Cichlid species diversity in naturally and anthropogenically turbid habitats of Lake Victoria, East Africa

Frans Witte,Tijs Goldschmidt,Ole Seehausen,Mary A Kishe-Machumu,Marjolein Rensing,Jan H Wanink

doi:10.1007/s00027-012-0265-4

Abstract

During the past decades, major anthropogenic environmental changes occurred in Lake Victoria, including increased predation pressure due to Nile perch introduction, and decreases in water transparency and dissolved oxygen concentrations due to eutrophication. This resulted in a collapse of the haplochromine cichlids in the sub-littoral waters of the Mwanza Gulf in 1986–1990, followed by a recovery of some species in the 1990s and 2000s, when Nile perch densities declined. We studied two data sets: (1) haplochromines from sand and mud bottoms in the pre-collapse period; (2) haplochromines from sub-littoral areas during the pre-collapse, collapse and recovery periods. Water over mud is murkier and poorer in oxygen than water over sand, and differences in haplochromine communities in these natural habitats during the pre-collapse period may predict the effects of anthropogenic eutrophication during the collapse and recovery periods. In the pre-collapse period, haplochromine densities over sand and mud did not differ, but species richness over sand was 1.6 times higher than over mud bottoms. Orange- and white-blotched colour morphs were most common at the shallowest sand station. More specifically, insectivores and mollusc-shellers had higher numbers of species over sand than over mud, whereas for mollusc-crushers no difference was found. Laboratory experiments revealed that mollusc shelling was more affected by decreased light intensities than mollusc crushing. During the pre-collapse period, spawning occurred year-round in shallow areas with hard substrates and relatively clear water. In deeper areas with mud bottoms, spawning mainly occurred during months in which water clarity was high. No effects of hypoxia on spawning periods were found. It follows that clearer water seems to support differentiation in feeding techniques as well as year-round spawning, and both may facilitate species coexistence. Water clarity is also known to be important for mate choice. These observations may explain why, since the decline of Nile perch, haplochromine densities have recovered, the numbers of hybrids increased and species diversity in the current eutrophic sub-littoral waters has remained 70 % lower than before the environmental changes.

Highlights

In Lake Victoria, more than 500 haplochromine cichlid species, 99 % of which are endemic, made up some 80 % of the bottom dwelling fish mass until the beginning of the 1980s (Greenwood 1974; Kudhongania and Cordone 1974; Witte et al 2007a)
During the past decades, major anthropogenic environmental changes occurred in Lake Victoria, including increased predation pressure due to Nile perch introduction, and decreases in water transparency and dissolved oxygen concentrations due to eutrophication
Impressive the ability of some populations to cope with changing environments through phenotypic plasticity and/or rapid evolution may be, the sad conclusion that we have to draw from our multi-decadal work on Lake Victoria is that human impacts on the environment have led to a roughly 70 % loss of species and ecological diversity in the sub-littoral waters in just three decades

Summary

Introduction

In Lake Victoria, more than 500 haplochromine cichlid species, 99 % of which are endemic, made up some 80 % of the bottom dwelling fish mass until the beginning of the 1980s (Greenwood 1974; Kudhongania and Cordone 1974; Witte et al 2007a). During the 1980s, the majority of the haplochromine species in the sub-littoral (6–20 m deep) and offshore areas disappeared (Ogutu-Ohwayo 1990; Witte et al 1992) This disappearance coincided with the upsurge of the introduced predator, Nile perch, Lates niloticus (L.) (Goudswaard et al 2008), and with strong eutrophication of the lake. The latter resulted in increased algal blooms and decreases in both water transparency and dissolved oxygen concentrations (Kaufman 1992; Hecky et al 1994; Seehausen et al 1997a; Wanink et al 2001). The initial diversity of haplochromines has not fully recovered

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Conclusion