Tijs Goldschmidt

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.

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

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).