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

Knowledge and Management of Aquatic Ecosystems (2015) 416, 06 c ONEMA, 2015  DOI: 10.1051/kmae/2015002 http://www.kmae-journal.org Population dynamics of freshwater oyster Etheria elliptica (Bivalvia: Etheriidae) in the Pendjari River (Benin-Western Africa) G.D. Akélé(1), , H. Agadjihouèdé(1) , G.A. Mensah(2) , P.A. Lalèyè(1) Received October 1, 2014 Revised January 3, 2015 Accepted January 5, 2015 ABSTRACT Key-words: Etheria elliptica, freshwater oyster, Bivalvia, population dynamics, Benin Etheria elliptica (Bivalvia: Etheriidae) is the only freshwater oyster occurring in Africa. The current study provides the first data on the population structure, growth, age, mortality and exploitation status of this species in the Pendjari River. E. elliptica length-frequency data were collected monthly from January to December 2009 and analyzed with FiSAT software. Population parameters including the asymptotic length (L∞ ) and growth coefficient (K) were assessed to evaluate the stock status. The recruitment pattern was modeled with a FiSAT routine. The asymptotic length (L∞ ) was 14.75 cm, while the growth coefficient (K) was 0.38 year−1. The growth performance index (ø ) reached 1.92. Specimens of Etheria elliptica reached a mean size of 4.66 cm and 6.41 cm at the end of one year and 1.5 years, respectively. We estimated total mortality (Z), natural mortality (M) and fishing mortality (F) to be 2.90 year−1 , 1.16 year−1 and 1.74 year−1, respectively. The recruitment pattern was continuous over the year with one major peak event during the rainy season (July). The exploitation rate (E = 0.60) revealed that the freshwater oyster was probably facing overexploitation due to lack of a minimum limit size and also due to an increase in the harvesting effort. Therefore, efficient management methods were urgently required to conserve the species. The return of empty shells into the water to increase the recruitment surface, rotation planning among harvesting sites and the imposition of a minimum limit size were recommendations made in order to ensure the sustainable exploitation of wild stocks. RÉSUMÉ Dynamique des populations de l’huître d’eau douce Etheria elliptica (Bivalvia : Etheriidae) de la rivière Pendjari (Bénin-Afrique de l’Ouest) Mots-clés : Etheria elliptica, huître d’eau douce, bivalve, Etheria elliptica (Bivalvia : Etheriidae) est la seule espèce d’huître rencontrée dans les eaux douces en Afrique. La présente étude a fourni les premières données sur la structure de la population, la croissance, l’âge, la mortalité et le niveau d’exploitation de l’huître dans la rivière Pendjari (Bénin). Les données de fréquence de taille de l’huître E. elliptica ont été recueillies chaque mois de janvier 2009 à décembre 2009 et analysées avec le logiciel FiSAT. Les paramètres de la population, y compris la longueur asymptotique (L∞ ), le coefficient de croissance (K) et le modèle de recrutement ont été estimés pour évaluer l’état du stock. (1) Laboratoire d’Hydrobiologie et d’Aquaculture, Faculté des Sciences Agronomiques, Université d’Abomey-Calavi, 01 BP 526 Cotonou, Bénin (2) Institut National des Recherches Agricoles du Bénin (INRAB), CRA-Agonkanmey, 01 BP. 884, Cotonou-Benin  Corresponding author: Article published by EDP Sciences G.D. Akélé et al.: Knowl. Manag. Aquat. Ecosyst. (2015) 416, 06 dynamique des populations, Bénin La longueur asymptotique (L∞ ) était de 14,75 cm et le cœfficient de croissance (K) atteignait 0,38 an−1 . L’indice de performance de croissance (ø ) s‘élevait à 1,92. L’huître atteignait la taille de 4,66 cm et de 6,41 cm à la fin d’une année et de 1,5 années, respectivement. La mortalité totale (Z), la mortalité naturelle (M) et la mortalité par pêche (F) ont été estimées à 2,90 an−1 , 1,16 an−1 et 1,74 an−1 , respectivement. Le recrutement était continu au cours de l’année avec un pic majeur pendant la saison des pluies (en juillet). Le taux d’exploitation (E = 0,60) a révélé que l’huître était probablement confrontée à la surexploitation due à l’absence d’une taille limite minimale d’exploitation et de l’augmentation de l’effort de pêche. Les stratégies de gestion, y compris le dépôt des coquilles vides dans la rivière pour augmenter le recrutement naturel, la planification de la rotation entre les sites de récolte et l’observation d’une taille minimale ont été recommandées pour l’exploitation durable des stocks sauvages. INTRODUCTION Bivalves, notably, clams and oysters, are harvested in many African fisheries for food and income (Mavuti et al., 2005; Adjei-Boateng and Wilson, 2012). In West Africa, freshwater bivalves play an important role as they represent a source of cheap protein and provide employment to riverine communities. Moreover, empty shells are used as a source of calcium in poultry feed and in lime manufacturing (Ampofo-yeboah, 2000; Adjei-Boateng and Wilson, 2012). This region is facing an exponential demographic pressure with limited resources, which is threatening bivalve natural stocks (Heck et al., 2007). Therefore, assessment of the exploitation level of exploited stocks is of great importance to implement relevant management strategies of bivalve mollusks. In Ghana, Adjei-Boateng and Wilson (2012) investigated the population dynamics of the freshwater clam Galatea paradoxa in the Volta River and noted the overexploitation of bivalve stocks. 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. Surprisingly, while many studies have investigated the exploitation status of the freshwater clam, there has been no research work focusing on the harvesting status of the freshwater oyster, despite a wide collection in many fisheries across Africa (Van Damme, 2011). The freshwater oyster Etheria elliptica (Lamarck, 1807) is a bivalve mollusk belonging to the order Unionoida (freshwater mussels), superfamily Etheroidea and family Etheriidae. This oyster occurs in a broad belt across tropical Africa and the north of Madagascar. E. elliptica is the only freshwater oyster encountered in African waters (Graf and Cummings, 2006). The bivalve is of great economic importance for the local people and has been exploited in many artisanal fisheries across Africa for food and income over the years (Ampofo-yeboah et al., 2009; Van Damme, 2011; Ikpi and Offem, 2012). In West Africa, oyster exploitation was reported in many fisheries in Nigeria (Abowei and Hart, 2008; Ikpi and Offem, 2012), in the Volta River in Ghana (Ampofo-yeboah et al., 2009) and in the Pendjari River in Benin (Kiansi, 2011). Surprisingly, no data were reported on the population dynamics of the species p (...truncated)