Lake Victoria fisheries: the Kenyan reality and environmental implications

Synopsis The introduction of the Nile perch into Lake Victoria has dramatically altered the fishery in that lake and contributed to the decline of the fishery for indigenous tilapias. One sector of the fishery in Lake Victoria has benefitted from the Nile perch introduction, although catches have declined in recent years. Inefficient enforcement of fisheries regulations has had a detrimental effect on indigenous species but may also have contributed to the recent decline in Nile perch catches. Fisheries development plans have tended to favour capital-intensive fisheries and to ignore small scale subsistence fisheries. A case study at Wichlum Beach on the Kenyan shores of Lake Victoria has revealed the efficiency of traditional fishing and fish drying methods as well as the high ecological costs of the practice of kiln-drying Nile perch. Forty-five tons of firewood are used per month at Wichlum Beach alone for kiln-drying perch. The increased economic viability of the fishery has attracted professionals into the industry and resulted in the development of an export-oriented trade. The Yala Swamp adjacent to Lake Victoria has been extensively drained as part of a large land reclamation scheme and more draining is planned. Increased environmental awareness in Kenya, and Kenya's membership in the Convention on Wetlands of International Importance, has resulted in a critical review of these plans.     

The tilapiine fish stock of Lake Victoria before and after the Nile perch upsurge

Since the beginning of fisheries in Lake Victoria, two native tilapiine species, Oreochromis esculentus and Oreochromis variabilis , were the main target of the local fishermen. A continuous increase in fishing pressure led initially to a declining catch per unit of effort, and a smaller average fish size; eventually, there was a reduced landing of tilapiines. To boost the fisheries, three alien tilapiine species and the Nile perch Lates niloticus were introduced. Thirty years after its introduction, Oreochromis niloticus appeared to be the most successful tilapiine species. It replaced the indigenous tilapiines almost completely before the Nile perch came to dominate the ecosystem of Lake Victoria. Reduced fishing pressure on the tilapiines in the 1980s, due to the shift of the local fishery towards the Nile perch, resulted in an increase in the stock of O. niloticus and an increase in average fish size. Subsequently, the total mass of O. niloticus landed increased. The stocks of the indigenous tilapiines did not recover but declined to extremely low levels, or vanished from the main lake. Currently, these species still occur in satellite lakes of Lake Victoria, from which O. niloticus is absent. Nile perch feed on O. niloticus; however, the limited overlap in distribution between piscivorous Nile perch and O. niloticus of consumable sizes is probably an important factor in explaining the coexistence of the two species. The main cause of the disappearance of the native tilapiine species is presumed to be competitive dominance by O. niloticus .     

The evolution of the fishery of Oreochromis niloticus (Pisces: Cichlidae) in Lake Victoria

The Lake Victoria ecosystem has experienced changes associated with fishing levels, a rise in lake level in the 1960s, fish introductions, and human activities in the drainage basin. Following the fish introductions of the 1950s and early 1960s, Oreochromis niloticus has become the most abundant and commercially important species among the tilapiines, and the only species which has managed to co-exist with the Nile perch in Lakes Victoria and Kyoga. There is, however, little published information on the biology and ecology of the specie in the new habitats. It has therefore been found necessary to initiate studies on the characteristics of O. niloticus in Lake Victoria.     

The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria

The piscivorous Nile perch was introduced into Lake Victoria some 30 years ago, since when it has completely transformed the fishing industry and the species composition of the fish fauna of the lake. The original multispecies fishery, based mostly on cichlids (haplochromines, tilapias), cyprinids ( Barbus, Labeo, Rastrineobola ) and siluroids ( Bagrus, Clarias, Synodontis, Schilbe ), has changed dramatically to one based on three species: the introduced Nile perch, the cyprinids, Rastrineobola argenrea (Pellegrin), and the introduced Nile tilapia, Oreochromis niloticus (Linnaeus).
Within 25 years of its introduction the Nile perch became ubiquitous and now occurs in virtually every habitat with the exception of swamps and affluent rivers. It has preyed on all other species with profound effects, especially on the stocks of haplochromines. These originally comprised 80% of the total fish biomass in Lake Victoria, but have now decreased to less than 1% offish catches from the Kenyan waters of the lake. The fishermen of Lake Victoria have adjusted to this ecological crisis by using large-meshed nets to catch Nile perch, which has become the most important commercial species. For the first time in the history of Lake Victoria, fish fillets are now being exported to several overseas countries: the fillets are all from Nile perch.     

