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The study was conducted from July to December, 2018, in the Volta Lake, with gillnets principally of mesh sizes, 7.62 cm, 8.89 cm and 10.16 cm, and an additional mesh size of 6.35 cm, using four indirect methods to investigate gillnet selectivity parameters for Oreochromis niloticus and Sarotherodon galileus and to estimate the true relative abundance of the fish populations from the gillnet catches. A total of 1,969 fish samples of both species was measured for standard length, and selectivity parameters determined by comparison of catches of length classes of successive pairs of gillnets. The study determined selection factors of 1.96 and 1.92 for Oreochromis niloticus and Saratherodon galileus respectively and being nearly equal was attributed to both species having similar body shape. The distribution of the selection curves was skewed, due to various factors, but the resultant asymmetry was corrected by the Beverton/ Holt method which minimized the standard deviations of the curves to one-tenth of the normal Holt method for selectivity determination. The gillnet selectivity parameters assessed provided the bench mark for guidance for re-formulation ofthe fisheries regulations on mesh size restrictions for sustainable management of the two tilapiine fish species. Gillnet catches of the two species were significantly different from the true relative abundance of the fish populations due to the selectivity of gillnets. To reflect true abundance and be qualified for more advanced fisheries resources management work, the gillnet data should be adjusted using the probability of capture generated through the selectivity studies.

journal of fisheriessciences.com

Estimate of Gillnet Selectivity Parameters and True RelativeAbundance of Oreochromis niloticus and Sarotherodon galileus,Using Four Indirect Methods, with Gillnets Principally of ThreeDifferent Meshes in Thevolta Lake, Ghana