Growth patterns and condition index of rock oysters Saccostrea cucullata in relation to river influence in southeastern Madagascar

Abstract

Saccostrea cucullata is the dominant species of rock oyster (family Ostreidae) in southeastern Madagascar. Despite its economic and ecological importance, information on its biology and ecology is scarce. This study investigated the growth, shell length–weight relationship, and condition index (tissue dry weight/shell dry weight) of S. cucullata in the Ambinanibe coastal lagoon system. Oysters were placed in cages and deployed at four sites: one site at the Efaho River mouth emptying into the main lagoon, two sites within the main lagoon (Ambinanibe lagoon), and one site out of river influence (Andranasy lagoon). Shell length, shell weight and the condition index were monitored at each site over one year. The results show significant increases in shell length and shell weight during the study period. The relationship between shell length and weight was nonlinear and best described by a power equation. The b value of the length–weight relationship ranged between 1.01 and 1.76, which is comparable to values reported in other studies. Oysters from the site at the river mouth exhibited the highest condition index (4.50 ± 2.82%), while those from the site out of river influence had the lowest (0.72 ± 0.55%), suggesting that freshwater inflow creates favourable environmental conditions for producing high-quality, marketable oysters. Overall, this study makes an important contribution to the current knowledge on the morphometric characteristics and growth parameters of S. cucullata in coastal zones. The findings provide valuable insights for developing effective conservation strategy and optimising the farming of this species, in this region and beyond.