The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria

Synopsis The Lake Victoria fish fauna included an endemic cichlid flock of more than 300 species. To boost fisheries, Nile perch (Lates sp.) was introduced into the lake in the 1950s. In the early 1980s an explosive increase of this predator was observed. Simultaneously, catches of haplochromines decreased. This paper describes the species composition of haplochromines in a research area in the Mwanza Gulf of Lake Victoria prior to the Nile perch upsurge. The decline of the haplochromines as a group and the decline of the number of species in various habitats in the Mwanza Gulf was monitored between 1979 and 1990. Of the 123+ species originally caught at a series of sampling stations ca. 80 had disappeared from the catches after 1986. In deepwater regions and in sub-littoral regions haplochromine catches decreased to virtually zero after the Nile perch boom. Haplochromines were still caught in the littoral regions where Nile perch densities were lower. However, a considerable decrease of species occurred in these regions too. It is expected that a remnant of the original haplochromine fauna will survive in the littoral region of the lake. Extrapolation of the data of the Mwanza Gulf to the entire lake would imply that approximately 200 of the 300+ endemic haplochromine species have already disappeared, or are threatened with extinction. Although fishing had an impact on the haplochromine stocks, the main cause of their decline was predation by Nile perch. The speed of decline differed between species and appeared to depend on their abundance and size, and on the degree of habitat overlap with Nile perch. Since the Nile perch upsurge, the food web of Lake Victoria has changed considerably and the total yield of the fishery has increased three to four times. Dramatic declines of native species have also been observed in other lakes as a result of the introduction of alien predators. However, such data concern less speciose communities and, in most cases, the actual process of extinction has not been monitored.     

Genetic affinities of an introduced predator: Nile perch in Lake Victoria, East Africa

Although the introduction of Nile perch, Lates niloticus , to Lake Victoria has received intense global attention, especially in relation to its impact on endemic cichlid species and on fishery yields, fundamental information on its taxonomy and population genetics is lacking. Most importantly, the introduced fish originated from two lakes (Lakes Albert and Turkana) containing three Lates species, and it has never been entirely clear which of these became established in Lake Victoria, or indeed whether the Lake Victoria population is derived from hybridization between Lates species. In addition, genetic drift caused by the relatively small founder population (≈ 400), the initially slow population increase followed by a period of explosive population growth, and selection pressures in the new environment may have resulted in substantial genetic changes. Allozyme data indicated that the introduced Nile perch of Lake Victoria were mainly L. niloticus from Lake Albert, although maximum likelihood estimates of stock contributions (GSI) suggested the presence of L. macrophthalmus. In contrast, introduced Nile perch in adjacent smaller lakes (Lakes Kyoga and Nabugabo) appeared to be entirely L. niloticus . The effect of the introductions on allozyme diversity varied among lakes and appeared to be uncorrelated to the number of fish introduced.     

Impact of the Nile perch on the fisheries of Lakes Victoria and Kyoga

The combined effects of lack of effective management, over-exploitation with destructive fishing gear and interspecific competition, particularly among tilapiines have had profound effects on the fish stocks of lakes Victoria and Kyoga. It has been proposed that these have been more important in the decline of the indigenous fisheries than predation or competition from Nile perch.     

