(the other two being Banaba in Kiribati and Makatea in French Polynesia). It is oval-shaped
and bounded by a reef platform which is exposed at low tide. The ground rises gently from
sandy beaches to a fertile coastal belt, 100-300 m wide, with soils consisting of a mixture of
sand and fine corals. Inland, a coral-limestone escarpment rises to a central plateau with an
average height of about 50 m and a high point at 71 m. This plateau, which covers about 75%
of the island’s total area, is composed largely of phosphate bearing rocks. These deposits of
tricalcium diphosphate, the second richest deposits in the world (after those of neighbouring
Banaba Island), occur in small isolated masses (nests) and pits, separated by walls and
pinnacles of very hard dolomite limestone. The undisturbed plateau soils are classified as
Lithic Haplustolls, Typic Haplustolls and Lithic Ustorthents mainly derived from phosphate
materials and, to a lesser extent, dolomitic limestone (Manner and Morrison, 1991)”.
“The climate is tropical with daily temperatures ranging between 24.4°C and 33.9°C,
and average humidity between 70 and 80%. The average annual temperature is 27°C, with
a seasonal variation of 1°C. Annual rainfall averages about 2,000 mm, much of it occurring
during the monsoon season from November to February. However, the annual rainfall is
subject to wide fluctuations, ranging from as little as 104 mm to over 4,572 mm. Droughts are
not uncommon, and several lasting more than 12 months have occurred this century. Streams
are non-existent. The tidal range is about 2.0 m”.
“Agriculture is very limited, with a few fruit trees, coconuts, Pandanus and breadfruit
planted or protected along the coastal belt and in the area around Buada Lagoon. There is also
some small-scale cultivation of bananas and vegetables in the coastal belt and in the swampy
area bordering Buada Lagoon”.
“The natural vegetation comprises mixed plateau forest, atoll forest and scrub with
Pandanus and Cocos in the coastal belt, and less than two hectares of mangroves (IUCN, 1991).
Stands of Pandanus species are occasionally interspersed amongst both forest types, but are
reported to have been deliberately planted for their edible fruits”.
“Phosphate mining has had a drastic effect on the topography and vegetation of the
plateau. Before mining can begin, the land is stripped of vegetation, and the topsoil and
contaminated phosphate are scraped off, thereby exposing the phosphate deposits. By 1989,
some 75% of the surface area of the island had been mined, and over 90% of the plateau forest
had been destroyed, leaving less than 200 ha of forest intact (SPREP, 1989). Virtually no
attempt has been made to rehabilitate any of the mined areas, and by the end of this century,
an estimated 80% of the total land area (1,760 ha) will have been transformed into pitted,
barren wastelands with scattered coral pinnacles (Manner et al., 1984”).
Attachment B, continued
49
“There is very little surface water on the Nauru’s highly permeable terrain, much the
largest permanent water body being Buada Lagoon. This is a brackish sunken lagoon, some
3-4 ha in extent, surrounded by a swampy area. It is situated near the centre of the limestone
plateau, and has a salinity of 2 ppt. and a pH of 8. Ranoemihardjo(1981) lists one other lagoon
(at Anabar), a small brackish lagoon with a salinity of 10 ppt and 28 tiny fresh to slightly
brackish ponds, most of which were formed in bomb craters during World War II”.
“ Many of the ponds and the two lagoons are used for the rearing of milkfish (Chanos
chanos). Fry are collected from the reef at low tide, acclimatized for 2-3 weeks, and then
released into the ponds and lagoons. Growth rates have, however, been slow, partly because
of competition with Tilapia and partly because of the insufficiency of natural food and
overcrowding (Ranoemihardjo, 1981). Tilapia mossambica was introduced into the island in
about 1960 to feed on mosquito larvae, and rapidly became abundant in the lagoons and ponds.
At the request of the Republic of Nauru, a Tilapia eradication program was implemented by
FAO in 1979 and 1980. This involved poisoning the lagoons and ponds with the highly toxic fish
poison rotenone (Ranoemihardjo, 1981). Both of the lagoons and most of the ponds are also
used as dumping grounds for rubbish”.
“There are two other wetland systems of note in Nauru. The first is a series of tiny
wetlands (0.25-0.33 ha in extent) along the inner edge of the reef lagoon at the base of the
limestone escarpment. These are small brackish marshes which sometimes dry out completely.
