Stock Assessment of Oreochromis niloticus (Linnaeus, 1758) and Chrysichthys nigrodigitatus (Lacépède, 1803) from the Buyo Hydroelectric Reservoir Bordering Taï National Park (Côte d'Ivoire)

Abstract

Fish stock assessment provides indicators for sustainable fisheries management. This study was conducted for this purpose in the Buyo hydroelectric reservoir bordering the Taï National Park. The objective was to assess the stocks of Oreochromis niloticus and Chrysichthys nigrodigitatus, two main fish species caught by fishermen. From May 2022 to June 2023, a survey has been carried out in order to identify the fishing gears used by fishermen. Some specimens of the two species have been sampled in their catches, measured in centimeters and weighed in grams individually. These data were processed using FISAT II program. The results indicated that fishermen used five fishing gears types, with preference for gillnets and traps. For the stocks of the two species, the growth parameters correspond to an asymptotic length L∞=4.13 cm and a growth rate K=0.39/year for Oreochromis niloticus and to L∞=57.23cm and K=0.230/year for Chrysichthys nigrodigitatus. This resulted in longevity of Tmax = 7.67 years and Tmax = 13.03 years respectively. The mortality parameters indicated that the populations of the two species decreased more of natural causes than fishing and their stocks were underexploited (E=0.20 for Oreochromis niloticus and E=0.02 for Chrysichthys nigrodigitatus). This was confirmed by the current exploitation rates below the exploitation rates that reduce unexploited biomass by half (E50), the economic exploitation rates (E10) and the exploitation rates that maximize the yield per recruit (Emax). Finally, the length of first capture estimated for Oreochromis niloticus (L50=13.65 cm) was slightly greater than the length at sexual maturity, and inferior for Chrysichthys nigrodigitatus with L50=12.37 cm. This requires a thorough study on fishing gears selectivity for the preservation of the spawners.