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

Abstract

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.

Keywords:

• Haplochromine cichlids
• detritivores
• zooplanktivores
• eutrophication
• fishery
• Nile perch

Acknowledgements

We thank our colleagues from the Haplochromis Ecology Survey Team (HEST), the Tanzania Fisheries Research Institute (TAFIRI) and the Freshwater Fisheries Training Institute at Nyegezi for support and co-operation during the fieldwork. We wish to thank the crews of the trawlers for their skilful labour. We are indebted to Kees Barel, Gertrude Namulemo and Mike Richardson for their comments on earlier drafts of the paper and to Martin Brittijn for making of the figures. The research of HEST was financially supported by The Netherlands Foundation for the Advancement of Tropical Research (WOTRO; grants W87-129, W87-161, W87-189, W84-282, W84-488), the Section for Research and Technology of The Netherlands Minister of Development Co-operation, the Schure Beijerinck-Popping Fonds, the van Tienhoven Stichting and by Yellow Springs Instruments.

Notes

*Also referred to as detritivores/phytoplanktivores (e.g. Witte and van Oijen, 1995).

*Data from 1987 were based on 32 (out of 69) hauls in which haplochromines were identified to the trophic level.