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

Abstract

Key-words:recruitment, mortality, growth parameters, Volta River, Bivalvia Population parameters such as asymptotic (L ∞ ), growth coefficient (K), mortality rates (Z, F and M), exploitation level (E) and recruitment pattern of the freshwater clam Galatea paradoxa were estimated using lengthfrequency data from the Volta River estuary, Ghana. The L ∞ for G. paradoxa at the Volta estuary was 105.7 mm, the growth coefficient (K) and the growth performance index (Ǿ) ranged between 0.14-0.18 year −1 and 3.108-3.192, respectively. Total mortality (Z) was 0.65-0.82 year −1 , while natural mortality (M) and fishing mortality (F) were 0.35-0.44 year −1 and 0.21-0.47 year −1 , respectively, with an exploitation level of 0.32-0.57. The recruitment pattern suggested that G. paradoxa has year-round recruitment with a single pulse over an extended period (October-March) in the Volta River. The Volta River stock of G. paradoxa is overfished and requires immediate action to conserve it. This can be achieved by implementing a minimum landing size restriction and intensifying the culture of smaller clams which is a traditional activity at the estuary. RÉSUMÉLa dynamique des populations de palourdes d'eau douce Galatea paradoxa de la rivière Volta, Ghana Mots-clés :recrutement, mortalité, paramètres de croissance, Rivière Volta, BivalveLes paramètres de population tels que (L∞) asymptotique, le coefficient de croissance (K), les taux de mortalité (Z, F et M), le niveau d'exploitation (E) et le modèle de recrutement des palourdes d'eau douce Galatea paradoxa ont été estimés à l'aide des données de fréquences de longueur dans l'estuaire du fleuve Volta, au Ghana. Le L∞ pour G. paradoxa dans l'estuaire de la Volta était de 105,7 mm, le coefficient de croissance (K) et l'indice de performance de croissance (Ǿ) variaient entre 0,14 à 0,18 an −1 et de 3,108 à 3,192, respectivement. La mortalité totale (Z) était de 0,65 à 0,82 an −1 , tandis que la mortalité naturelle (M) et la mortalité par pêche (F) étaient de 0,35 à 0,44 an −1 et de 0,21 à 0,47 an −1 , respectivement, avec un niveau d'exploitation de 0,32 à 0,57. Le schéma de recrutement suppose que G. paradoxa a du recrutement tout au long de l'année avec un maximum sur une longue période (octobre-mars) dans le bassin de la Volta. Le stock de G. paradoxa de la rivière Volta est surexploité et exige une action immédiate pour le conserver. Ceci peut être réalisé en mettant en oeuvre une restriction de taille minimale de débarquement et en intensifiant la culture de petites palourdes qui est une activité traditionnelle dans l'estuaire.

Growth of freshwater Etheria elliptica oyster (Lamarck, 1807) reared in cages in the Pendjari River (Benin, West Africa)

“…In West Africa, most commercially important bivalve populations are overexploited due not only to the harvest of immature individuals (Adjei-Boateng and Wilson, 2013;Akélé et al, 2015) but also to local human population growth (Ajana, 1980;Wilson, 2012, 2013). The traditional method of culturing bivalves under natural conditions without food supplementation has been reported to be beneficial in several studies (Ansa and Bashir, 2007;Adjei-Boateng and Wilson, 2012;Adjei-Boateng and Wilson, 2013;Crow and Carney, 2013).…”

“…Developing bivalve culture requires technical data on the growth performance and survival of species under culture systems (Angell, 1986;Urban, 2000;Adjei-Boateng and Wilson, 2013). Various studies have focused on the mangrove oyster Crassostrea gasar (Ajana, 1980;Ansa and Bashir, 2007) and the freshwater clam Galatea paradoxa (King, 2000;Adjei-Boateng and Wilson, 2012;Adjei-Boateng and Wilson, 2013) to ascertain their growth rates and culture potential. Mangrove oysters, C. gasar, grow quickly and are a suitable candidate for culture in African estuaries (Diadhiou, 1995;Ansa and Bashir, 2007).…”

Population dynamics of the Brown mussel Perna perna at a Rocky beach near Cape Coast, Ghana

“…It could be inferred that the P. perna population at Iture rocky beach approaches its hypothetical maximum length at a faster rate than P. viridis at Hong Kong and India, but much slower than the P. viridis population at Malaysia and Bangladesh. The growth performance index (Φ') allows inter‐ and intra‐specific comparison of growth performance in bivalve species of different stocks (Abohweyere & Falaye, ; Adjei‐Boateng & Wilson, ). The growth performance index of P. perna (Φ'= 3.49) at Iture rocky shore of Ghana was higher than P. viridis population (Φ'= 2.38) in Bangladesh (Khan et al, ), but lower than that of P. viridis (Φ'= 4.197) population in coastal waters of Malacca, Peninsular Malaysia (Al‐Barwani et al, ).…”

“…Berry and Schlerrer Gayanilo, Soriano, and Pauly, (1989) defined recruitment as a fully metamorphosed young fish whose growth is described Khan et al (2010) adequately by the VBGF and occurs at the fishing ground(s) with the instantaneous rate of natural mortality similar to that of the adults. Recruitment has been described as a continuous phenomenon for tropical species because of the relatively stable and elevated water temperatures allowing year-round breeding (Adjei-Boateng & Wilson, 2012;Qasim, 1973;Weber, 1976). The recruitment pattern exhibited by P. perna suggested a year-round recruitment with a single seasonal peak from April to June (see Figure 5), which coincided with the major spawning period of the species (Krampah et al, 2016) Bivalves in the larval, juvenile, and adult stages can die from a variety of causes, which can be environmental or biological in origin (Helm, Bourne, & Lovatelli, 2004).…”

“…Generally, studies on population dynamics of bivalves are usually conducted with the objective of sustainable management and conservation (Adjei-Boateng & Wilson, 2012). Management of molluscan resources requires knowledge of various population parameters and exploitation level of populations (Adjei-Boateng & Wilson, 2012;Al-Barwani et al, 2007). There are many tools for assessing exploitation levels and population dynamics of a stock.…”