The exploited population of the brackish river prawn (Macrobrachium macrobrachion Herklots 1851) in the Cross River estuary, Nigeria

Abstract

The dynamics of the exploited population of Macrobrachium macrobrachion in the Cross River estuary, Nigeria, were studied based on monthly length-frequency data collected from January 1997 to June 1998 (18 months), from the landings of the artisanal Macrobrachium fishery. Sexual dimorphism was indicated in the growth and mortality parameters. For the males, the von Bertalanffy growth parameters were estimated as L∞ = 141.35 mm, K = 1.21 year-1, C = 1.0 and WP = 0.15. For the females, they were L∞ = 117.55 mm, K = 1.60 year-1, C = 0.81 and WP = 0.51. The instantaneous rate of total mortality (Z) was estimated as 9.53 year-1 (males) and 9.14 year-1 (females). The instantaneous rate of natural mortality (M) was estimated as 2.44 year-1 (males) and 3.09 year-1 (females), while the instantaneous rate of fishing mortality (F) was estimated as 7.09 year-1 (males) and 6.05 year-1 (females). The exploitation rate (E) obtained was 0.74 for the males and 0.66 for the females, suggesting that the prawn population was over-fished for both sexes. It is necessary to analyse the catch and effort data for the last 10 years and to apply other methods of stock assessment in order to estimate the long term trends in the fishery.