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

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.

Author(s): Lawrence Issah Braimah

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