Human-induced changes in the composition of fish communities in the African Great Lakes

Summary Large natural African lakes contain unique and diverse fish faunas which have evolved within each lake in a comparatively short period of time. members of the family Cichlidae are particularly diverse, although there is strong evidence to show that the haplochromines in Lake Victoria, and possibly Lake Malawi, are monophyletic. The unique faunas in Lakes Victoria and Kyoga have been subject to perturbations from the introduction of exotic fish, and the faunas in these and other lakes have been disturbed by fishing activities and other human endeavours.Factors governing the establishment of exotic species are not clearly understood. The exotic fish must be physiologically adapted to their new environment, able to compete successfully both for habitat and for food at each stage of their life history, able to avoid predation and must have a suitable reproductive potential. Although about 50 species of fish have been introduced into African inland waters, including reservoirs, only comparatively few, in particular Nile perch (Lates niloticus), various cichlids (especially tilapias) and clupeids (Limnothrissa miodon), have been successful in establishing themselves. Those that have become established have had obvious but unquantifiable impacts on the indigenous faunas.It is difficult to differentiate between the effects of fishing and of the presence of alien fish on the fish species composition of the lakes (Witte et al., 1992). Many of the lakes were overfished before introductions were made, with a resultant decline in some species, especially the larger ones, and the virtual disappearance of others. Some lake fish faunas, such as those of Lakes Kyoga and Victoria, which have been subjected to the perturbations described above, continue to change rapidly (Ogutu-Owayo, 1990b).There is a fundamental need to collect biological information on the fish communities of African lakes for effective management, resulting not only in the conservation of unique fish faunas but also the production of sustainable fish yields for the people relying on this source of protein. This information is required before any more introductions of exotic fish are made.     

Changes in the prey ingested and the variations in the Nile perch and other fish stocks of Lake Kyoga and the northern waters of Lake Victoria (Uganda)

The fish stocks of Lakes Kyoga and Victoria have changed since Nile perch, Lates niloticus (L.), was introduced, and this is reflected in the prey ingested by the predator. Initially, haplochromine cichlids constituted the main prey of most sizes of Nile perch. As the stocks of these have declined, Caridina nilotica (Roux) and Anisopteran nymphs have become the dominant food of the juveniles, while Rastrineobola argentea (Pellegrin), juvenile Nile perch and Oreochromis niloticus (L.) have become the main food of larger Nile perch. Apart from R. argentea , most of the native fish species of these lakes have disappeared. The stocks of Nile perch in Lake Kyoga, to which it was introduced earlier than to Lake Victoria, have declined after dominating the fishery since 1965. and have been superseded by O. niloricus . an introduced herbivore. Similar changes are now occurring in Lake Victoria. The Nile perch might not maintain the high yield realized in the two lakes when haplochromines were abundant. It is therefore necessary to exercise caution with high and long-term investments aimed specifically at developing the Nile perch fishery.     

The decline of the native fishes of lakes Victoria and Kyoga (East Africa) and the impact of introduced species,especially the Nile perch,Lates niloticus,and the Nile tilapia,Oreochromis niloticus

Synopsis There has been a decline, and in some cases an almost total disappearance, of many of the native fish species of lakes Victoria and Kyoga in East Africa since the development of the fisheries of these lakes was initiated at the beginning of this century. The Nile perch, Lates niloticus, a large, voracious predator which was introduced into these lakes about the middle of the century along with several tilapiine species, is thought to have caused the reduction in the stocks of several species. But overfishing and competition between different species also appear to have contributed to this decline. By the time the Nile perch had become well established, stocks of the native tilapiine species had already been reduced by overfishing. The Labeo victorianus fishery had also deteriorated following intensive gillnetting of gravid individuals on breeding migrations. L. niloticus is, however, capable of preying on the species which haven been overfished and could have prevented their stocks from recovering from overfishing. L. niloticus is also directly responsible for the decline in populations of haplochromine cichlids which were abundant in these lakes before the Nile perch became established. Even without predation by Nile perch, it has been shown that the haplochromine cichlids could not have withstood heavy commercial exploitation if a trawl fishery had been established throughout Lake Victoria. Their utilisation for human food has also posed some problems. The abundance of the native tilapiine species may also have been reduced through competition with introduced species which have similar ecological requirements. At present, the Nile perch and one of the introduced tilapiine species, Oreochromis niloticus, form the basis of the fisheries of lakes Victoria and Kyoga.Invited editorial     

