Abstract

The study investigated the seasonal relationship between the Condition Index (CI) and concentrations of six trace metals in the tissues of three bivalves: Crassostrea tulipa (n = 275), Anadara (Senilia) senilis (n = 310) from two 'open' lagoons (Benya and Ningo), a 'closed' lagoon (Sakumo), and Perna perna (n = 155) from rocky shores adjacent to Benya and Sakumo. These bivalves were analyzed for their total Cu, Zn, Fe, Mn, Cd and Hg concentrations across three size classes, two seasons, and three location sites on the Ghanaian coast. When all samples were pooled together, oysters exhibited the highest CI (147, range: 35 to 446), while mussels displayed the least (124, range: 48 to 252). Overall, CI was higher in the dry season compared to the wet season. CI varied based on body size, season, level of parasitic infestation, and local environmental conditions. In this study, CI was used to assess the plumpness of bivalves, defined as the ratio between soft tissue dry weight (mg) and shell length (mm). Food availability appeared to control CI in the results, while trace metal concentrations varied with size or CI and the season of sampling.

Key words: Trace elements, bivalves, lagoons, seasonal variation, condition index.