Hypoxia Tolerance in Twelve Species of East African Cichlids: Potential for Low Oxygen Refugia in

The speciesflock of haplochromine cichlidfishes 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 species have vanished, andpredation by the introduced Nile perch (Lates niloticus) is thought to be one of the most significant factors underlying this mass extinction. Information on the hypoxia tolerance of haplo- chromines from Lake Victoria is valuable for predicting their response to the increasing anoxia within the lake and in evaluating theirpotential use of low oxygen regions as refugia from predation by introduced Nile perch. This study examines the response of nine cichlid species from Lake Victoria (eight indigenous, one in- troduced) and three cichlid species from Lake Tanganyika to different low-oxygen regimes under laboratory conditions. Fish were exposed to progressive and acute hypoxia, with and without access to the surface. All species used aquatic surface respiration at very low P02. Buccal bubble holding and active swimming at the surface during aquatic surface respiration were used by many species and may serve to increase its effi- ciency. Lacustrine cichlids endemic to Lake Victoria were more tolerant of hypoxia than ecologically similar species from Lake Tanganyika. The two species examined that are widespread in a variety of aquatic habitats exhibited a relatively high tolerance to hypoxia, with well-developed aquatic surface respiration and bubble- holding capabilities and no loss of equilibrium during progressive hypoxia. Species strongly affected by recent changes in Lake Victoria were not consistently poorer in their hypoxia tolerance than less-affected species. But, two of the less-affected species are inhabitants of shallow, rocky habitats, an environment that may be both rich in oxygen and well 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 examined from Lake Victoria suggest that these species potentially could use low-oxygen refugia to escape Nile perch predation. Some species that are thought to have disappeared may currently inhabit low-oxygen refugia that have not been adequately sampled.