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Lake Victoria Case study
University of Liverpool
Case Study - Lake Victoria
The fish populations of Lake Victoria were very abundant and diverse when the lake was first
discovered by Europeans. Within a relatively short time, the biodiversity was destroyed with
very large adverse consequences. This case study examines three key issues:
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Introduction of Exotic Fish Species
The impact of Exploitation on fish populations
The effect of Habitat Change due to Human Population Pressures
Background
Lake Victoria
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2nd largest lake in world and largest source of the Nile
Three countries and 65000 km2 (1/2 size of England)
Catchment (184000 km2 bigger than England) and population in that catchment 8 million
The two issues are well illustrated here
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Traditionally regarded as a cichlid lake
300 sp. of endemic haplochomids (a genus of the cichlid family)
2 indigenous tilapia spp. (Oreochromis variabilis and O. esculentus [also cichlids
50 non-cichlid spp
Total 350 coexisting species
History of Lake Victoria Fishery
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Despite this high diversity, traditional fishery has depended on relatively few taxa
Exploitation
 Used simple fishing gear (baskets, hooks and papyrus nets)
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Exerted little fishing effort and required high maintenance
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However introduction of flax gill nets (1905) and non-selective beach seine nets (1920’s)
intensified fishing
Urban centres developed and railway stimulated greater fishing throughout lake (1930’s)
In particularly intensive fishing of littoral zone with beach seine nets
This damaged the breeding and nursery grounds of cichlids (most notably the
haplochromids and tilapia stocks)
Found that availability of spawning sites was most important factor limiting tilapiine
populations in the lake
Hence increased fishing effort and efficiency lead to localised overfishing for the most
popular fish
Also resulted in dramatic decline in spp. diversity and in addition
Simultaneous huge increase in total annual fish yield
More efficient fibre gill nets 1952 and arrival of outboard motor (1953) lead to a 100%
increase in fishing effort
But yield only went up 10%
Therefore existing nets hardly caught any fish hence illegal use of smaller mesh occurred
Exploitation of immature fish greatly affected recruitment processes (recruitment
overfishing)
But the demand for fish lead to fishery biologists advice being ignored and further
reduction in mesh size granted
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Lake Victoria Case study
University of Liverpool
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All a consequence of unlimited entry and unrestricted no. of fishermen, fishing boats,
gear and of course capitalism
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They didn’t stop there though. They added to the fish competition
Competition
 To replenish stocks of the 2 indigenous tilapine, 4 exotics were introduced
 O. niloticus, O. leucostictus, Tilapia zillii, T. melanopleura in the 1950’s
 Increased interspecific (between species) competition especially
 Competition for spawning sites which limited population recruitment
 Prior to the introductions the 2 indigenous species were spatially segregated
 Smaller O. variabilis found more inshore than O. esculentus
 T. zillii and O. leucostictus established and occupied same habitat as O. variabilis
 Direct competition for nursery grounds between T. zillii and O. variabilis
 Competition for breeding grounds between adult O. niloticus and O. variabilis
 Subsequent decline at two points in life history
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As well as competition
Hybridisation
 The restructuring of tilapiine communities was also aided by increased genetic dilution
through hybridisation
 Hybrids occurred between O. variabilis and O. niloticus
 O. niloticus and O. esculentus
 T. zillii and T. melanopleura
 In crosses involving O. niloticus the morphological characteristics of hybrids were
dominated by features of O. niloticus
 Probable that over time stocks lumped together as O. niloticus will in fact be a mixed
population of hybrids
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However these are not what L. Victoria is famous for that is the introduction of the Nile
Perch (Lates spp.) a predatory fish
Predation
 L. niloticus, L. macrophthalmus, L. longispinis were added (although the exact difference
between these species is under question and they may be hybrids of one species, genetic
work be underway)
 Numerous predators already existed in lake before the intro of Lates spp.
 e.g. Protopterus sp. lungfish made up >40% of trawls upto 1984
 Barbus sp.), Bagrus sp. , Schilbe sp. , Clarias sp. all catfish and some Haplochromis spp.
 Since 1985 food fish catches have fallen and remained low
 Combined pressure exerted by these species on stocks in lakes not considered significant
as they are only part time predators and no problems in the past
 Finger pointed at Lates spp.
 Lates spp.
