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TECHNOLOGY, FEDERAL UNIVERSITY OF TECHNOLOGY, MINNA.
TECHNOLOGY, SCHOOL OF AGRICULTURE AND AGRICULTURAL
DEPARTMENT OF WATER RESOURCES, AQUACULTURE AND
FISHERIES TECHNOLOGY
FISH ECOLOGY NOTE
COURSE CODE: WAFT 314
CREDIT UNIT: 2
COURSE CONTENT:
Ecology of fishes with special reference to –
 Distribution and natural history and application of this knowledge for
fisheries management and obtaining maximum returns from fishery
resources.
 Characteristics of aquatic environment, organic production in aquatic fauna
and flora – algal blooms and eutrophication, plankton, benthic and biomass
assessment.

Food and feeding habit of fish, fish food and habitat selection, population,
niche concept, food chains, reproductive behavious and life cycle of some
selected species.
DEFINITION:
Ecology is the study of the interrelationship between an organism and its entire
environment. Thus, fish ecology is the study of relationship that exists between fish
and its entire environment.
ECOLOGICAL FACTORS THAT
ABUNDANCE OF FISH IN WATER
AFFECT
DISTRIBUTION
AND
The major habitat or environment of fish is water and within water, there are other
environmental factors which are physio-ch emical factors which are interacting
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within themselves. These factors are sometimes not seperatable because of their
inter-actions for instance one cannot remove temperature while talking about pH
alone. Thus it is not easy to say this is a particular factor limiting fish distribution
and abundance at a particular period. The major factors affecting fish distribution
and abundance are temperature, light, water current, food and social factors such as
competition, predation, parasitism and population density.
(A)
TEMPERATURE: Temperature is a factor which is of wide and varied
significance. This is true because it affects food, growth of fish and also the
general metabolism of the fish. Temperature affects fish in the following
ways;
i.
The day to day activities of the fish is also affected by the
temperature of the environment. I.e. the fish are sluggish at
low temperatures but very active at warm temperature.
ii.
Extreme or sudden changes in temperature are often very lethal e.g.
removing fish from temperate or polar region to tropical region. The
fish will be greatly affected as a result of sudden temperature
changes.
Temperature is critical to spawning and development of embryo i.e.
for fish to spawn, there should be ideal or suitable temperature –
similarly, when the eggs are hatched the larva can't develop further
if there is not suitable temperature.
iii.
Physical reflexes are controlled by temperature, thus variations or
changes in temperature may block simple reflexes of the fish.
iv.
Unstable temperatures can affect the swimming ability or speed of
small fry.
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v.
The depth of distribution of fishes in water can also be affected by
temperature i.e. those that require higher temperature for their
higher metabolism will be found in hypolimnion.
vi.
Temperature also affects general production of the fish (amount of
flesh or biomass which the fish put on at a particular time of the
year).
vii.
Fish migration either for spawning or homing, is controlled by
temperature.
viii.
Furthermore, temperature also affects distribution of fish, where
temperature is high, there are more fishes whereas in areas
with
perpetual low temperature, fewer fishes are found.
The ways the fishes react to temperature changes generally if temperature is
critically high; fishes can move down into the lower water zones where temperature
is normal. But when temperature is extremely low some fishes can undergo
aestivation or hibernation or migration until the temperature is normal.
B. Light: This is another ecological factor of importance in the life of fishes. Light
usually have two effects; (i) Direct effect (ii) Indirect effect.
i)
Direct effect on the fish:
Direct effect on vision (ability to see) hence it is important for sight feeders –
those who need to see their prey before feeding, thus affecting distribution
and abundance of fish.
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Direct effect on the pituitary organ: This is located below the brain and is of
vital importance. The amount of light shining on individual fish usually
affects the organ for various endocrinal and physiological activities.
ii)
Indirect effect of light on the fish:
(i)
Timing role in the reproduction of fishes, pituitary organ triggers up
the reproduction. Hence, the amount of light shined on the fish will
stimulate the pituitary gland which releases reproductive hormone.
(ii)
Light also triggers migration of fishes
(iii)
Light influences the rate and pattern of growth. This is also tied on
pituitary organ which release growth hormones. If light is
perpetually low, growth will be affected.
(iv)
Light also affects the productivity of aquatic environment and thus
affecting the food available to fishes. The more the sunlight, the
more the production of natural food.
C.
WATER MOVEMENT:-
This can simply be defined as the speed of the water movement. It can be described
as slow or fast current. Water current affects fish habitat in the following ways;
(i).
it usually shapes the banks of the water body.
(ii)
it can cause turbidity in water resulting from erosion and this may be
lethal to fish by clogging the gills of these fishes which affects respiratory system,
sight feeders are also affected as they will not be able to feed. Furthermore, as light
penetration is reduced it will cause low primary production of the water body and
finally turbidity enhances rate at which heat is absorbed, hence, warming up the
water faster.
(ii)
Turbid water would result in silt deposition which may cover the
feeding and spawning ground thereby leading to food insufficiency and
reduction in reproductive rate.
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Water current can be divided into head water region which is the source and very
fast and the down region which is down stream and slow.

Ways by which current affects the distribution of fishes
i.
The stronger the current, the poorer the fish farmer since fewer fishes
can be caught in that water body.
ii.
Fishes living in the strong current region have developed hold fast
features, so that they will not be easily swept off.
ii.
The current helps in mixing up the water temperature and also helps in
the recirculation of the nutrients.
(D) FOOD: Food is one of the several important biological factors in the
environment of the fishes. Its abundance and variety will determine fish population
density and composition. Also, food affects habitat of fish and their distribution.
Hence, distribution of fishes as surface feeders, benthic feeder, pelagic and demersal
species. Thus, the more the variety of food in a habitat, the more the variety of fish
in that habitat.
