Assessment of the ecological, growth and mortality parameters of crassostrea tulipa in selected water bodies in Ghana and the Gambia

Abstract

This study carried out a comparative assessment of the shellfish fishery in six water bodies; three in Ghana (Densu, Narkwa and Whin) and three in The Gambia (Allahein, Bullock and Tanbi). It assessed the ecological, growth and mortality parameters of Crassostrea tulipa in these waterbodies for 12 months from March 2021 to March 2022. Findings revealed that, the water bodies in The Gambia were generally deeper, less turbid and with better salinity ranges that enhanced the survival of the oysters than the Ghanaian waterbodies. This contributed to better condition indices and bigger sized oysters at the sites in The Gambia. Crassostrea tulipa at all the six sites significantly deviated from isometric growth. They were short-lived species with higher growth rate (K/yr) values and higher natural mortality (M/yr) values. Z/K ratios showed that the populations were mortality induced (natural mortality) although Bullock and Narkwa showed fishing induced mortality. The Ecurrent at these two sites also showed an overfished population. Lc50/L∞ ratio suggested growth overfishing in Ghana while those in The Gambia were of appropriate sizes. All the populations significantly deviated from a 1:1 sex ratio except Allahein in The Gambia and Densu in Ghana. The estimated maturity size indicated that the species in both countries all attained maturity before they were harvested except Densu in Ghana. All the harvesting sites in The Gambia were running some form of co-management structure in conjunction with the Gambian government where restrictions such as seasonal closures, size restrictions, were in place. In Ghana, Densu also has co-management modalities led mainly by the women shellfishers and supported by the Government, Densu and Tanbi have co management plans and have been granted exclusive use rights. The fishers in these jurisdictions were better off in income than oyster fishers in Ghana.