Population dynamics and gillnets selectivity of Chrysichthys nigrodigitatus (Lalepede 1803) in lower reaches of the Cross River estuary, Nigeria

Abstract

Population Dynamics and gillnet selectivity of Chrysichthys nigrodigitatus (Lalepede 1803) in the lower reaches of the Cross River estuary of Nigeria was studied for two years (January 2008 – December 2009), with respect to selectivity, gillnet sizes and population of the species based on length frequency analysis. Estimates of growth parameters, asymptotic length (L∞) = 98.25cm and growth constant (K/year) = 0.96yr^-1. Total mortality rate (Z) was 6.27 yr^-1, natural mortality (M) = 1.22 yr-1, Fishing mortality (F) = 5.05 yr^-1 and Exploitation rate (E) = 0.81. Significant correlation (r = 0.9140 – 0.9770 and r = 0.976 – 0.977). (P < Z0.05) was observed between mesh sizes of gillnets and mean values of the morphometric characteristics (such as half-circumference, total length and total weight) of the fish. There is low percentage frequency range of the number of fish caught in different mesh sizes, possibly with low hanging coefficient range (0.4-0.5). Gilling, snagging, wedging, entangling were observed in the selection of the fishes. In conclusion, selectivity in Chrysichthys nigrodigitatus gillnet fishery in the Cross River estuary may be moderately adequate despite the influence of several other factors. The selection distribution curve using 17.5cm estimated modal total length recommends the usage of 50mm mesh size and above for exploitation of the species.

Keywords: Gillnet, Selectivity, Mortality, Length, Population