The biology and abundance of three Cichlid species from the Kavango and Caprivi regions, Namibia
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Date
2012
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Abstract
River-floodplains are highly productive environments capable of supporting large fisheries. In the Kavango and Caprivi Regions in Namibia, artisanal gillnet fisheries catch is dominated by three cichlids: Oreochromis andersonii, Oreochromis macrochir and Tilapia rendalli. Their abundance is declining because of increased fishing pressure and their stocks are becoming vulnerable to overfishing. The aim of this thesis was to provide management recommendations for the three cichlids by investigating and comparing their age, growth, reproduction and relative abundance in Lake Liambezi and the Kavango, Kwando and Zambezi Rivers. These four systems present different scenarios whereby Lake Liambezi is a highly productive but heavily exploited area, the Zambezi River is considered heavily exploited and the samples from the Kavango and Kwando Rivers were collected from protected areas where exploitation was low. Fish community composition and relative abundance were assessed using experimental gillnet fleets in each system. Gillnet catch composition was similar between the three river systems but differed markedly in Lake Liambezi because of habitat differences. Schilbe intermedius dominated gillnet catches in the three river systems, while Brycinus lateralis was most abundant in Lake Liambezi. Cichlid abundance was low despite their importance in artisanal fisheries. Catch per unit effort (CPUE) for O. andersonii and O. macrochir was highest in the unexploited systems and lowest in the exploited systems. Tilapia rendalli CPUE was highest in Lake Liambezi and was lowest in the Zambezi River. Age and growth were estimated using sectioned sagittal otoliths. Growth zone deposition rate was validated as annual using edge analysis (EA) and occurred in summer between October and January. Initial growth rates for each species were fastest in exploited populations, and O. andersonii and O. macrochir attained larger sizes in the Zambezi River. While the three species attained older ages in protected areas, their growth rates were slower in these areas. This may be a consequence of intra-specific competition as well as of general system productivity. Age-at-50% maturity (tm50) differed significantly between populations of each species as a result of differences in growth rates, but length-at-50% maturity (Lm50) did not differ significantly between populations. Reproduction of the three assessed species peaked between September and April.
These results indicate that cichlid fisheries in the Kavango and Caprivi Regions could be managed using similar exploitation rules in all systems. It was recommended that the legal minimum mesh size be increased from 3” (76 mm) to 3.5” (89 mm) to ensure that juvenile cichlids are not targeted by the fishery. The implementation of this would reduce the risk of overfishing. The establishment of Fish Protection Areas (FPA’s) on the Zambezi River was endorsed and it was recommended that further areas be established on the Zambezi and Kavango Rivers to maintain an adequate spawner stock of large cichlids.
Description
A thesis submitted in fulfilment of the requirements for the Degree of Master of Science.
Keywords
Cichlid