The habitat, fisheries and aquaculture of the Volta clam, Galatea paradoxa in the lower Volta river in Ghana: An example of the worldwide importance of brackishwater clams

Abstract

The Volta Clam, Galatea paradoxa (=Egeria radiata) occurs in the lower reaches of certain large West African rivers including the Volta River. The industry surrounding the Volta clam supports a very large riparian population and provides a substantial protein source for people in the lower Volta area of Ghana. The species is ideal for aquaculture because it is a sedentary filter feeder requiring no feeding apart from the natural algae content of the surrounding water and generally minimum husbandry. Although described as being brackish water species, it appears to be excluded from the lowermost 10 kilometres of the course of the Volta river. The lowermost limit of the range of Galatea appears to be the approximate limit of penetration of saline water from the sea. The factors which prevent Galatea from colonizing the whole length of the river is not clearly understood but could include the optimal grain size and organic matter content of the substratum.The monthly catch assessment surveys indicate an average catch of about 5,400 kg of clam per month per aqualung diving fisher. At an estimated cost of US$0.12 per kg, an aqualung diver could therefore make an estimated income of about US$648.0 per canoe per month. This suggests that the clam fishery is a multi-million business for supporting the livelihood of the riparian communities and urgently needs to be salvaged and developed to contribute sustainably to the income of the local fishing communities.The closure of the clam fishery by traditional authorities from (December to March) coincides with the period when the clams have lower Condition Index emphasizing the importance of local knowledge in the development of the clam fishery and culture practices.Observations from bottom grab samples indicate that the distribution of the clam in the sandy substratum is patchy. The gastropod, Pachymelania aurita which inhabits similar grounds as the clam could possess competitive advantage over the clam for habitable space due to their relatively higher abundance a situation which may not augur well for settlement of the larval stages of the clams.Two size frequency distributions were found for the clam's population, which is attributed to possible display of protandric hermaphroditism-a typical phenomenon in bivalve populations. Protandric hermaphroditism has not been confirmed in G. paradoxa. Further studies on the sexuality of the species are needed to confirm this phenomenon, although it has been found in other bivalve that inhabit similar habitat.Recent studies have shown by molecular technique that the endosymbiotic bacteria, Wolbachia, which could influence sex change in the clam, have been found in 59% of the clam mantle samples. The smaller size frequency distribution with average shell length <20 mm could represent males. These findings may influence the future of the industry in Ghana.Preliminary experiments on clam culturing did not show appreciable growth in shell length and fresh weight per month. © 2013 Nova Science Publishers, Inc. All rights reserved.