Changes in abundance of Nile shrimp, Caridina nilotica (Roux) following the decline of Nile perch and recovery of native haplochromine fishes, Lake Victoria, Tanzanian waters
Keywords:
catch rates, fishing pressure, selectivity of fishery, Rastrineobola argentea fishery, bottom trawlsAbstract
The abundance of Nile shrimp, Caridina nilotica (Roux) in Lake Victoria has fluctuated significantly over time, from the periods of 1900–1950s, 1980s–1990s and from 2000s to present day. To elucidate its current abundance, its contribution to the Rastrineobola argentea fishery catches, and its importance to the composition of the diet of Nile perch (Lates niloticus L.), and to the bottom trawl catches, were investigated. From August 2006 to January 2007, the mean C. nilotica CPUE was 1.68 ± 1.20 kg boat−1 day−1 at Kijiweni and Igombe, and its 0.37% contribution to the R. argentea total catches was insignificant. Of the 230 Nile perch stomachs containing prey items, C. nilotica comprised 54% by volume of the diet of fish <50 cm TL; while haplochromines contributed 41% to the diet of fish >50 cm TL. Mean catch rates of C. nilotica in bottom trawls in 8 months between 2005 and 2008 ranged between 0 and 2.45 ± 2.50 kg hr−1. Low catches of the C. nilotica in the R. argentea fishery, and the dietary shift of Nile perch of >50 cm TL to once again include haplochromines, may indicate a decrease in C. nilotica's abundance in the lake, and vice versa. Overfishing and the selectivity of the fishery to take only large Nile perch for fish filleting factories, have resulted in reduced stocks and dominance of juveniles in the perch's populations. The highly reduced Nile perch stocks are currently leading to an apparent reversal of the 1980s regime with a shift to a new cichlid-dominated and C. nilotica low abundance state. Increase in predation on juvenile C. nilotica by the recovering haplochromines, and juvenile perches, as well as environmental degradation, especially eutrophication and pollution, along with the effects of global warming impacts, account for the observed decline in C. nilotica.
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