(E) Social factors: These are factors which affect fish distribution and abundance
indirectly. Such as competition, predation, predators – prey relationship, mutualism,
commensalisms, parasitism (carnivous fishes e.g. shark, claridea are normally found
with fish in their stomachs) and population density. In competition when two
individual compete, one is forced to migrate, or due to forces of evolution adapt to
new way of life. Competition could be for spawning site, food, space and shelter.
Those who have not gotten the upper hand will be found thinly scattered within the
population.
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MANAGEMENT OF FISHERY RESOURCES
There is no general rule for management of an eco-system, rather each major
class of resources has its own set of management principles or techniques.
Nevertheless the following principles are common to all;
(i)
(ii)
There should be wise use of the resources to avoid waste.
One should have a general knowledge of the ecology of the species one is
trying to manage:
(a)
Understand the behaviour of the resources
(b)
Know the reproductive biology of the resource
(c)
The food habit, of the resource
(d)
The growth pattern of the resource
(e)
The life span of that resource
(f)
The natural enemies of the resource
(g)
The diurnal movement of the resource
(h)
The production of that resource
(i)
Education of the general public about the resource.
FISHERY CONSERVATION AND MANAGEMENT
In the past, there were no problems of fish production and conservation. There were
very few people inhabiting water areas and also the method of catching fishes was
very primitive and the gears were very inefficient. So at that time the water resource
seems very inexhaustible, but as time goes on civilization begins to improve with
modern techniques coupled with the steady rise in human population.
These result in increase in amount of efforts needed to harvest fish, decrease in the
weight of the annual harvest, and ultimately decrease in size of fish landed as a
result of over fishing hence, the need for conservation and management of fisheries
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resource. Fishermen and scientists generally have come to recognize that aquatic
resources are not in exhaustible based on the knowledge of factors limiting
biological productivities. They also know that man's activities can alter natural
processes. Alteration to the environment can be hazardous to the fishes and
fisheries. For example diversion of water for irrigation, obstruction of streams flow
by damming and pollution can all diminish the productivity of the water. Equally so
is the productivity of future generation of fisheries resource which may be
contingent upon how it is presently used, treatment of fish population and the
ecosystems that produce them. Hence, management of the fisheries to protect their
capacity for producing a sustained yield is highly desirable.
We have to manage and conserve our fisheries resources so that we can get
what we need today and also for the future. So to conserve and manage a fishery
resource, the following are to be done.
(i)
The knowledge of the habitat
(ii)
The knowledge of the fish
(iii)
Training of the manager
(iv)
Involvement of the government
(I)
the knowledge of the habitat: Since habitat is the supportive factor of
resource of which we are going to manage, a thorough knowledge of the habitat is
important. You have to know the physical and chemical properties of the water. The
physical properties may include temperature of the water, wind direction, type of
grasses and weeds which are found in this water, the (aquatic macrophysics) the
water current, soil type and the topography.
a.
Temperature: the temperature is very critical to development of fish because it
determines their metabolic rate, determines some physiological process like
reproduction, migration and aestivation.
b.
Transparency: this is very important because fishes depend on the transparency
of the water which is a factor to light penetration and water freshness.
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c.
Wind direction: the wind direction will determine the water current; hence,
when you need fish, you follow or study the direction of the current and the
current will determine the type of fish that can survive in a particular water
body either river or stream.
d.
Aquatic weeds: they serve as food for fish, serves as substrate for laying eggs.
The weed can provide oxygen in the process of respiration. The same goes for
macrophytes.
e.
Soil type: the soil should not be porous. You have to know the chemical
composition of the soil; some soils can have some poisonous elements. So one
needs to do soil analysis and soil pH test to confirm the condition of the soil.
f.
Topography: bottom type of soil, relief feature etc we need to know these
because they directly affect the survival of fish. It is a known fact that where
some of the heavy metal exist in large size such as lead, it can bring problem,
also, small amount of molybdenum can cause problem. Also the amount of
oxygen present must be known.
(ii)
KNOWLEGE OF FISH
-
The knowledge of the following about fish is essential to ensure proper
management of the resource;
food and feeling habit of the species
life cycles of the fish species
-
Fecundity (the amount of eggs carry by the fish) this will determine the
survival rate of the fish.
-
The spawning time: when does she shed the eggs, how many times per year,
what happens to the eggs after laying -are they covered, or attached to the
vegetation? The biology of the eggs as per how long they take to hatch. The
size at first maturity, life expectancy –long life expectancy doesn’t mature on
time.
-
Ages and growth rate of the fish.
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-
Physiological processes of the fish including reproduction strategy of the fish
does it need to migrate during certain seasons of the year?
(iii)
TRAINING OF THE MANAGER: The managers need to be trained
because it is through his own knowledge that proper management of the resource
can be done.
(iv)
THE INVOLMENT OF THE GOVERNMENT: it’s difficult to manage
resources without the support of the Government. For instance, if someone comes to
dump refuse or toxic substance in your pond situated in a public water body and the
government has not enacted law against the act, the culprit may go free. Hence, the
needs for Government to promulgate laws to protect your pond by protecting the big
water body and more importantly to enforce its implementation. It also creates
awareness to the public by the use of mass media.
MANAGEMENT OF RUNING WATER RESOURCE: (Rivers Streams,
Springs etc.)
Management of the running water is not easy because it flows through many
territories, settlements and villages. However, in urban centers factories use water
from the stream or river to cool machinery engines which is in turn released back to
the rivers as boiling water. Textile industries also use water from streams and rivers
during the process of dyeing, and the waste including dyes will be returned to the
water body. Moreover, in some countries where there are factory laws,
manufacturing industries are forced to recycle the water before releasing it into the
river.
Urban centers also have sewage disposing problem, this is normally dumped
directly into the water. Although the waste at beginning enriches the river but at
later end it turns out as pollution. Urban centers also dam rivers for purposes like
electricity, irrigation and the likes.
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In rural areas, water is used for so many purposes such as disposer of feaces,
washing of clothes, drinking and cooking. However, some of the activities have
negative effects on the fishery management. For example, dumping of waste into
the water body will affect its food cycle. At tolerable stage, the algae will bloom.