They are virtually unused by the islanders, and remain in an almost unspoiled condition. The
second system is a small patch of mangroves, probably less than two hectares in extent, on the
island’s northeast coast. This very isolated stand of mangroves, of unknown origin, contains
only a single species, Bruguiera gymnorrhiza (Woodroffe, 1987). The mangrove fruits were
apparently once used as a food by the Nauruans”.
“Nauru’s marine systems have been described by UNEP/IUCN (1988) as having no
true reef and no lagoon; rather, the island is surrounded by an intertidal reef platform, some
150-200 m wide, cut into the original limestone of the island and typified by the presence of
numerous emergent coral pinnacles. The platform is dominated by large yellow-brown algae
and little or no coral growth occurs on the reef flat. Coastal waters are relatively unpolluted,
although there may have been one or two instances of silt accumulating on some parts of the
reef flat”.
“The only research specifically related to wetlands has focused on the development of
fish culture in the island’s ponds and lagoons (Ranoemihardjo, 1981). Soil surveys have been
undertaken by John Morrison of the University of the South Pacific, while Manner et al. (1984,
1985) have studied the natural vegetation of the plateau and plant succession after phosphate
mining”.
“There is no legislation relating specifically to the inland aquatic systems, and indeed
no legislation concerning the conservation of terrestrial ecosystems. No protected areas have
been established, and none is proposed (IUCN, 1991). The Marine Resources Act 1978 makes
provisions for the exploitation, conservation and management of fish and aquatic resources in
territorial waters and the exclusive fisheries zone. In general, customary rights over the reefs
restrict over-harvesting, and allow the recovery of exploited resources, especially on the reef
slopes (UNEP/IUCN, 1988”).
Attachment B, continued
50
“The Department of Island Development and Industry is the government body concerned
with the island’s natural resources, and has been involved with the development of fish farming
in the lagoons and ponds. However, no specific government entity is directly assigned to take
charge of the inland aquatic habitats or the fishery which they support”.
“Only one of Nauru’s tiny wetlands would appear to be of international importance on
the basis of the Ramsar criteria, namely Buada Lagoon. The following site account has been
compiled from the literature”.
Buada Lagoon. Location: 0°32’S, 166°55’E; near the south end of the island of Nauru,
approximately 1.3 km from the coast; Area: 3-4 ha.;Altitude: Near sea level;.Overview: An
enclosed brackish lagoon in the interior of a raised coral limestone island.
Physical features: A small, brackish, sunken lagoon, some 3-4 ha in extent, surrounded by
a swampy area. The lagoon is situated in a depression near the southwest end of Nauru’s
limestone plateau, and has a salinity of 2 p.p.t. and a pH of 8 (Ranoemhardjo, 1981). The water
is slightly greenish in colour. The lake is fed by local run-off, principally during the monsoon
season from November to February.
The climate is tropical, with an average annual rainfall of about 2,000 mm. There are, however,
wide fluctuations in rainfall from year to year, and droughts are not uncommon.
Ecological features: No information is available on the aquatic vegetation. The natural
vegetation on the surrounding plateau comprises plateau forest dominated almost entirely
by Calophyllum inophyllum. However, most of this forest has now been cleared for phosphate
mining, leaving barren wastelands with scattered coral pinnacles (Manner et al., 1984 &
1985).
Land tenure: Customary ownership.
Conservation measures taken: None.
Land use: The lagoon was formerly used for the rearing of milkfish (Chanos chanos). Fry
were collected from the reef at low tide, acclimatized for 2-3 weeks, and then released into the
lagoon. Growth rates were reported to be slow, partly because of competition with Tilapia
and partly because of the insufficiency of natural food and overcrowding (Ranoemihardjo,
1981). There is some small-scale cultivation of fruit trees, Pandanus, breadfruit, bananas and
vegetables in the swampy area bordering the lagoon.
Disturbances and threats: Tilapia mossambica were introduced into the lagoon in about
1960 to feed on mosquito larvae. They increased rapidly and were thought to be limiting
production of milkfish through competition. At the request of the Republic of Nauru, a Tilapia
eradication programme was implemented by FAO in 1979 and 1980. This involved poisoning
the lagoon with the highly toxic fish poison rotenone (Ranoemihardjo, 1981). The lagoon is
used as dumping grounds for rubbish.
