Murky water may be causing fish extinctions in Africa's Lake Victoria by making it impossible for female fish to recognize and mate with the brightly colored males. Ecologists say the finding, reported tomorrow in Science, explains why the lake has lost dozens of native fish species--and illustrates how human activities, such as logging and farming, can interfere with the sometimes subtle forms of animal communication that help maintain species diversity.
Lake Victoria--a 69,216-square-kilometer body of water that lies between Kenya, Uganda, and Tanzania--was once home to at least 500 species of small fish called haplochromine cichlids. Over the last 70 years, however, hundreds of these species have become extinct. Initially, scientists blamed the losses on predation by Nile perch--large, voracious fish introduced into the lake in the 1950s. In recent years, however, scientists noticed that even cichlid species that are rarely eaten by perch continued to become extinct.
Now, ecologists Ole Seehausen, Jacques J. M. van Alphen, and Frans Witte of the University of Leiden in the Netherlands have shown that Lake Victoria's increasingly turbid waters, caused by runoff from farm fields and logged forests, could be responsible for the loss of these cichlid species. In their study, the researchers demonstrated that, in aquariums, female cichlids use the distinct, bright colors displayed by male fish to recognize mates from within their own species. In the wild, such visual cues help cichlids remain genetically distinct by preventing interbreeding, which can occur in these closely related fishes.
In Lake Victoria, however, the researchers found evidence that turbid water interferes with the visual communication system. In shallow, rocky regions of the lake--where visibility has been cut in half over the last decade--the researchers discovered that murkier water supported fewer distinct, brightly colored species than did clearer water. For instance, they found just five distinct cichlid species in murky water that allowed the transmission of 100 nanometers of the light spectrum, but about 20 species in clearer water that allowed the transmission of 500 nanometers. The researchers believe the murky water "turns the lights off," making even the most brightly colored males appear drab and indistinct to females, leading to breeding mistakes, the loss of distinct species, and an increase in hybrids.
"The study is very elegant and demonstrates that the barriers that keep species separate are often quite delicate and vulnerable to environmental changes caused by people," says Les Kaufman of Boston University. But Ron Coleman of the University of California, Berkeley, wonders whether the researchers have overstated the impact that turbidity is having on the mating process. "These fish court up close," Coleman explains. "I think that once the female gets within 6 inches [15 centimeters], she would be able to realize, 'Hey, I've got the wrong guy.' "