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 T o = -0.10 year.The instantaneous total mortality coefficient (Z) by the catch curve method was 0.82 yr-'; natural mortality coefficient (M) was estimated at 0.32 yr-. 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-for flesh (i.e. shell-free) production units. The production-to-meanbiomass (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-' for total and 112.2 kg ha-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 (To) 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. 2

Population dynamics of the freshwater clamGalatea paradoxafrom the Volta River, Ghana

“…It was, however, lower than the 145.1 mm obtained by Vakily (1992) from the re-analysed data collected by Kwei (1965) at an upstream station (Akosombo) on the Volta River and the 111.0 mm obtained by King et al (1992) on the Cross River stock of G. paradoxa. The L ∞ estimate in this study was higher than that of Moses (1990) and Etim and Brey (1994) for G. paradoxa stocks in the Cross River in Nigeria (Table II). The growth coefficient (K) estimate of 0.14-0.18 year −1 in this study was the lowest ever recorded in studies of the population dynamics of G. paradoxa (Table II).…”

“…It is endemic to the West African sub-region with a range that extends from the Gulf of Guinea to the Congo (Moses, 1990). Galatea Paradoxa is the basis of an artisanal clam fishery at the Volta River estuary providing employment and an affordable protein source to the riparian human communities and beyond (Amador, 1997).…”

Relationship Between Heavy Metal Concentrations in Bottom Sediments and the Clam, Galatea Paradoxa (Born 1778) from the Volta Estuary, Ghana

“…It is a filter-feeding organism with a wide distribution extending from the Gulf of Guinea to the Congo [10]. Limited information about the prevalence and commercial exploitation of this clam is available from only a few countries, including Ghana, Nigeria and Cameroon, despite its extensive distribution in the wider West African region [11].…”

Population dynamics of freshwater oysterEtheria elliptica(Bivalvia: Etheriidae) in the Pendjari River (Benin-Western Africa)

“…In the present study, the value reported for L ∞ (14.75 cm) was higher than for other freshwater mussels. The lowest value (9.3 cm) was reported for the freshwater clam G. paradoxa (Donacidae: Veneroida) in the Cross River system in Nigeria (Moses, 1990). In the genus Crassostrea, the highest value (20.88 cm) was reported f or C. madrasensis (Amin et al, 2008) and was 1.5 times higher than that for E. elliptica.…”

“…Therefore, urgent strategies are required for stock management, such as imposing a minimum landing size on fishermen (Adjei-Boateng and Wilson, 2012). Previously, Moses (1990) reported overexploitation of the freshwater clam G. paradoxa in Cross River in Nigeria with an exploitation ratio (E = 0.60) higher than the optimum value (E = 0.5) according to Gulland (1971). Overfishing was mainly due to juvenile exploitation.…”