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Attachment C
General Courses Of Action That May Be Considered In Preventing Infestation Of
Undesirable Species Including Tilapia
(Source: Mississippi State University Coastal Research and Extension Center
2710 Beach Blvd Suite 1E Biloxi, MS 39531 601/388-4710 and Auburn University
Marine Extension and Research Center 4170 Commanders Drive Mobile, AL
334/438-5670)
1. Treatment with hot water/steam.. Contaminated equipment like hauling-tanks or vats
should be steam cleaned or immersed in hot water (140°F or 60°C).
2. Desiccation. Seines, nets, aerators, floats, and other contaminated equipment should
be allowed to air dry at least one week in humid climates.
3. Infested ponds should be drained and allowed to dry thoroughly for at least two weeks,
preferably during very cold or very hot weather.
4. Salt. This is a particularly promising control measure for tilapia producers with access
to saline or brackish ground water or bulk supplies of rock salt because tilapia generally benefit
from intermediate concentrations of chlorides. Whenever possible, a 1% treatment of sodium
chloride (24 hours) should be used when transporting fish to or from other facilities.
5. Disinfection. Traditional aquaculture disinfectants, calcium hypochlorite and iodine,
may be used.
6. Benzalkonium chloride is effective against effective against all stages of the zebra mussel
at 100 mg/L for three hours and at 250 mg/L for 15 minutes. It can be used to disinfect haulingtanks, stainless steel troughs, vats, nets and other equipment. This compound is commercially
available as ROCCAL™ (Benzalkonium chloride) which is highly toxic to most fish species and
should be used with extreme care. Thorough rinsing and proper disposal of runoff is essential to
avoid impacts to fish stocks within and outside the facility.
7. Treatment with traditional fisheries chemicals. Rotenone™ (15 mg/L for 24 hours)
or chelated copper (2 mg/L for 48 hours) have been shown to kill fishes when applied for other
control purposes in infested ponds. Note that Rotenone is classified as a restricted use pesticide,
and can be purchased and applied only by a certified pesticide applicator.
8. One fishery chemical that could be effective is potassium chloride.Research suggests
that exposure to KCl concentrations as low as 100-200 mg/L for 24 hours can be effective in
killing fishes.
9. Hydrated lime (CaOH). Addition of calcium hydroxide (hydrated lime) to newly drained
ponds at 1000-2000 lb/acre will kill all unwanted organisms. However, potential environmental
impacts to adjacent aquatic habitats whenever these types of compounds are applied must be
observed. The use of any of these chemicals requires specific permission from state and federal
agencies.
10. Molluscicides/toxic compounds. Although a number of molluscicides have been
investigated or permitted for use in eradicating zebra mussel infestations at public and private
52
utilities and industries throughout the Great Lakes and Mississippi valley, regulatory agencies
are understandably concerned with their effect on the environment. (Rick Kastner, Greg Lutz
and Marilyn Barrett-O’Leary, Mississippi Sea Grant Extension Service, Louisiana Cooperative
Extension Service and Louisiana Sea Grant College Program)
Some undertakings in the control of pest and nuisance species may apply in tilapia
control initiatives by developing Tilapia Critical Control Points (TCCP) Program. This TCCP
program is a proactive, common sense approach to address potential impacts of tilapia on warm
water aquaculture. This goal can be achieved for specific production systems by following these
guidelines:
1. Identify Critical Control Points (CCPs), that is, areas where tilapia could inadvertently
enter production facilities.
2. Determine appropriate measures to avoid infestation and establish monitoring
procedures based on the identified problem areas.
3. Formulate control actions and/or remediation in the event an infestation does occur.
4. Establish a record keeping system to facilitate these activities.
Identify Critical Control Points In Tilapia Aquaculture. Given the record of
tilapia in the island of Nauru, as well as its widespread presence in most water bodies in the
island, it may only be a matter of time in southern waters when all tilapia can be eliminated from
the island. It would help considerably if the residents of the island are especially aware of these
Critical Control Points (CCPs) such as (The statements were modified in order that it can be
referred to tilapia):
1. Contaminated surface water sources. Tilapia populations are already well established
in the island’s bodies of water and could potentially exist longer in these areas if nothing will be
done about it.
2. Widespread proliferation of tilapia fry and fingerlings. This is a particular problem in
the spread of tilapia in any place in the world. Tilapia can reproduce up to 12 times in a year and
their larvae can be passed on to other areas in the island.
