Jan H. Wanink

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

SynopsisThe 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.

Biodiversity and Fishery Sustainability in the Lake Victoria Basin: An Unexpected Marriage?

Abstract Lake Victoria is Africas single most important source of inland fishery production. After it was initially fished down in the first half of the 20th century, Lake Victoria became home to a series of introduced food fishes, culminating in the eventual demographic dominance of the Nile perch, Lates niloticus. Simultaneously with the changes in fish stocks, Lake Victoria experienced dramatic changes in its ecology. The lake fishery during most of the 20th century was a multispecies fishery resting on a diverse lake ecosystem, in which native food fishes were targeted. The lake ended the century with a much more productive fishery, but one in which three species—two of them introduced—made up the majority of the catch. Although many fish stocks in Lake Victoria had declined before the expansion of the Nile perch population, a dramatic increase in the population size of Nile perch in the 1980s roughly coincided with the drastic decline or disappearance of many indigenous species. Now, two decades after the rise of Nile perch in Lake Victoria, this species has shown signs of being overfished, and some of the native species that were in retreat—or even thought extinct—are now reemerging. Data on the resurgence of the indigenous species suggest that heavy fishing of Nile perch may enhance biodiversity; this has spawned renewed interest in management options that promote both fishery sustainability and biodiversity conservation.

Species Extinction and Concomitant Ecological Changes in Lake Victoria

Lake Victorias fish fauna included a large endemic flock of 300+ haplochromine cichlid species. About two-thirds of these species have disappeared or are threatened with extinction. The main cause of this large extinction event is predation by Nile perch, an introduced predator. We describe the decline of the haplochromine species and demonstrate that the rate and sequence of their decline was determined by their relative abundance, their adult size and their habitat overlap with Nile perch. Many non-haplochromine species declined as well, but in contrast, stocks of the native pelagic cyprinid Rastrineobola argentea and the introduced Oreochromis niloticus increased. There are also indications of an increase in phytoplankton, macrophytes, prawns and benthic organisms. Many of these rapid changes in the ecosystem were probably effects of the increase of the Nile perch and the disappearance of the haplochromines. The original fish fauna included many primary and secondary consumers. Currently secondary and tertiary consumers dominate. The food web in the sub-littoral and offshore areas of the lake changed considerably due to the stock replacements.

Recovery of cichlid species in Lake Victoria: an examination of factors leading to differential extinction

234 Bottom trawl catches in the northern part of the Mwanza Gulf Fish landings of nocturnal light fishery in the Speke Gulf Discussion 236 Effects of predator decline Predictions about survival and recovery of haplochromines Potentials of comparative studies on closely related species Possible effects of the recovery of zooplanktivorous haplochromines

Species Distinction and the Biodiversity Crisis in Lake Victoria

Abstract Until the 1970s, the fish fauna of Lake Victoria in East Africa was dominated by about 500 endemic haplochromine cichlid species, which comprised about 80% of the demersal fish mass. The cichlids were extremely diverse ecologically; however, the small diversity in gross morphology and the presence of intraspecific variation made it difficult to distinguish among species. In the first half of the 1980s, the Nile perch Lates niloticus, an introduced predator, suddenly boomed and cichlids declined dramatically. During the same period eutrophication increased strongly. With the decline of Nile perch catches in the 1990s, the cichlids showed some recovery. These events have triggered many studies and debates. Disagreements about the severity and causes of the decline often stemmed from considering the cichlid flock as a single unit owing to the lack of proper taxonomic and ecological knowledge. By studying cichlid communities, trophic groups, and individual species, researchers uncovered differential ...

Prospects for the fishery on the small pelagic Rastrineobola argentea in Lake Victoria

The pelagic cyprinid dagaa plays a crucial role in the disrupted ecosystem of Lake Victoria. It is the main utilizer of zooplankton, a major prey for the introduced Nile perch and, after Nile perch, economically the second-most important species in the fishery. Light fishery for dagaa was started in the 1960s and boosted during the 1980s. In spite of an intensified exploitation by man, Nile perch and piscivorous birds, the dagaa population increased significantly. Spatial and temporal distribution patterns of dagaa and its potential predators restricted the harvestable fraction of the dagaa stock mainly to mature fish. An increase in recruitment to the reproducing part of the population and a reduction in generation time enhanced the prospects for a sustainable fishery. However, a recent increase in the use of mosquito seines forms a potential danger for the fishery, since dagaa seems to use the inshore waters as spawning areas and nurseries.

