Growth, biomass, mortality, production and potential yield of the West African clam, Egeria radiata (Lamarck) (Lamellibranchia, Donacidae) in the Cross River system, Nigeria

Abstract

An important fishery for the West African hemi-estuarine clam, Egeria radiata exists in the lower reaches of the Cross River (CR) system. The stock is heavily exploited but its productivity and potential yield have not been studied. This paper discusses the distribution and ecology of, and presents data on growth, abundance, mortality, biological production and potential yield of the CR stock of E. radiata based principally on length structured relative age. Growth was exponential and conformed to the von Bertalanffy's growth model (VBGM), with the parameters L∞ = 9.3 cm, K = 0.36 and To = −0.10 year. The instantaneous total mortality coefficient (Z) by the catch curve method was 0.82 yr−1; natural mortality coefficient (M) was estimated at 0.32 yr−1. Total mean biomass computed from data obtained in two month's grab sample surveys in the area of occurrence of E. radiata was 1923 kg ha−1 for total (i.e. flesh + shell) and 628 kg ha−1 for flesh (i.e. shell-free) production units. The production-to-mean-biomass (or P/B ratio, R′) were 0.31 and 0.52 for total and shell-free production units respectively. The maximum sustainable yield (Ymax) was estimated at 398.4 kg ha−1 for total and 112.2 kg ha−1 for shell-free production units. With an exploitation ratio (E) of 0.61 and an estimated present catch rate of 1970 tonnes(t) yr−1 total weight (or about 82% of Ymax) the Egeria stock of the CR is currently being fished above the level of its optimum yield. Much of the fishing pressure is on the very young clams, thus lowering the age at first capture (Tc) to 1 year. This is an unhealthy situation for a species with limited area of distribution and low (< 1.0) P/B ratio; it can lead to rapid extinction of the stock if for some reason there is a substantial reduction in annual recruitment. It is recommended that in order to conserve this valuable resource and keep it at a level of sustained optimum yield, there should be a regulation prohibiting the landing of young clams (< 6.0 cm shell length). Furthermore the fishermen should learn to cultivate the animal by transplanting the young clams to new beds upstream to give them time to grow larger before harvesting. This type of management has been practised in Ghana for a long time and has helped to conserve the Egeria stock of the Volta River. The need for a continuous monitoring of the effluents from the Nigerian Newsprint Manufacturing Company's factory located at the bank of the CR in the centre of the clam industry is stressed; such monitoring would ensure that dangerous toxic chemicals are not discharged into the river to damage the Egeria industry.