 It was introduced in the early 1960’s
 Numbers rose only significantly from late 1970’s but different parts of lake different
numbers (its so big)
 In fact role of Lates in decline of commercially caught fish in L. Victoria appears less
significant than always assumed because
 Even though most fish sp. In lake have been found in stomachs of the piscivorous stages
of Lates
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University of Liverpool
Lates is a generalist predator that selects most abundant prey not greater than 1/3 of its
length
Now as Lates became fully established only when traditional food fish species had
already declined to only 15% of total ichthyomass
It seems unlikely that Lates would search out relatively rare food fish (15%) when
haplochromids were dominant (85%)
However there appears much evidence that Lates does have significant impact on
haplochromid populations
Studies have found they constitiuted 80% of weight of food of Lates
Mirrored by a decline in trawl catches of haplochromids following establishment and rise
in populations of Lates
This links well with the fact that Lates prefer to feed on the most abundant prey
Lates vs. Fishing
 However this evidence on its own gives impression that Lates predation has been major
cause of decline of Haplochromis spp.
 It does not distinguish between effects of predation and fishing
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Establishment of trawl fishery in Southern lake in 1973 for moderate fish meal production
provided opportunity to separate out the impact by predation and exploitation by humans
1978, 5 years after the introduction of industrial trawl fishing, larger haplochromids
became depleted and catch rates were low
Depleted from 1750 kg ha-1 in 1976 to 680 kg ha-1 in 1985
Two years later in 1987 trawlable haplochromids extremely rare
In North no commercial trawling was established
Experimental trawl catch rates in North declined only slightly after a period of 10 years
from 670 kg ha-1 in 1970 to 549 kg ha-1 in 1981
Lates was fully established in North Lake in 1975 but much later in the South (1982)
However the trawl catch data suggests exploitation by man was more intensive and
damaging to haplochromid stocks in South than predation by Lates
Lates has been blamed for all deleterious changes for years
So it shows how important it is to look at all aspects of a fishery especially the timing of
events (canal survey day one and two different numbers so important is timing of last
survey)
Also the relative size of the system being studied (It took many years for Lates to
become established in the South it is not just a big lake found in UK its bigger than
England often difficult to appreciate the size of something like this)
Present Status
 The lake is now dominated by three fish species
1, Lates spp. _ introduced predator
2, O. niloticus – an exotic herbivore
3, Rastrineobola argentea – indigenous zooplanktivore
Lates spp. Originally ‘Nile Perch’ current taxonomic status uncertain
 Very fecund (16 million eggs in one breeding cycle)
 Can feed at different trophic levels at each stage in its life history
 Adaptable feeding behaviour able to switch to most abundant prey
 In piscivorous stage R. argentea, O. niloticus and juvenile Lates constitute major prey
species
 This situation appears to restrain overpopulation of Lates beyond available prey sources
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Oreochromis niloticus - Tilapia
 Fecund, fast growth rate, wide food spectrum, flexible habitat selection
 Only sp. now feeding on primary productivity
 Filling trophic positions previously occupied by several original tilapiine and
haplochromine herbivores and detritivores
 Sound ecological base to sustain stocks to prevent phytoplankton dominance and
eutrophication
Rastrineobola argentea ‘Dagaa’ or ‘Omena’ – actually a cyprinid
 Indigenous zooplanktivore – r-selected life history
 Rise in abundance closely associated with decline in haplochromine stocks
 Species now occupies wide ecological range previously dominated by many
haplochromine species
 Has become the main trophic link between secondary production (eg Zooplankton) and
tertiary levels (Lates )
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The decrease in diversity of native species has been accompanied by tremendous increase
in landings of these three species
From 42000 tons 1970 to 120000 tons 1990
If consider these species as the new fishery for L. Victoria then can see that unlikely that
they would be affected by competition/hybridisation among themselves
Human exploitation and possibly predation OR environmental changes are the main
processes that could influence the fishery in future
As seen predation by Lates appears unlikely to be limiting fishery as stocks of O.
niloticus and R. argentea increased despite increase in Lates
But can this current catch level and species composition be maintained in the long term?
Current fishing practices use uncontrolled mesh sizes
Definite signs of decrease in Lates landings both in terms of weight and individual size
The Future
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Future is therefore uncertain if current practices continue
There is increasing evidence that the lake is suffering from eutrophication because of
agriculture, deforestation and population pressure, although it is possible that biotic
interactions are increasing apparent eutrophication through ecosystem changes.
Fishery biologists and ecologists need to be listened to at an equal or higher level than
commerce and politics for even this altered, ‘artificial’ ecosystem to be sustained
Further Information
Can be obtained from the course website at :
http://pcwww.liv.ac.uk/aquabiol/BIOL468/
including specific references plus links to additional websites with detailed up to date
information. Cichlid fish and their evolution have attracted a lot of attention resulting in many
superb resources to which links are provided on the site.
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