Growth and population structure of Odaxothrissa mento and Brycinus leuciscus from the lower reaches of the White Volta in Ghana

Abstract

The growth and population structure of two small pelagics: Odaxothrissa mento and Brycinus leuciscus were studied based on 1-year data collected from the White Volta at Yapei in northern Ghana. The species were assessed using an R package TropFishR. The von Bertalanffy growth parameters were estimated with an asymptotic length (L_∞) of 13.65 cm TL (CI_95%: 11.29–17.29), growth curvature of .32/year (CI: .14–.63) for Odaxothrissa mento and L_∞ of 8.85 cm TL (CI_95%: 8.17–11.02) and growth curvature of 1.57/year (CI_95%: .64–1.57) for Brycinus leuciscus. The computed total mortality (Z) was 2.25/year, the natural mortality (M) was .91/year and fishing mortality (F) was 1.34/year for Odaxothrissa mento while Z = 4.5/year, M = 2.8/year and F = 1.7/year were estimated for Brycinus leuciscus. Results indicate that the stock of Odaxothrissa mento is slightly overexploited with a current exploitation rate (E) of .60/year which is above the optimum exploitation rate (E_opt) of .5. However, the stock of Brycinus leuciscus was found to be underexploited with a current exploitation rate (E) of .38 year⁻¹. The estimated age at 50% capture for Odaxothrissa mento and Brycinus leuciscus were 3.4 years and .65 years, respectively, with their corresponding lengths at 50% capture of 9.14 cm and 5.67 cm, which are close to the mean lengths of the population in the present study. Mean length at first sexual maturity (L_m) for Odaxothrissa mento is estimated as 5.2 and 4.2 cm TL for females and males while L_m of 3.5 cm and 4.5 cm were estimated for females and males of Brycinus leuciscus, respectively. The estimated longevity (t_max) of Odaxothrissa mento and Brycinus leuciscus was 9.7 years and 1.7 years, respectively, indicating that Odaxothrissa mento is a long-lived species while Brycinus leuciscus is a short-lived species. This conforms to the exploitation pattern of the species and thus requires species-specific management interventions.