This will boost the population of Tilapia in that water body as food chain will be
improved correspondingly but will be followed by decline due to the reasons
bellow:
(waste – Algae – Tilapia (Oreochromis) – lates niloticus)
OVER SHADING EFFECTS: algae bloom will compete for oxygen, hence
oxygen becomes short in that water, and other aquatic organisms in that water will
have shortage of oxygen assimilation which could lead to (1) their death (2) A
depletion of algae may also occur as a result of dumping of hot water or chemicals
such as lead into the water, the algae will die subsequently the food chain level
would decline, the process could be rapid or gradual. As a user of resources, you
begin to have decline in the harvest you then have to think out the cause. And by
this, you have to move beyond your vicinity to find the cause. It could be town 'A'
on the upstream that may be causing the decline. Thus testing of the algae could be
done to see what the likely cause could be.
Ways of managing river fishery. For effective management of river fisheries
resource, knowledge of the followings is essential;
(i) You have to know the various uses of the water throughout it course.
(ii) Knowledge of the physical and chemical properties of the water body.
- For the oxygen content of the water to be increased, population of the algae
have to be increased, this can be done by fertilizing the water by either
organic (manure, poultry) or inorganic (chemical fertilizers). In a running
water body, it is not easy to increase the algae level because of the rate of
flow down the stream.
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- When silt is accumulated at the bottom of the river, it will decrease the
volume of water, hence, dredging have to be done. This will increase oxygen
content of the river.
- Also co-operation of the other towns sited at the bank of the river is necessary
for instance, when dam is to be constructed environmental impact assessment
has to be done to ensure that passages of fish would not be hindered. In
addition, some time there are trees by the river, they need to be cut down for
general circulation of oxygen and to avoid shade for maximum light
penetration. Furthermore, trees are also cut down to prevent organic matter
decaying which could cause oxygen shortage. And lastly the trees can impede
movement of boat, setting of the net and tree stumps will destroy nets, as
such they have to be cut down for smooth operation of these activities.
(iii) FISHERY REGULATION As a manager, you have to be able to make fishery
regulation so as to boost the fishery business such as by application of the
following;
(a)
Close season; Time to stop fishing e.g. gravid fishes migrating to spawning
areas may be fished out hence, such season should be closed to fishing.
(b)
Regulation on the use of appropriate mesh size in order to preserve gravid
female and fries.
(C)
Prohibition of certain fishing techniques such as the use of explosives, toxic
chemicals (e.g. gamalin 20), pounded poisonous roots etc.
(d)
Regulating the length of fishing seasons i.e. 3 month in one year.
(e)
Regulating number of fishing gears i.e. out of 10 fisher men 5 have to use
5cm mesh size type of net, 3 to use gill net, 2 to use traps and so on.
(f)
Close areas; that is you don’t want fishing to be done in such area. This may
include spawning grounds, nursery ground to protect the young ones.
(g)
Regulating size of fishermen; i.e. you can say this river can support only 10
fishermen hence only 10 are allowed to fish, you do this by issuing license to
the permitted individuals.
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(h)
Protection of females because they are responsible for continuation of
generation i.e. 1 male to about 100 females is tolerable. Hence the knowledge
of sexual dimorphism in fish is essential to be able to identify the male and
the female (this is done by exerting gentle pressure on the belly, if it is a
female, eggs will come out)
(i)
Anti-pollution regulation: You should be able to control waste, either solid or
liquid from being dumped in the river.
(vi) ENVIRONMENTAL CONTROL
This involves building of fish shelter which may make the place calm and
vegetation naturally provides this.
i.
Weeds: Weeds can be responsible for loss of 1/2 of small water body through
evapotranspiration. They also occupy space which limits oxygen because of their
rapid removal during respiration, they prevent light which is the source of energy
from penetrating to the deep water, when they die they consume a lot of oxygen
when decaying, water weeds also hinders navigation as well as fishing activities of
gears.
But weeds have some merits, such as being the source of food for some fishes
e.g. grass carp and tilapia galilea. Some of these fishes feed on the leaves or roots of
the weeds. Weeds also act as fish cover from predators, and help to oxygenate the
water.
Weed control
Weeds can be controlled by the use of chemicals like 24 b, 245T, Dalapon,
Diquat etc. also by mechanical means i.e. by picking up the weeds or harvest those
that are capable of floating. This method is suitable but more labourous and more
costly. The weeds harvested can be recycled as livestock feeds or sometimes for
human consumption.
Weeds can also be controlled by biological means. This is done by
introducing some animal species which are capable of eating these weeds in large
quantity e.g. grass cap, tilapia galilea.
II.
PREDATOR – PREY CONTROL: Must be able to determine the number
of prey and predators in that water body. The number of prey should be more than
the predator, usually 10 times the predator.
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Management of the fishes:
(a)
You should be able to know diseases that affect them and their control.
(b)
The diseased fish, weakened and albino fishes should be removed as they
occupy space and spread diseases.
(c)
Castration of males (sex reverser), so that there won’t be any population
explosion. But in low density of fish population you encourage it. For the
survival of the male and the young ones fingerlings can be purchased and
stocked in the water.
(d)
Creation of awareness of fisheries management among people i.e. through
extension services.
(e)
The fishery officer should be aware of current trends through researches.
FOOD CHAINS
A linear feeding relationship between different organisms in a particular
ecosystem is called food chain. Intricate relationship between different organisms is
referred to as food webs.
An example of food chain in fish ecology is given below.
Algae
Tilapia
(1,000,000)
(5,000)
Clarias
Hydrocynus
(100)
(50)
The organism at the lower base of the chain pyramid is usually larger in
number and smaller in size than those of higher feeding cadre. That is, those at the
higher tropic level are few in number and larger in size.
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Typical food chain - sizes
A generalized food web of fish community:
Plankton feeders
Herbivores
Omnivores
Generalized predators
Benthic predators
Piscivorous predators
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Natural food web
fish density
Increasing fish production
feeds
$$
nutrients
fertilizers
manure
detritus
oxygen
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FOOD AND FEEDING HABITS
FOOD:
Food is any material that can be digested, assimilated and utilized for the production
of energy.