3. Water flow from one area to another that could be a cause of infestation.
53
Attachment D
Things to Consider in the Use of Various Control Measures for Different
Organisms
(Source: Queensland Fisheries Service, 2003. Department of Primary Industries,
Queensland, Australia)
In general, the principles behind controlling/managing of pests and nuisance species
may be considered in controlling tilapia population. It should always be borne in mind that the
most effective way to prevent infestations of noxious organisms are to determine monitoring and
verification procedures for the critical control points (CCP) and formulate preventive measures.
Therefore, careful surveillance and monitoring coupled with a regular procedure to prevent entry
into other water bodies is the preferred approach. The general methods of control are as follows:
physical/mechanical, chemical and biological control; genetic engineering, and environmental
management and cultural control.
1. Physical control. This control method involves the use of human labor in the
physical removal/destruction of the pests. This may involve netting or electro-fishing. Physical
control of pests is suitable for the management of small areas but large-scale infestations are
better managed using other methods.
2. Chemical control. Relying solely on chemicals, such as the fish poison rotenone, as
a means of control is not a desirable practice. When the need arises, the use of chemicals should
form part of an integrated pest management strategy. The key is to use pesticides in a way that
complements rather than hinders other elements in the strategy and which also limits negative
environmental effects. It is important to understand the life cycle of a pest so that the chemical
control can be applied when the pest is most vulnerable to attack — the aim is to achieve the
maximum effect at minimum levels of pesticide so that there is minimal impact on the associated
environment. Chemical control should be undertaken using registered products specific to the
pest and situation. It is important that chemicals are applied at registered application rates and
in the manner specified by the product label.
3. Biological control. This process requires an intensive search for predators and
pathogens (in the case of disease-bearing pests) from the native country where the pest originates.
Strict measures are in place to ensure these agents do not affect native fish, plants and animals or
crops and livestock. The process is long and time-consuming, but the benefits can be substantial.
The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Department of
Natural Resources and other agencies undertake predator and pathogen research for the control
of significant pests in Queensland.
There are three general approaches to biological pest control. The first of these is the
importation of a biological agent. For example, the janitor fish in the Philippines was
imported as a “biological cleaning agent” in aquaria. It made aquarium maintenance easier. But
there are dangers with this approach because when they were accidentally released into bodies
of water the population increased tremendously and now they are pest.
The second approach to biological control is augmentation, which is the manipulation of
existing natural enemies to increase their effectiveness. This can be achieved by mass production
and periodic release of natural enemies of the pest, and by genetic enhancement of the enemies
to increase their effectiveness at control. Also the release of genetically altered sterile pests (e.g.
54
male fruit fly and screw worm fly) can be used to disrupt the breeding of some pest organisms. It
is not presently known if any native fish are effective predators of tilapia and carp. It is thought
that barramundi and mangrove jack in stocked impoundments may impact on tilapia numbers,
and there is some conjecture that Australian bass may prey on carp. However, it appears that
neither carp nor tilapia is a favoured prey species and introduction of predators can lead to
increased predation pressure on native fish.
The third approach is conservation. This involves identifying and modifying factors that
may limit the effectiveness of the natural enemy. In some situations, this may include reducing
the application of pesticides, since such pesticides may kill predators at the same time as killing
the pests.
4. Environmental management. This involves changing the environmental
conditions in an area in which a pest plant or animal has invaded such that pest populations
are reduced. Environmental management may also include the re-vegetation of areas using
a combination of native grasses, trees and shrubs. Re-vegetation of native species creates
competition for pest plant species and provides a habitat for native animals, which prey upon
other pest plants and animals. Pest fish seem to have a competitive advantage over native fish in
disturbed environments, so habitat rehabilitation may reduce their impact.
5. Genetic modification. Genetic modification of pest fish is a possible method of
control. The idea is to engineer a disadvantageous trait in a pest and then release modified
individuals into the outside world. The sterile insect release method is an example of this
approach. The genetic engineering of organisms is controversial. Some people argue that toxins
produced as a result of gene transfer may have harmful effects on beneficial organisms or on
human health, while others suggest that the transferred gene might ‘escape’ into wild, related
species of the organism, with possible ecological implications.
Integrated pest management is aimed not so much at trying to eradicate pests totally, as
this is almost always impossible, but more at keeping pests under control so that the extent of
their damage is kept within acceptable boundaries.
55