Population parameter estimates for Chrysichthys auratus and C. nigrodigitatus (Pisces: Claroteidae) in lake Volta, Ghana

Abstract

Estimation of population parameters of two commercially important claroteids (Chrysichthys auratus and C. nigrodigitatus) in lake Volta were obtained from length composition data compiled in 1995 and 1996. The von Bertalanffy growth constants for C. auratus were computed as L∞ = 31.0 cm SL, K = 0.60 yr-1 and t0 = -0.12 yr with derived growth performance index (ψ′ = 2.76). Corresponding estimation for C. nigrodigitatus were L∞ = 44.5 cm, K = 0.65 yr-1, t0 = -0.22 yr and ψ′ = 3.12. The two species live for about 5 years and attain at least 50% of the asymptotic length at the first age class. The growth patterns determined by the growth performance index (ψ′) were higher for C. nigrodigitatus than for C. auratus of similar sizes. Total mortality rate, Z, for the fish ranging 6.0-30.0 cm SL was estimated as 2.63 yr-1 and the natural mortality rate, M, estimated as 1.30 yr-1 with average fishing mortality, F, calculated as 1.33 yr-1. From the mortality estimation the exploitation ratio (E = F/Z) for C. auratus was 0.51. Mortality rate estimates for C. nigrodigitatus were Z = 3.77 yr-1, M = 1.24 yr-1 and F = 2.53 yr-1 and exploitation ratio, E, calculated as 0.67 which is indicative of over-fishing. The management implications of the results to the fishery are discussed and suggestions made for sustainable exploitation of the stocks. © 2002 Elsevier Science B.V. All rights reserved.