Food is the major source of energy for locomotion, searching for food, and escapes
from predation, employed by migration of fishes, required for gonad production
which is essential for the continuity of existence. Food is also required for
elaboration of cells/ organelles.
Basic foods
(j)
Carbohydrate- simplest form of glucose
(ii) Protein- ammo acids
(iii) Fats- fatly acids and glycerol
Various food items are required in various proportions; hence there is the need for
combination in right proportion for a balanced diet necessary for optimal growth of
fishes.
REASONS FOR FOOD STUDIES:
(i)
to determine population levels because the number of organism in any
population depends on the available food
(ii)
to determine rate of growth of fish species because rate of growth of
individual fish represent the food they are able to consume in the course of
struggle for survival.
(iii)
to determine forage pattern of fish whether predatory, planktonphagus,
herbivores or omnivores.
(iv)
the type and magnitude of food available gives information on the seasonal
life history of fishes.
(v)
reveals ecological relationship of different fish species on one hand and the
different fauna and flora that constitute any aquatic habitat on the other.
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Methods of studying food: They include:
(i)
Collection or sampling
(ii)
Preservation
(iii)
Preparation/processing
(iv)
Analysis
Note: Best collection method is by the use of such gear that will produce an
unbiased sample. Why? Because food varies with sex, environment, time, size and
age.
(i)
Sampling Method: Sampling must include all sizes, sexes and ages sampling
should be done around the feeding period of the fish. It helps to remove bias
posed by differential digestive materials; sampling should be done at different
feeding periods, in order to avoid mainly studying empty stomachs and to
remove post humus digestion.
(ii)
Preservation method: Method of preservation must be consistent throughout
the food studies. Reason for preserving is to maintain food in the same
condition of moistness.
(iii)
Preparation: involves dissecting the stomach or intestine. Before dissection –
measure the length and the weight, time of capture, location of capture and
method of collection. There is controversy between the use of stomach material
and intestinal contents because cellulose or chitin will not digest in the
stomach. Hence it is recommended to use both stomach and intestinal contents.
However, no matter the method used to analyze the content, the stomach is
usually classify into 0/4, 1/4, 2/4, 3/4, 4/4 fullness.
IV ANALYSIS OF FOOD:
(a)
Numerical method
(b)
Frequency of occurrence method
(c)
Volumetric method
(d)
Gravimetric method
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(e)
Points methods
(a) Numerical method:
This involves counting the number of individual food items and expressed it as a
percentage of total food items available. presented as:
Number of individual food items x 100
Grand total of food items 1
Limitations
Conclusions can be drawn on the numerical significant of food items. But the
organism occurring in large number may not necessarily be the most important food
items.
(b) Frequency of occurrence: This involves counting the number of times each
food item occur and expressing it as total number of times the food item occur in
stomach.
Occurrence percentage (%) = Frequency of food item
Number of food within stomach.
x
100
Significance and limitations
Its significance is that you can obtain the frequency of the food and thus the relative
importance of that food to the population can be guessed. However, the method is
biased due to accumulation of digestive resistance materials. When there are such
materials, the suppose frequency with which they occur will seemingly be greater
than the actual frequency. It will also be difficult to know the number of feeding
(foraging) fish spp. Both of the two methods do not show "Bulk" relationship of
food items, in order words, will not show the biome sic importance of food items.
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(c) Volumetric Method:
Volume of different food items and expressing them as percentage of grand total
volume of all the food items.
Volumetric percentage (v %) = Volume of food items
Grand total of all food items.
x 100
1
The volumetric is very important because it removes wrong impression created by less
important, but abundant food materials. That is it shows bulk relationship of the food
materials. However it is known to cause large voluminous food items mostly from
cumulative resistant digestion food items of different feeding periods, Errors may arise
from this differential digestion of different food materials.
(d) Gravimetric method:This is obtaining dry weight of different food items and expressing them as
gravimetric percentage
(G %)
= Dry weight of food items
Grand total dry weight of all food
x
100
1
(e) Point’s methods: involves scoring points to different food items and summing
up the number of points. Usually, it is summed up to 5 points. Moreover, to take
care of limitations in each method it is advisable to study food and feeding of
fish with 2 or more methods. Because to compare number of food, organisms,
frequency of occurrence and volumetric is important.
Reasons for using more than one method
(i)
Numbers alone can not give true picture of importance of food items.
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(ii)
Frequencies are said to be biased in favour of smaller food items, while.
(iii)
Volumetric are biased in favour of large food items. Hence any combination
which will express number of occurrence and bulk importance of food items is
ideal.
Factors affecting feeding habits of fish
These are size, sex, season, temperature, competition and habitat.
(i)
Size: Different size of fish feed on different food items; the reason for this is
as a result of competition. It could also be as a result sizes in different
locations, Fagade and Olaniyan (1972) on revealed variation of food with
different sizes variation of food with different sizes.
(ii)
Sex: Food habit also vary with sex of fish, it is a known fact that spawning
fish may change their location, habitat and depth of water in which they are
found. In all these places, there are different food items.
(iii)
Season: The seasonal distribution of food items like preys, phytoplankton,
zooplankton, have been known to control the rate at which they are
consumed. It is a known fact that fishes (predators) makes use of the most
available food items, hence at any season when one item is in abundance,
fishes would make use of it as food.
(iv)
Temperature: temperature is known to regulate spatial distribution of many
fishes as well as their food organisms. This will undoubtedly change the
feeding habit of such fishes. Bard et al (1916) noted that tropical fishes eat
more hence they grow fast than temperate fishes.
(v)
Habitat and competition: The two are inter – related to size, sex and
seasonal distribution
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Food Habits
FOOD AND HABITAT SELECTION
Fishes, generally make use of the available fauna and flora in their habitat as foods,
because of this, fishes could be divided into different group as to what they feed on
and habitat they dwell.
