Cichlid fish species of Lake Victoria can interbreed without loss of fertility but are sexually isolated by mate choice. Mate choice is determined on the basis of coloration, and strong assortative mating can quickly lead to sexual isolation of color morphs. Dull fish coloration, few color morphs, and low species diversity are found in areas that have become turbid as a result of recent eutrophication. By constraining color vision, turbidity interferes with mate choice, relaxes sexual selection, and blocks the mechanism of reproductive isolation. In this way, human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity.In the Great Lakes of Africa, large and diverse species flocks of cichlid fish have evolved rapidly (1, 2). Lake Victoria, the largest of these lakes, had until recently at least 500 species of haplochromine cichlids (3). They were ecologically so diverse that thev utilized almost all resources available to freshwater fishes in general (2), despite having evolved in perhaps as little as 12,400 years (I ) and from a single ancestral species (4). This s~ecies flock is the most notable . . example of vertebrate explosive evolution known today. Many of its species have vanished within two decades (5, 6 ) , which can only partly be explained by predation by the introduced Nile perch (Lates spp.). Stenotopic rock-dwelling cichlids, of which there are more than 200 species (7), are rarely eaten by Nile perch (8). Yet, many such species have disappeared in the past 10 years (6, 7). Because gene flow between island ~o~ulations of these cichlids is effectivelv iiGited by stretches of sand and mud botto4 (9), populations underlie local selection regimes. Here we demonstrate that increasing turbidity, by curbing the impact of sexual selection on sexual isolation, is responsible for the decline in cichlid diversity.The seven Great Lake basins of tro~ical Africa in which haplochromine cichlids formed endemic s~ecies flocks have distinctly clearer waters than the five in which they did not (300 to 2200 versus 20 to 130 cm maximum Secchi disc readings; n = 12, t = 2.99, P = 0.015). Significance increases when the three very large lakes Victoria, 'Malawi, and Tanganyika are excluded (n = 9, t = 3.83, P = 0.009), ruling out the alternative hypothesis that lake size explains the difference. Lake Victoria has rapidly eutrophied (10) and become turbid. Water transparency decreased in deep open waters from 5.5 to 8 m in the 1920s to 1.3 to 3 m in the 1990s and decreased in the littoral zone from 3 to 1.5 m within the past decade (I I). We have investigated the effect of these changes on the cichlids. Postmating reproductive barriers have not been found among Lake Victoria cichlids (12), presumably because of their phylogenetic youth. Reproductive isolation among sympatric species is maintained only by mate choice. Haplochromines have vibrantly colored males and usually cryptically colored females (2,7). Their eyes are equipped with three retinal cone pigments ...

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Articles Overfishing, environmental degradation, and redistribution of surface water have placed great stress on inland fisheries throughout the world. Human activities usually shift the balance among fish species, causing the extirpation of many indigenous species and the dominance of a reduced set of often introduced fish species. The result has been a massive reshaping of fish communities in the world's fresh waters over the past few centuries, with the pace of change quickening of late in the tropics.It has been known for some time that fishes react to environmental degradation and fishing pressures with a characteristic series of changes. If too much of the brood stock is caught, fewer and fewer recruits appear in the population in succeeding years. This is called recruitment overfishing. The impact is somewhat different if the large fish in a population are taken first, then smaller ones, and so on. The mean size of individuals drops, and there is selection for individuals that mature at a smaller and less fecund size. This is growth overfishing. Each of these phenomena has a counterpart corresponding to effects that become apparent when more than one species or stock is taken into consideration.Three decades ago, Regier and Loftus (1972) observed a multispecies analog to growth overfishing while they were researching the anthropogenic transformation of fish communities in the Great Lakes of North America. What they described-the successive removal of the largest-bodied species-was later generalized Henderson 1973, Welcomme 1995) and has been called the "fishing-down Uganda, Box 343, Jinja, process." Greater fishing pressure can initially bring about a higher catch, followed by a plateau over a range of increasing exploitation as component fish stocks are serially depleted. First large, and then successively smaller, species are removed and their places taken by even smaller and faster-growing ones, producing an illusion of sustained productivity that conceals deep changes in community and food web structure. Eventually there are no more economically exploitable stocks, and both the fishery and the fish community collapse or are changed beyond recognition (Welcomme 1995(Welcomme , 2003. John S. Balirwa is acting director of the Fisheries Resources Research Institute ofIn Africa, overfishing is a recurrent problem closely tied to environmental conditions. Africa has suffered food crises for decades, exacerbated in the Sahelian zones by prolonged drought through the late 1970s and the 1980s. When severe drought compromises production of livestock, rural communities turn to hunting and fishing to satisfy their protein needs. What worked for a long time when people were relatively few can have quite different impacts on wildlife at the currently very high human population densities. Thus, food crises, together with a political orientation of open access to wild resources such as fish, have led to a rapid increase in fishing pressure. This effect is further compounded by the rapid improvement and disseminat...

In the 1980s an explosive increase of the introduced Nile perch (Lates sp.; Harrison 1991) in Lake Victoria caused the destruction of approximately 65% of the endemic haplochromine cichlids. The eradication of approximately 200 vertebrate species in less than a decade may well represent the largest extinction event among vertebrates during this century. The introduction of the Nile perch can be considered as a large‐scale, albeit unintended, experiment. Farreaching changes in the food web are taking place. We present data on the importance of haplochromines in the ecosystem prior to the Nile perch boom. An analysis of the pelagic community in the sublittoral area of the Mwanza Gulf revealed that the phytoplanktivores formed 18% of the biomass of the total haplochromine community. In the benthic community, the detritivores that frequently included phytoplankton in their diet comprised 31% of the biomass. We examine the hypothesis that algal grazing was reduced by the disappearance of haplochromine phytoplanktivores and detritivores. The disappearance of these groups may have contributed to the recent algal blooms. To investigate this hypothesis we consider a second major change in the system. Coinciding with the decrease of the haplochromines, the number of atyid prawns, Caridina nilotica, has strongly increased. We present preliminary evidence that the stock of the detritivorous haplochromines that formerly comprised most of the demersal ichthyomass has (partially?) been replaced by the prawn. We discuss the possible mechanisms underlying this major change in the food web. With respect to the conservation of the Great African Lakes, tbe collapse of Lake Victoria’s ecosystem should serve as an example of how easily a complex ecosystem can be irreversibly destroyed.