The Lake Victoria environment: Its fisheries and wetlands — a review

Recent ecological changes in the Lake Victoria ecosystem have been attributed to the effects of species stockings and, in particular, from predation pressure by the Nile perch. Evidence for the decline of haplochromines due to predation by the Nile perch, while overwhelming, does not necessarily account for these gross ecological changes. Ecological theory predicts that natural fluctuation would occur in a fisheries where predator and prey species could alternate in abundance. The absence of a substantial recovery of the endemic species within cyclical abundance patterns in Lake Victoria (even though they could be delayed), particularly in the pelagic and profundal zones, points to other causal factors.Ecological changes have occurred in Lake Victoria since the turn of the century when modern fishing methods and techniques were introduced. As the human population increased and the catchment became more exposed to diverse socio-economic activities, further pressure on the ecological functioning of the lake was compounded by exotic species stockings. Thus, declining fisheries, wetland degradation and eutrophication are part of gross environmental changes that are likely to become more manifest with the increasing impact of the water hyacinth. It therefore seems appropriate to consider the ecological changes at both temporal and spatial scales and to re-examine some of the paradigms for ecological change.Although cataclysmic impacts may have occurred between the 1960's and the early 1980's, the basic cause of ecological changes, at least in the inshore zone, appears to result from human activities partly associated with a degradation of the riparian wetlands that exposes the aquatic ecosystem to catchment activities. Overexploitation and a reduction in habitat quality and quantity in this zone could be major factors in the ecological transformations. However, the importance of the littoral zone and the ecological impact of the Nile tilapia, another stocked species, are hardly known. An additional hypothesis is therefore put forward to determine whether or not wetland vegetation types in Lake Victoria are important fish habitats. This hypothesis allows for testing several interrelated sub-hypotheses about the relationships between wetland vegetation and the ecology of Nile tilapia. The water hyacinth, a recent feature of the shoreline and an additional strain to the already endangered buffer zone, may have to be regarded as a dynamic, but permanent, part of most of the littoral zone.Corresponding Editor: Prof. P. Denny     

Effects of resources and mortality on the growth and reproduction of Nile perch in Lake Victoria

1. A collapse of Nile perch stocks of Lake Victoria could affect up to 30 million people. Furthermore, changes in Nile perch population size‐structure and stocks make the threat of collapse imminent. However, whether eutrophication or fishing will be the bane of Nile perch is still debated. 2. Here, we attempt to unravel how changes in food resources, a side effect of eutrophication, and fishing mortality determine fish population growth and size structures. We parameterised a physiologically structured model to Nile perch, analysed the influence of ontogenetic diet shifts and relative resource abundances on existence boundaries of Nile perch and described the populations on either side of these boundaries. 3. Our results showed that ignoring ontogenetic diet shifts can lead to over‐estimating the maximum sustainable mortality of a fish population. Size distributions can be indicators of processes driving population dynamics. However, the vulnerability of stocks to fishing mortality is dependent on its environment and is not always reflected in size distributions. 4. We suggest that the ecosystem, instead of populations, should be used to monitor long‐term effects of human impact.     