Differential decline and recovery of haplochromine trophic groups in the Mwanza Gulf of Lake Victoria

Lake Victoria had a fish fauna dominated by 500+ species of haplochromine cichlids that made up more than 80% of the fish mass. The five main trophic groups caught with bottom trawlers in the sub-littoral areas of the Mwanza Gulf were: detritivores, zooplanktivores, insectivores, molluscivores and piscivores. The detritivores (13+ species) formed the most important guild, making up 60–80% of the number of individuals, followed by the zooplanktivores (12+ species), which comprised 10–30%. In the 1980s the haplochromines from the sub-littoral and offshore areas (estimated at some 200 species) vanished almost completely. Commercial trawl fishery, the upsurge of the introduced Nile perch, and an increase of eutrophication were potential causes of this decline. In the 1990s, when Nile perch was heavily fished, a recovery of some haplochromine species was observed. We studied the decline and partial recovery of the different haplochromine trophic groups in the northern part of the Mwanza Gulf. The rate at which the trophic groups declined differed; the relatively large piscivores, insectivores and molluscivores were the first to disappear from the catches. The small detritivores and zooplanktivores declined at lower rates, especially the latter group. From the beginning of the 1990s a resurgence of both groups was observed. By 2001, the zooplanktivores had reached their previous level of abundance, but their diversity declined from more than 12 species to only three. Though four detritivorous species began being regularly caught again, they constituted only about 15% of the number of individuals, while the zooplanktivores made up more than 80%. The patterns of decline and recovery indicate that, though fishery played a role locally, predation by Nile perch and eutrophication were the main factors determining the fate of the haplochromines. However, it has so far been impossible to establish the causal relationship between the two, and the relative impact of each of these phenomena separately. The potential effects of the changed trophic dominance, and the importance of the haplochromines for the ecosystem and a sustainable fishery, are discussed.

Dynamics of the haplochromine cichlid fauna and other ecological changes in the Mwanza Gulf of Lake Victoria

Lake Victoria, the largest tropical lake in the world (Fig. 6.1), was until recently a typical cichlid lake. Eighty percent of the demersal ichthyomass of this East African lake consisted of haplochromine cichlids (Kudhongania and Cordone, 1974a, b). The more than 300 haplochromine species, of which 99% were endemic, exploited virtually all food sources in the lake (Greenwood, 1974, 1981; van Oijen et al., 1981; Witte and van Oijen, 1990). Each species had its own unique combination of food and habitat preferences (e.g. van Oijen, 1982; Goldschmidt et al., 1990). The haplochromines were only locally important to fisheries (Kudhongania and Cordone, 1974a). In the 1970s a small-scale trawl fishery on these fishes was developed in the southern part of the lake near Mwanza. At that time the mean catch rate was more than 1000 kg h-1 (Goudswaard and Ligtvoet, 1988; Barel et al., 1991; Witte et al., 1992b).

Dietary shift in benthivorous cichlids after the ecological changes in Lake Victoria

Cichlid fishes of Lake Victoria are well known for their trophic specializations. In our study area, the sub-littoral waters of the Mwanza Gulf (Tanzania), at least 12 trophic groups coexisted in the 1970s. After the Nile perch upsurge and eutrophication in the 1980s, most cichlids disappeared from the area. During the 1990s, heavy exploitation of Nile perch reduced the abundance of this predator. Subsequently, some cichlid species, mainly zooplanktivores and small benthivores, recovered. To establish their ecological role in the changed environment, among other factors we studied their diet. Stomach contents of fish caught before and after the ecological changes were compared. The small benthivores shifted from a diet dominated by detritus and phytoplankton, supplemented with small quantities of midge larvae and zooplankton, to a diet of invertebrate preys of larger sizes. Currently, the diet includes zooplankton, midge larvae, shrimps and molluscs, and a very small amount of detritus and phytoplankton. Apparently, the recovering benthivores have a more carnivorous than a herbivorous diet. Concomitantly, the ratio of intestine length to standard length of the benthivores decreased with 30% and the average stomach fullness decreased as well. The observed dietary shifts may be due to (1) the changed availability of food types and a decline of the nutritious quality of detritus and phytoplankton due to the strong increase of cyanobacteria in the lake, (2) the decline of the former trophic specialists, and (3) the reduced water clarity. The diet of the former detritivores is currently very similar to that of the zooplanktivores. Consequently, if diet plays a role in the relatively slow recovery of the small benthivores compared to the zooplanktivores, this would only hold if they are relatively poorly adapted to their new diet.

Effects of Nile perch (Lates niloticus) introduction into Lake Victoria, East Africa, on the diet of Pied Kingfishers (Ceryle rudis)

In recent years the ichthyofauna of Lake Victoria, the worlds largest tropical lake, has gone through dramatic changes. The population of Nile perch, a large predator which has been introduced into the lake by man, increased explosively at the expense of many haplochromine cichlid species. At the same time, numbers of a small cyprinid (dagaa) rose sharply.Previously Pied Kingfishers on Lake Victoria fed mainly on haplochromines. Only the youngest nestlings depended on dagaa as primary food. The current diet of adult birds clearly reflects the changes which have occurred in the fish community. Pellet analysis reveals a shift towards a diet composed of almost 100% dagaa.The change in prey species composition has increased the number of fish a kingfisher needs to catch daily in order to meet its energetic demands, because:(1)the mean size of haplochromines is larger than that of dagaa;(2)(2) the mean size of dagaa has decreased since the increase in Nile perch;(3)(3) the weight of dagaa is lower than that of haplochromines of equal size;(4)(4) mainly juvenile dagaa and adults in poor condition are accessible to kingfishers.