(a)
Predators: Those whose food comprise mostly of animal matter. They can
either be piscivorous or non-piscivorous feed on other animal matter. They are
mostly surface or profundal dwellers where they can get their prey easily.- e.g.
hydrocynus,
(b)
Plankton feeders: Those that feed on algae only are referred to as
phytoplanktonphaygus. They feed mainly on plankton including zooplankton.
They dwells in the surface waters where algae are readily available .e.g.
heterotis niloticus
(c)
Herbivores: Those that feed mainly on vegetable matter hence dwells in the
littoral zone of the aquatic environment e.g tilapia – macrophagus and
microphahus species.
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(d)
Detritus feeders: Those that feed on bottom deposit or detritus. There is
another group referred to as detritopelagic feeder, these feed on vegetable
matter as well as detritus.
(e)
Omnivorous feeders: these are known to subsist on mixed diets of vegetable
and animal matters. Most fishes are known to overlap in their feeding habit;
this is referred to as feeding inter-relationship between the fish flora and fauna
of the habitat. The over lapping in their dietary needs is aimed at reducing
competition. Studies have shown that plankton feeds form the main food for
the piscivores, hence, the occurrence, abundance and distribution of these
species can be regulated by the plankton feeders and vice – versa. Non –
piscivores are usually connected by their prey to both the plankton and the
bottom deposit. Most at times to avoid inter and intra species competition
fishes not only overlap in their food habit, they also argument their food with
different food items, when their major food is not available. Because of these
reasons various food items have been separated into various or different
categories;
(i)
Main/Major Basic Food: This is the food which the fish usually consume and
comprise the main part of the gut content.
(ii)
Incidental Food: These are those food items that are rarely occur in the gut.
(iii)
Secondary food: These are the kind of food which is frequently found in the
gut of the fish but in smaller amounts
FISH POPULATION ESTIMATION
Population is the number of organism in a given area. It could be
individuals of the same species e.g. Tilapia population in a given area, or of
different species e.g. Total fish population in an aquatic environment.
There are a number of factors that affects the fish population size. These are
biotic and a biotic factors. They affect the rate of increase or decrease in fish
population of a particular environment. These are extrinsic environmental
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resistance (such as climate, predators, food supply, disease and inter specific
competition). For each of the biotic environmental factors, fish have a range within
which it can survive. Towards the upper and lower limits of the range survival is
either increased or reduced.
ESTIMATING THE SIZE OF FISH POPULATION IN A GIVEN AREA.
Two techniques can be used in estimating the fish population in a given area
a.
Direct method
b.
Indirect method
DIRECT METHOD: Involves counting of individual fish in a given environment.
This method can be aided by applying fish toxicant in that environment and thereby
doing direct counting but it is often not possible to count all the individuals of a
given species in a given area, especially when the number of individuals involved is
large. Fishery Biologists, therefore, often employ suitable techniques to estimating
fish population when desired for species which have low mobility but inhabit fairly
uniform habitats.
INDIRECT METHOD: The population can be estimated by a simple or indirect
method - the capture – mark – recapture technique.
The technique makes some assumptions.
(i)
That there is neither emigration nor immigration, or death and birth of
individuals during the sampling period.
(ii)
That after each trapping session, the trapped individuals mix thoroughly with
the rest of the population and that all individuals remain equally trappable
throughout the exercise.
(iii)
That the marking techniques must not be harmful to the organism or in any
way confer a disadvantage or advantage on marked individuals or affect their
behaviour.
23
APPARATUS: Capturing equipment (e.g. electro fishing, sweep net or live traps),
marker (i.e. string to tag them)
Procedure: The capture – mark – recapture method: Within the given
environment for a specific time (i.e. 1hour) mark the caught fish with a
suitable marker. Count these marked individuals and designate them as "n"
then release them into the area where they had been caught. On the next day
of sampling period after the captured individuals had mixed thoroughly with
the uncaptured; the second sampling or trapping can be done within that same
given area and for same period (1 hr) as in the first trapping. Count the total
number of fish caught on this second sampling and designate the total as M.
After obtaining the total count M, remove the marked individuals and count
them. Let the number of these individual be m.
n=1st captured & marked;
M=2nd captured (marked + unmarked)
m= counted marked from M
N= total estimate of fish in the environment.
If N is the estimate of the total number of fish in the given area, then:
n=m
N M
The above relationship was independently discovered by two researchers, Peterson
and Lincoln and is thus referred to as the Peterson – Lincoln equation.
From the above equation, N=nM,
m
Thus, it is possible to get an estimate of the total number of fish in a given
environment.
An advantage of this census technique is that it saves time, labour and cost.
e.g. if n=90 M=81 and m=3
: N = nM =90x81 = 2430
m
3
ECOLOGICAL NICHES CONCEPT
24
The ecological niche in any organism cannot be violated. In aquatic environment,
there are different species of fish, all may have different needs or behaviour though
living together in the same aquatic environment. The niche of every population
determines its structural, physical and behavioural adaptations.
Ecological niche of fish is affected by some changes in the environment, this could
be biotic or a biotic changes for example: Age may bring about changes in niche of
a fish i.e. fingerlings enters shallow zone of the water while the adult lives in the
deeper zone. As fish migrate, may be The space or area or microhabitat occupied by
fish or an organism which is suitable for its establishment, development and
continuity of generation is referred to as Ecological Niche.
as a result of competition or reproductive requirement, its niche automatically
changes. Also, extreme winter and summer temperatures can cause changes in
ecological niche of some fishes. Availability of food can also bring about changes in
niche of some fishes.
NICHE DIFFERENCIATION
This is a phenomenon whereby two species of organisms in inter- specific
competition for space through some behavioural adaptation brings about difference
in their niche requirement. For example, if two different species are naturally
surface feeders, but due to competition between then, one decides to adapt to
feeding in profounder region this is niche differentiation brought about as a result of
inter-specific competition.
