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

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 precollapse, 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.

Two new species of zooplanktivorous haplochromine cichlids from Lake Victoria, Tanzania

“…However, since the early 1990s a gradual recovery has been observed for three of these species: Haplochromis pyrrhocephalus , Haplochromis laparogramma and Haplochromis tanaos (Seehausen et al 1997b, Witte et al 2000). Since 2005, these species are even more common than they used to be in the 1970s (Witte et al 2007a, 2012a, Kishe-Machumu 2012). In contrast, the other five species, including Haplochromis argens , remain rare or are absent.…”

“…All type material was collected between May 1975 and August 1986, before the collapse of the haplochromines in the Tanzanian part of the lake due to the Nile perch upsurge and eutrophication (e.g. Witte et al 1992a, b, 2012a; Seehausen et al 1997a; Wanink et al 2001; Goudswaard et al 2008). Haplochromis “argens” was collected in the Mwanza Gulf (between Nyamatala Island and Hippo Island, depth range about 6–16 m) and in the Emin Pasha Gulf (Fig.…”

Fast adaptive responses in the oral jaw of Lake Victoria cichlids

“…Concurrently, eutrophication resulted in lower dissolved oxygen levels (Hecky et al ; van Rijssel 2014) and a turbidity increase (Seehausen et al ; van Rijssel 2014). All these changes combined contributed to the decline of the haplochromine cichlid species and numbers (Witte et al , , ). The piscivores, molluscivores, and insectivores were the first to disappear during the early 1980s, whereas the detritivores showed a steep decline during that time and disappeared only after 1985/1986.…”

Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s ecosystem services

“…Indeed, 65-70% of both benthic and pelagic species dwelling over soft bottoms in the sublittoral waters (6-14 m deep) of the Mwanza Gulf seem to have been irreversibly lost (KisheMachumu 2012, Witte et al 2013) (Fig. 4).…”