Bokhutlo T., Mosepele K. 2015. Gillnet selectivity for the north African catfish, Clarias gariepinus (Actinopterygii: Siluriformes: Clariidae), from the Upper Okavango Delta, Botswana. Acta Ichthyol. Piscat. 45 (2): 133–141.
Successful fisheries 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 fishery catch. The aim of this study was to simulate the harvesting pattern of the commercial gillnet fishery and provide gillnet selectivity parameters for C. gariepinus. This will help fishery managers with information on the appropriate mesh sizes needed for sustainable utilisation of the catfish 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).
The 93 mm and 118 mm mesh sizes were the most efficient when capturing C. gariepinus accounting for 44.6% and 21.9% of the total catch, respectively. Mean fish length increased with increasing mesh size and was significantly different between mesh sizes (P < 0.001). The modal fish 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.
The modal fish lengths for meshes 93 mm and 118 mm are greater than the size at maturity and therefore the current commercial gillnet fishery 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 fishery 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.
growth, overfishing, mean length, mesh size, catch per unit effort, selectivity