Niche differentiation brings about territoriality which is defending area
against conspecific or individual of some species and sometimes against members
of other species
ALGAL BLOOMS
Under favourable or suitable environmental conditions algal colonies may
grow to considerable cell densities 20 – 100 – 103 cells (m/s). This condition is
called algal blooms or tides. During or after the blooms, toxins, may be produced
25
which are lethal to fish. Algal blooms also cause oxygen depletion in water which
can also result in mass mortality of fish. Microcystis aeruginasa is the example of
algal species in which many cells combined to form colonies and congregate in
numbers sufficient to produce patches of blue mat on the water. Concentration of
about 500,000 colonies of microcystis/litre of water are sufficient to cause the death
of fish while concentration of 1,000,000/litre are likely to cause very extensive
mortalities.
PLANKTON
A mixed assemblage of long floating plants and animals. They are not real life
plants and animals in totality as they do not possess chlorophyll which characterized
them as plants species like Euglena. They are sometimes classified as phytoplankton
and zooplankton: Plankton serves as food for some fish species. These may be fed
upon by zooplanktons which are in turn fed upon by certain fish species. Most
importantly all fish in the post –embryonic stage feed on zooplankton.
BENTHOS
These are bottom dwelling organism. These are phytobenths which are plant
–like and zoobenths which are animal – like. The study of benthic involves the use
of sieves for separating organisms from the sediments. The sieve usually has
openings of 0.833 – 0.47mm. Mesh opening ranges from US sieve series number 20
– 40. The sample to be studied is made to pass through the largest sieve size through
the 5 series. Organisms that are retained by the sieve (largest size) are called macro
benthic forms and those retained in the finest mesh opening are referred to as
microbe tic forms.
BIOMASS ASSESSMENT
Biomass is the standing crop or the weight of any particular organisms per
unit area at a given time.
26
This does not take into account relative productive rate of the component
making up the food web or the duration of their life span. A high standing crop does
not necessarily mean that the ecosystem has a high rate of production.
Vertebrate biomass is usually greater in smaller stream than in large stream.
The invertebrate biomass is grater in rapids water but lowest in muddy bottom.
There is variation in biomass in respect to seasons of the year and size of the stream.
Biomass assessment is usually by direct counting of the standing crop. This could
be expressed in fresh weight or dry weight per unit area.
ORGANIC PRODUCTION IN AQUATIC FLORA AND FAUNA
Production is defined as addition of new biomass to an organisms or group of
organisms per unit time. Productivity is the potential rate under ideal or stated
condition of incorporation of organic matter or energy. For a single individual,
production represents growth in weight as well as the weight of lost parts.
Production could be expressed as the gross of carbon produce per unit area per unit
time g/m2/time, could also be expressed in term of caloric content or energy fraction
expresses as kcal/m2/time.
i.
PRIMARY PRODUCTIVITY is that part of production derives from plant
photosynthesis.
Gross primary production (GPP) Is that derived when chlorophyll
contained in plants make use of light energy and combined them with various
substances like water and nutrients to synthesize complex organic molecule.
Net primary productivity (NPP) is obtained by minusing or deducting the
amount used for respiration and reproduction from the gross productivity.
II.
SECONDARY PRODUCTION Is the formation of tissues in animal from
the consumption of plant material.
III.
TERTIARY PRODUCTION Is the tissue production by carnivores as a
result of feeding on heterotrophs.
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CHARACTERISTICS OF AQUATIC ENVIRONMENT
The aquatic environment encompasses a wide variety of parameters, virtually all of
which influence the maintenance of homeostasis essential for growth and
reproduction of fishes. If this is altered beyond acceptable limit, may expose the fish
to diseases or actually cause disease.
Among the most importance of them are physical factors such as the
temperature, the intensity and periodicity of light (including shading), the chemical
composition of the water, its biological content, the availability of space and food
and the frequency of fright stimuli such as moving shadows. Another important
factor for wild fish and those farmed in extensive systems is the production of the
ecosystem which sustains their food supply.
Physical and chemical aspects of water quality
a.
TEMPERATURE: Fish have upper and lower thermal tolerance limits and
optimum temperatures for growth, egg incubation, food conversion and
resistance to specific diseases. These optima may all be different and may
change according to water pH. water temperature also affects properties of
the aquatic environment important for fish health. Surface waters are
subject to temperature fluctuations of up to 400c caused by latitude, season,
altitude, time of day, depth and other factors. The range of temperature
change of sea water is much less, due to water circulation in the seas and
oceans and the large volumes of water involved dissolved gases generally
decrease in solubility with increasing temperature, whereas the solubility of
toxic compounds which are only sparingly soluble in water, such as crude
oil and pesticides, increases with temperature rise. The toxicity of some
substances such as heavy metals increases with temperature.
b.
LIGHT: In natural waters and extensive farming systems, light levels can
only be changed indirectly by methods such as increasing water depth and
controlling unicellular algae, macrophytes and tree shade. Poor light
penetration caused by absorbent or reflecting pollutants, such as clays, coal,
28
washings and paper wastes diminishes algal productivity and may decrease
the availability levels of food for fish. In intensive system the light intensity,
photoperiod, shaded areas and light absorption by background is more readily
controlled. All of these parameters may contribute to aspects of the growth
and maturation rate of fish. In shallow intensive culture systems ultra – violet
light from excessive sunlight can result in sunburn of the dorsal surface of the
head.
c.
DISSOLVED GASES: Of these gases dissolved in water, two are of
particular interest, oxygen and nitrogen. The occurrence of carbon dioxide,
ammonia and hydrogen sulphide are special cases in a mixture of gases such
as air, each gas dissolves in water according to its solubility. This in turn is
controlled by the total air pressure and the partial pressure of the gas in the air
mixture in contact with water. If fish is held in water that is supersaturated
with oxygen and nitrogen, the condition known as gas-bubble disease may
develop. The effect of dissolved salt content is that as a general rule, gases are
less soluble in water containing dissolved salts. Increasing temperature also
decreases the solubility of most gases in water.
d.
