Management Implications of the Response of Two Tilapiine Cichlids to Long-Term Changes in Lake Level, Allodiversity and Exploitation in an Equatorial Lake
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The tilapiine cichlids Oreochromis leucostictus and Tilapia zillii were introduced into Lake Naivasha, Kenya, in 1956. Previous studies on data collected to 1987 revealed they were persistent following establishment, despite environmental variability and exploitation. Recent data, however, suggest this persistence is under threat as data indicate some significant declines in aspects of their abundance since 1999. The influence of changes in lake level, allodiversity and fishing effort on this decline was tested and showed that a decline in lake level was a significant causal factor. The recent change in allodiversity, with the establishment and dominance of Cyprinus carpio in the fishery, was not significant on the catch per unit effort of O. leucostictus but was on T. zillii. Since 1999, catches of tilapiines in the fishery have been independent of fishing effort, contrary to between 1975 and 1987, suggesting their management through application of fishery models may no longer be applicable. As it was anthropogenic-mediated lake level changes that were mainly responsible for their decline, then lake management should focus on sustainable water utilization that maximizes lake levels in accordance with the basin-wide water balance.
KeywordsOreochromis leucostictus Tilapia zillii Catch per unit effort Fishery catch Environmental change
This work was conducted under the Kenya Government Research Permit NCST 5/002/R/020-D to D. M. Harper and was undertaken with strong support from the Fisheries Department, the Kenya Marine and Fisheries Research Institute and the Water Resources Management Authority. Our appreciation is also due to The British Council for a Commonwealth Scholarship award to D. Oyugi to facilitate part of this work. Sponsors of funding also include the Earthwatch Institute (to 2007), the Darwin Initiative (logistical support of vehicles) and the British Council DelPHE project “Field IT” (2009-11). The authors would also like to sincerely thank Vincent I. Kinyua and George N. Morara for providing fishery data, Dominic Wambua for lake level data, George E. Owiti for laboratory working space and the entire “Lakes, Wetlands and People of East Africa’s Rift Valley” field team for their support in the sampling expeditions.
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