HYPOXIA TOLERANCE IN 12 SPECIES OF EAST-AFRICAN CICHLIDS - POTENTIAL FOR LOW-OXYGEN REFUGIA IN LAKE-VICTORIA

The species flock of haplochromine cichlid fishes in Lake Victoria is one of the most extensive and recent radiations of vertebrates known. Over the past 15 years, however, many of the haplochromine cichlid spe cies have vanished, and predation by the introduced Nile perch (Lates niloticus) is thought to be one of the most significant factors underl ying this mass extinction. Information on the hypoxia tolerance of hap lochromines from Late Victoria is valuable for predicting their respon se to the increasing anoxia within the lake and in evaluating their po tential use of low oxygen regions as refugia from predation by introdu ced Nile perch. This study examines the response of nine cichlid speci es from Lake Victoria (eight indigenous, one introduced) and three cic hlid species from Late Tanganyika to different low-oxygen regimes unde r laboratory conditions. Fish were exposed to progressive and acute hy poxia, with and without access to the surface. All species used aquati c surface respiration at very low P-O2. Buccal bubble holding and acti ve swimming at the surface during aquatic surface respiration were use d by many species and may serve to increase its efficiency. Lacustrine cichlids endemic to Lake Victoria were more tolerant of hypoxia than ecologically similar species from Lake Tanganyika. The two species ex; unmined that are widespread in a variety of aquatic habitats exhibited a relatively high tolerance to hypoxia with well-developed aquatic su rface respiration and bubble-holding capabilities and no loss of equil ibrium during progressive hypoxia. Species strongly affected by recent changes in Lake Victoria were not consistently poorer in their hypoxi a tolerance than less-affected species. But, two of the less-affected species are inhabitants of shallow, rocky habitats, an environment tha t may be both rich in oxygen and roell defended against the Nile perch because of the structural complexity of the rocky, littoral area. The generally high levels of hypoxia tolerance in the cichlid species exa mined from Lake Victoria suggest that these species potentially could use low-oxygen refugia to escape Nile perch predation. Some species th at are thought to have disappeared may currently inhabit low-oxygen re fugia that have not been adequately sampled