PH: pH is another factor in aquatic environment. The pH scale is a negative
logarithmic scale, meaning that for a decrease of 1 pH unit, there is a ten-fold
increase of hydrogen ion concentration. Neutrality on pH scale is the point
where equal amount of hydrogen and hydroxyl ions exist. This value changes
with the salt content and the temperature. Where hydrogen ions are in excess
of hydroxyl ions, the solution is said to be acidic and in the reverse situation,
alkaline.
e.
CARBONATE, ALKAINITY AND HARDNESS: The buffering capacity
of fresh water is defined by the carbonate alkalinity measured in mg/liter of
equivalent of calcium carbonate. Hardness is a measure of calcium,
magnesium and other metals in freshwater and is expressed similarly as
mg/litre of calcium carbonate. Soft waters are classified as containing 0 –
60mg/l moderately hard, 60 – 120mg/l and hard as in excess of 120 mg/l.
29
Freshwaters with a significant carbonate alkalinity are commonly of alkaline
pH and are characteristics of limestone areas or outcrops. Seawater has a high
carbonate alkalinity.
f.
ACIDITY: In unpolluted freshwater, acidity is caused by carbonic acid and
the organic acid derived from soils, forests, swamps and bogs. Natural
acidity, carbonate alkalinity and pH are important in defining the quality of
the aquatic environment for fish health. The range of pH values found in
freshwater is wide and can fluctuate, but fish commonly live in the range 5.0
– 9.5. Although fish may tolerate and even reproduce in environments with a
wide range of pH values, their optimum performance, defined in terms of fast
growth rate or maximal reproductive capacity may well be restricted to within
a much narrower range of pH values.
g.
CARBONDIOXIDE: In natural waters and the waters of extensive fish
rearing systems, the dissolved carbon dioxide bicarbonate system forms a
reservoir of carbon for photosynthesis by aquatic plant life. The natural plant
productivity provides the basis for fish food production and is closely
correlated with adequate bicarbonate – carbonate buffering capacity.
Removal of carbon dioxide during photosynthesis in sunlight causes an
increase of pH. In most natural water, carbon dioxide levels do not exceed
6mg/l. Increasing amount depress fish respiration but provided the increase
is not too rapid, acclimatization is possible. high Co2 levels of the order of
30mg/l may occur in acidic ground water.
h.
AMMONIA: The undissociated ammonia molecule, NH3, is highly toxic for
fish. Even low levels of ammonia can cause bronchial hyperplasia. High –
protein diets fed to fish in intensive culture systems results in high levels of
ammonia as the principal nitrogen containing excretory product, so that
where alkaline or neutral water is reused without treatment, oxygenated toxic
ammonia levels may build up.
i.
MINERAL CONTENT: naturally occurring freshwaters may vary largely in
mineral contents depending on the source and location.
30
j.
SALINITY: Is the measure of the total salt in seawater. Expressed as g/kg or
part per thousand. Coastal water commonly has a variable salinity due to
freshwater run-off.
k.
CHLORINITY: This is a measure of the total halides in a given weight of
seawater. Oceanic water has a chlorinity of approximately 19%. The content
relationship between salinity and chlorinity is expressed by
5% = 0.30 + 1.8050CL%.
Fish are most at risk when variation in salinity occurs to the extent that the
gill and kidney are unable to control the osmolarity of the body fluid. The mineral
content of freshwater is largely determined by the composition of the soil and rocks
through which they have run. Rain water itself contains traces of many elements
derived from atmospheric dusts. An excess of a particular mineral or ion may
endanger fish health, such toxic situations are more commonly associated with man
– made pollution, thus natural water supplies a reduction in the quality of the
aquatic environment. Acceptable concentration of toxicants to which organisms are
to be exposed continually must take into account enhanced concentrations which
may be reached occasionally during brief period and the long term effects of
cumulative poisons.
THERMAL POLLUTION: Increased water temperature may be beneficial for
fish culture as it could aid faster growth but only if the increases is to a level below
the thermal unit for the species. Tropical fish often live environmental temperature
closer to their upper thermal limit than fish living in temperate water. Temperature
modifies the impact of pollutants. Many are more toxic in warmer water and since
they are also insoluble in high temperature.
THE METAL: The commonest causes of metal poisoning are the heavy metals.
Cu, Pb, Zn, Cr, Mn, Cd and Fe. Industrial discharges and seepage from industrial
and mining waste are the commonest sources, although sometimes they occur
naturally. The pathology of metallic poisoning varies according to the
concentrations and length of exposure. Many non – metals are toxic if present in
sufficient quantity. Some of those encountered commonly are NH3, Fluorides,
31
Cyanides, Phosphorus, sulphide, Arsenate and Halogens. Many organic compounds
used in agriculture and industry are also toxic to fish.
Sewage discharges may reduce water quality depending on the degrees of dilution
achieved. This discharges cause oxygen depletion, hence toxic production. Natural
water – contains a certain amount of naturally occurring suspended solids. High
concentration causes mechanical damage to the gills, silt deposition inhibits
respiration and encourages microbial growth and association with certain industries
such as quarry surface disturbance from civil engineering, can introduce large
amount of particulate matter whose effects on fish health may be observed many
miles downstream from their sources. As well as it effects on gills, it also reduces
the light penetration in water resulting in less energy in the food web leading to
decrease in fish production.
OIL POLLUTION: Spills of crude and refined oils can have highly toxic effects
on ponds and other enclosed waters where dilution of the water soluble component
is not rapid. The use of oil dispersants and their solvent greatly increase the toxicity
unless dilution is made it makes conditions terrible for aquiculture.
TAINTS: A wide variety of objectionable tastes, odour and colour have been noted
in fish flesh. Industrial waste implicated in causing taints include oil products,
phenolic, disinfectants and domestic sewage. Some taint can be removed or reduced
by holding fish in clean water for long periods but taints are more rapidly acquired
than dominated.