Nile perch and the transformation of Lake Victoria

The transformation of Lake Victoria that began in 1980 followed the population explosion of Nile perch Lates niloticus, causing the apparent extirpation of 500+ endemic haplochromine species and dramatic physico-chemical changes. Officially introduced in 1962–1963, but present earlier, the reasons for the long delay before its population exploded are discussed. The hypothesis that it occurred only after the haplochromine decline is evaluated, but haplochromines declined only after the Nile perch expansion began. The sudden eutrophication of the lake was attributed to Nile perch, but evidence of eutrophication from 1950 onwards led some researchers to conclude that it was the result of climatic changes. We conclude that the haplochromine destruction disrupted the complex food webs that existed prior to the upsurge of Nile perch. The depletion of fish biomass by Nile perch may have been the source of extra phosphorus responsible for the eutrophication of the lake. After the Nile perch explosion in 1980 the fish population came to be dominated by only three species, but fisheries productivity increased at least 10-fold. Fishing has caused demographic changes in Nile perch, which may have allowed some haplochromine species to recover. The condition of the lake appears to have stabilised since 2000, partly because the fish biomass has risen to at least 2 × 10⁶ t, replacing the ‘lost’ biomass and restoring some ecosystem functioning.     

Trophic shifts in predatory catfishes following the introduction of Nile perch into Lake Victoria

This study looked for evidence of trophic shifts in the diet of two predatory catfishes ( Bagrus docmac and Schilbe intermedius ) following the establishment of introduced Nile perch ( Lates niloticus ) into lakes of the Lake Victoria basin. Bagrus docmac exhibited a shift from a primarily piscivorous diet dominated by haplochromine cichlids to a broader diet that included a significant proportion of invertebrates and the cyprinid fish, Rastrineobola argentea , which became abundant following depletion of the haplochromines. Schilbe intermedius exhibited a trophic shift from a piscivorous diet dominated by haplochromines to an insectivorous diet. The flexibility in diet exhibited by these two catfishes may have permitted these species to persist, albeit in reduced numbers, subsequent to the introduction of Nile perch and may facilitate resurgence as fishing pressure reduces numbers of large Nile perch.     

Small pelagic fish assemblages in relation to environmental regimes in the Central Mediterranean

Stable isotope analyses and derived population-level metrics were used to quantitatively analyse spatial and seasonal heterogeneity in the fish trophic dynamics in relation to environmental variables in Mwanza Gulf, Lake Victoria (Tanzania). The fish community in Lake Victoria, including the top predator Nile perch, is generally omnivorous with a heavy reliance on invertebrates. This is in contrast to findings based on stomach content analyses of Nile perch, which showed a stronger reliance on fish. We tested two hypotheses: (1) during the rainy seasons multiple carbon sources influence the food-web structure inside the Gulf, leading to increased carbon ranges and trophic diversity. (2) During dry periods, the food-web structure mainly relies on pelagic primary production, reducing carbon ranges and trophic diversity. Carbon sources indeed varied seasonally and spatially, affecting the fish community at the highest trophic levels. With the onset of rains, carbon sources became spatially highly differentiated with enriched δ¹³C values of fish in shallow water inside the Gulf and depleted δ¹³C values in open waters. Metrics associated with niche size correlated significantly with seasonally varying environmental variables, while δ¹³C ranges correlated with spatially varying environmental variables.     

Little evidence for morphological change in a resilient endemic species following the introduction of a novel predator

Human activities, such as species introductions, are dramatically and rapidly altering natural ecological processes and often result in novel selection regimes. To date, we still have a limited understanding of the extent to which such anthropogenic selection may be driving contemporary phenotypic change in natural populations. Here, we test whether the introduction of the piscivorous Nile perch, Lates niloticus, into East Africa's Lake Victoria and nearby lakes coincided with morphological change in one resilient native prey species, the cyprinid fish Rastrineobola argentea. Drawing on prior ecomorphological research, we predicted that this novel predator would select for increased allocation to the caudal region in R. argentea to enhance burst‐swimming performance and hence escape ability. To test this prediction, we compared body morphology of R. argentea across space (nine Ugandan lakes differing in Nile perch invasion history) and through time (before and after establishment of Nile perch in Lake Victoria). Spatial comparisons of contemporary populations only partially supported our predictions, with R. argentea from some invaded lakes having larger caudal regions and smaller heads compared to R. argentea from uninvaded lakes. There was no clear evidence of predator‐associated change in body shape over time in Lake Victoria. We conclude that R. argentea have not responded to the presence of Nile perch with consistent morphological changes and that other factors are driving observed patterns of body shape variation in R. argentea.     

