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Background. Successful fi sheries management requires estimation of gillnet selectivity for optimum exploitation of the resource. In the Okavango Delta, no study has assessed the selectivity of gillnets for Clarias gariepinus (Burchell, 1822) which is an important component of both the subsistence and the commercial gillnet fi shery catch. The aim of this study was to simulate the harvesting pattern of the commercial gillnet fi shery and provide gillnet selectivity parameters for C. gariepinus. This will help fi shery managers with information on the appropriate mesh sizes needed for sustainable utilisation of the catfi sh resource. Materials and methods. Monthly gillnet sampling was conducted over a period of 8 years from 2001 to 2009. The SELECT method was used to estimate gillnet selectivity for C. gariepinus using catch data from four mesh sizes (73 mm, 93 mm, 118 mm, and 150 mm). Results. The 93 mm and 118 mm mesh sizes were the most effi cient when capturing C. gariepinus accounting for 44.6% and 21.9% of the total catch, respectively. Mean fi sh length increased with increasing mesh size and was signifi cantly different between mesh sizes (P < 0.001). The modal fi sh lengths for the four mesh sizes were estimated at 41.63 cm, 53.23 cm, 66.35 cm, and 85.54 cm in order of increasing mesh size. Conclusion. The modal fi sh lengths for meshes 93 mm and 118 mm are greater than the size at maturity and therefore the current commercial gillnet fi shery which uses 100 mm mesh size may not be a threat to the C. gariepinus population in the Upper Okavango Delta. Future studies should aim to conduct fi shery dependent selectivity studies in the lower Okavango Delta to determine if selectivity changes with location and gear. Moreover, a comprehensive age-based stock assessment is required to establish the mesh size that optimizes yield without adversely depleting the spawning stock biomass.

acta ichthyologica et piscatoria

Gillnet selectivity for the north African catfish, Clarias gariepinus (Actinopterygii: Siluriformes: Clariidae), from the Upper Okavango Delta, Botswana