ADVERSE BIOLOGICAL FACTORS FOR FISH HEALTH
Aquatic Animals: All surface water may contain species of wild fish which can act
as reservoir of infectious disease. Animals other than fish may be reservoir of
infection as well as intermediate in the life cycles of many parasites.
Micro Organism: Toxic producing algae are found in marine, brackish and
freshwaters throughout the world. Under suitable environmental conditions, they
grow to considerable cell densities (20–10 x 103 cell/ml) called blooms or tides.
Examples are Deflagellates, the cryophyte (Phytoflagellate) and the cyanophyta
32
(Blue green algal). Some cyanophytes produce toxins which cause mortality to fish
and other animals. Several genera including macrocystis, which often grow to form
thickly scum 5cm 10 – 20cm thickness on the surface of ponds. The algae cause two
direct effects on fish production-i) Poisoning by excretion of Inchthyotoxins and ii)
asphyxiation by the rapid depletion of oxygen due to algal respiration, the sudden
death of the bloom.
EUTROPHICATION
AS
AN
ECOLOGICAL
PERSPECTIVE
OF
POLLUTED WATERS
DEFINITION: The enrichment of aquatic environment with plant nutrient
especially phosphorus and nitrogen until such environment becomes increasingly
very productive in plant life. It is aging process of aquatic environment.
NATURAL EUTROPHICATION: The process of pollution of water bodies
resulting from slow natural geological or biological process such as siltation or
encroachment of vegetation or accumulation of detritus.
PROCESS: The Chief cause of eutrophication is an increase in the supply of plant
nutrient, specifically the chemical nitrogen and phosphorus and other organic
growth promoting substances in the lake. Such nutrients normally reach the lakes
from springs and through erosion or drainage from land. The nutrient supports the
growth of enormous amount of algae and aquatic plants which in turn provide food
for the rest of the food chain community such as fish and aquatic invertebrates in the
lake. When these organisms die, their bodies accumulate in the bottom of the lake
along with silt covered from inflowing water. The decomposition of these
organisms decrease the level of dissolved 0xygen in water and gradually over a
period of many thousand years, the lake may fill to extinction.
RATE:
Small lakes with large fertile drainage basin are shallower, more productive in
proportion to the surface area of the lakes. Lakes that are located in impoverished
33
basin never become eutrophic. Human activities often alter the natural state of lake
and increase the rate of eutrophication. Raw and treated sewage effluents, industrial
wastes, erosion from rich agricultural lands, manure scattered on grazen soil that
wastes off into lakes in winter and spring, manure from field lots and urban land
run-off add nutrients.
Effect: the impact of eutrophication is most clearly reflected in biological and
chemical changes that occur in the affected water body. In relation to organisms
living in the environment the effects will vary under the following;
Effects on water quality:
i.
The algae bloom; there is less oxygen in the water as a result of the
decomposition of the dead algae which remove oxygen from the
environment.
ii.
Creation of nuisance such as appearance of colour and odour due to
production of gases such as hydrogen sulphide (H2S).
iii.
Restriction of re-creational use of such water bodies e.g. swimming,
boating, sport fishing
iv.
Causes hindrance to navigation by blocking water ways and bank of
deposited solids and scum in and on the water. It also leads to additional
load or cost in water treatment plant.
v.
Finally all the above effects may give rise to undesirable change in water
quality, thereby rendering it unfit for domestic, agricultural and
industrial usage and more importantly aquacultural purpose.
Effects on aquatic organisms:
As a result of removal of dissolved oxygen by the dead decomposing algae, this
may cause extinction of many species of aquatic organism living in the lake and
then increases the rate of eutrophication.
Eutrophication causes the following negative effects;
i.
It inhibits reproduction, photosynthesis and the survival of phytoplankton.
34
ii.
It also causes reduction of light penetration, thereby reducing photosynthetic
activities of phytoplankton which in turn reduces the number and species of
zooplankton that feeds on them.
ii.
It favors the growth of phytoplankton e.g. Cyanophyta which often appear to
be nuisance on water body, many of the blue-green algae have odour that are
indigestible.
iii.
It results in rapid increase in bacteria and other saprophytic microorganisms.
iv.
Eutrophication by heavy metals are very toxic to organism even at low
concentration can cause destruction of several physiological and biological organs
of aquatic organisms.
v. It also inhibits the feeding rate of protozoa e.g. condittus and copepods.
Effects on fish and fishing industries:
i.
Fishes suffer greatly from effects of heavy water pollution. Inflow of metals
into water bodies inhibits the hatchlings of fish eggs and development of embryo.
ii.
Can cause elimination of fishes from a particular water body
iii.
It can also cause gas bubble diseases which may also lead to mortality.
iv.
Can lead to clogging of gills due to suspended and dissolved solids.
Abrasion in fish gills especially the ones from the mining industries can
cause smothering of the organ.
v.
Effluents can cover the spawning ground of fish and feeding sites.
vi.
And finally, there is elimination of endemic species giving room for large
scavengers to take full advantage of the situation.
Solution:
The solution to the eutrophication problems can be divided into two viz; a) Long
term and b) Short term solutions.
Long term solutions:
35
This requires measure designed to prevent nutrients from getting into the lake. Such
measures would include divergence of sewage effluents, advanced waste water
treatment. Improvement of agricultural practices to prevent rich soil from running
into lakes.
Short term
This includes the removal of toxic chemicals to reduce objectionable plant growth
or to eliminate unwanted fish.
The procedure may however distort the complex ecological balance of the lake. A
better method to reduce eutophication is to harvest unwanted plant growth.
REPRODUCTIVE STRATEGY OF SOME SELECTED SPECIES
R
R =External Fertilization
K
K= Mammalian, one or few progeny
Multiple Progeny,
High Productivity
E.G. Cichlids, Cyprinids
36
Substrate spawner
ChCharacteristis:
Female – Care
Monogamy
Male – guard
•
•
•
Mouth brooder
ChCharacteristic:
Polygamy
Male – establish nest
Female – oral
incubation
•
•
•
37