1 Institute of Evolutionary and Ecological Sciences, University of Leiden, P.O.Box 9516, 2300 RA Leiden, The Netherlands
2 Haplochromis Ecology Survey Team, P.O.Box 1866, Mwanza, Tanzania
3 Molecular Ecology and Evolution Group, Department of Biological Sciences, University of Hull, Hull HU6 &RX, U.K.

Habitat related species diversity in Lake Victoria cichlids: effects of water transparency and oxygen concentration

Pre-1980s, Lake Victoria used to have a diverse fish community, dominated by more than 500 haplochromine cichlid species. The diversity of the cichlid species in various habitats differed strikingly. The number of species was ca 1.5 times higher over sand than over adjacent mud bottoms of comparable depth. We surveyed environmental and biological parameters that may explain these differences. Many cichlids in sandy and rocky habitats spawn year-round. Whereas, spawning of most mud-dwelling species is strongly seasonal and correlates to periods with high water transparency. Mud bottoms in Lake Victoria are covered with a thick layer of organic ooze, which can easily stir-up. These layers are hypoxic, restrict light penetration close to the bottom, and seem unsuitable for making spawning pits. Oxygen concentration and water transparency are likely to have an impact on reproductive strategies. A high correlation was found between dissolved oxygen and egg size in a number of species, indicating that hypoxic environments may constrain survival of species that need large larvae. The high water transparency over sand bottoms implies a wide transmission spectrum that could explain the high species packing in this habitat. Finally, possibilities to spawn year-round in clear water over sand may promote niche segregation between species.
In the 1980s, the haplochromine stocks decreased dramatically as a result of Nile perch upsurge and concomitant ecological changes. Since a decline of the Nile perch in the 1990s, a recovery of some species has been observed. Preliminary results indicate that these recovering species adapted to murky water. Further conservation and rehabilitation of the cichlid diversity could be facilitated by a reduction of siltation and eutrophication caused by erosion and land use in the catchment.