Was Lates Late? A Null Model for the Nile Perch Boom in Lake Victoria

Nile perch (Lates niloticus) suddenly invaded Lake Victoria between 1979 and 1987, 25 years after its introduction in the Ugandan side of the lake. Nile perch then replaced the native fish diversity and irreversibly altered the ecosystem and its role to lakeshore societies: it is now a prised export product that supports millions of livelihoods. The delay in the Nile perch boom led to a hunt for triggers of the sudden boom and generated several hypotheses regarding its growth at low abundances – all hypotheses having important implications for the management of Nile perch stocks. We use logistic growth as a parsimonious null model to predict when the Nile perch invasion should have been expected, given its growth rate, initial stock size and introduction year. We find the first exponential growth phase can explain the timing of the perch boom at the scale of Lake Victoria, suggesting that complex mechanisms are not necessary to explain the Nile perch invasion or its timing. However, the boom started in Kenya before Uganda, indicating perhaps that Allee effects act at smaller scales than that of the whole Lake. The Nile perch invasion of other lakes indicates that habitat differences may also have an effect on invasion success. Our results suggest there is probably no single management strategy applicable to the whole lake that would lead to both efficient and sustainable exploitation of its resources.     

Nile perch (Lates niloticus,L.) and cichlids (Haplochromis spp.) in Lake Victoria: could prey mortality promote invasion of its predator?

The invasion of Nile perch into Lake Victoria is one of the iconic examples of the destructive effect of an introduced species on an ecosystem but no convincing explanation exists of why Nile perch only increased dramatically after a 25 year lag. Here, we consider this problem using a mathematical model that takes into account interactions between Nile perch and its cichlid prey. We examined competing hypotheses to explain Nile perch invasion and show that suppression of juvenile Nile perch by cichlids may cause the system to have two alternative stable states: one with only cichlids and one with coexistence of cichlids and Nile perch. Without cichlid predation on Nile perch, alternative stable states did not occur. Our analysis indicates that cichlid mortality, for example fishing mortality, may have induced the observed shift between the states.     

Yellow perch genetic structure and habitat use among connected habitats in eastern Lake Michigan

Maintenance of genetic and phenotypic diversity is widely recognized as an important conservation priority, yet managers often lack basic information about spatial patterns of population structure and its relationship with habitat heterogeneity and species movement within it. To address this knowledge gap, we focused on the economically and ecologically prominent yellow perch (Perca flavescens). In the Lake Michigan basin, yellow perch reside in nearshore Lake Michigan, including drowned river mouths (DRMs)—protected, lake‐like habitats that link tributaries to Lake Michigan. The goal of this study was to examine the extent that population structure is associated with Great Lakes connected habitats (i.e., DRMs) in a mobile fish species using yellow perch as a model. Specifically, we tested whether DRMs and eastern Lake Michigan constitute distinct genetic stocks of yellow perch, and if so, whether those stocks migrate between the two connected habitats throughout the year. To do so, we genotyped yellow perch at 14 microsatellite loci collected from 10 DRMs in both deep and littoral habitats during spring, summer, and autumn and two nearshore sites in Lake Michigan (spring and autumn) during 2015–2016 and supplemented our sampling with fish collected in 2013. We found that yellow perch from littoral‐DRM habitats were genetically distinct from fish captured in nearshore Lake Michigan. Our data also suggested that Lake Michigan yellow perch likely use deep‐DRM habitats during autumn. Further, we found genetic structuring among DRMs. These patterns support hypotheses of fishery managers that yellow perch seasonally migrate to and from Lake Michigan, yet, interestingly, these fish do not appear to interbreed with littoral fish despite occupying the same DRM. We recommend that fisheries managers account for this complex population structure and movement when setting fishing regulations and assessing the effects of harvest in Lake Michigan.