Evolutionary and Conservation Biology of Cichlid Fishes as Revealed by Faunal Remnants in Northern Lake Victoria

Abstract

Lake Victoria until recently harbored the world’s second richest lacustrine fish assemblage, but it is now experiencing a mass extinction. Here we report on the current status of the endemic cichlid fishes from a preliminary biotic inventory conducted as part of limnological reconnaissance in Kenyan and Ugandan waters during 1989–1992. Sixty‐two haplochromine taxa were recovered, half of them new. Some are local endemics, while others are more widely distributed, based on review of earlier collections. Comparison with data from 1982–1984 and 1988 suggests that some shoreline assemblages have been locally stable over the past decade. In addition to littoral rock and vegetation, four kinds of refugia for indigenous fishes are recognized: (1) schools of Rastrineobola, (2) benthic microbial mats in deep water, (3) satellite lakes, (4) the oxycline. Reconnaissance by remote operated vehicle (ROV) documented fish kills due to deoxygenation of the water column, indicating the risk of using the oxycline as a refugium. Several species formerly abundant in Lake Victoria were found only in satellite lakes, revealing the importance of these lakes in the conservation of indigenous species. Recent events in Lake Victoria shed light on mechanisms of speciation and mass extinction, insights relevant to conservation planning for the valuable remnants of the lake’s indigenous fauna.

Nile perch distribution in south-east Lake Victoria is more strongly driven by abiotic factors, than by prey densities

“…5). Such a threshold value seems very high as critical oxygen levels for Nile perch are much lower (i.e., are between 2.5 and 3 mg l -1 ) (Kaufman & Ochumba, 1993;Wanink et al, 2001). For fish of 20-40 cm, densities were even negatively related with oxygen.…”

Increase in Anoxia in Lake Victoria and Its Effects on the Fishery

“…Further, Kaufman and Ochumba (1993) using a remote operated vehicle in Lake Victoria found large numbers of dead or dying R. argentea below the oxycline. The authors suggested that rapid shifts in the oxycline may have caused the mass mortality.…”

“…Support for this 'hypoxic predation refugium' hypothesis came from exploratory dives by a remote operated vehicle, which filmed dagaa shoaling near the oxycline (Kaufman & Ochumba, 1993). Nile perch was in waters with a higher DO, but made short foraging grips into the hypoxic layer.…”

Interspecific Hybridization of Tilapiines in Lake Victoria, Kenya

“…Of the three satellites, L. Kanyaboli is the largest (10.5 km 2 ) and most remote from L. Victoria [39], with a depth of 2.5-4.5 m [40]. Indigenous tilapias, O. esculentus, O. variabilis and O. leucostictus, which have been extirpated from L. Victoria, form the main fishery of L. Kanyaboli [31,36,41]. However, the invasion of the lake by exotic O. niloticus could threaten the survival of the indigenous tilapias, and negatively affect the fishery and livelihoods of the local community.…”

“…Lake Victoria has a number of satellites: Kanyaboli, Namboyo and Sare, on the Kenyan side and Nabugabo on the Ugandan side. Satellite lakes have been recognized as important functional refugia for fish fauna, since they are connected to main lakes by extensive papyrus swamps that are anoxic to invasive predators like L. niloticus [29,31,[35][36][37]. Anoxic conditions prevent the natural entry of L. niloticus to these habitats, which are inhabited by indigenous fish fauna tolerant to low dissolved oxygen conditions, and this helps to conserve indigenous fish stocks (a valuable food resource for the local communities) from predation pressure, unless the invasive predator is introduced there by man.…”