Influence of the proximity to the ocean and seasonality on the growth performance of farmed mangrove oysters (Crassostrea gasar) in tropical environments

In tropical coastal and estuarine environments, such as the mangroves of the Amazon region, salinity is considered to be one of the principal factors limiting the farming of bivalve mollusks. In this context, the present study evaluated the effects of the location of the farm (proximity to the ocean) and seasonality on the growth of the mangrove oyster Crassostrea gasar, and identified the environmental factors that most influenced the growth performance of the oysters over the course of the year. The oysters analyzed in this study were raised over a one-year period on farms in three areas of mangrove on the Brazilian Amazon coast, located at different distances from the ocean. These farms rear oysters using a fixed-tray system. The study period was divided into four seasonal divisions - the rainy-dry transition period, the dry season, dry-rainy transition, and the rainy season. The salinity, pH, temperature, and dissolved oxygen concentration of the water were measured in situ using a multiparameter probe during specimen collection. A mean of 20 specimens were collected each month from each of the three study areas. Once collected, the oysters were weighed and measured. Salinity and precipitation were the environmental parameters that most varied over the course of the study period, with significant differences being found among study areas during this period. The best growth performance was recorded for the farms in areas 1 and 2, which were closest to the ocean, and were significantly different from area 3 (the furthest from the ocean), where the oysters were smallest in size. The evidence indicates that areas located closest to the coast, are the most appropriate for commercial-scale oyster farming, while areas furthest from the ocean and influenced primarily by fluvial discharge, is much less favorable for the production of oysters. Overall, the data showed that, when raised in its natural environment, C. gasar presents a better growth performance during the rainy-dry transition period and the dry season, especially on farms located close to the coast, where salinity is between 25 and 35, given that this condition favors the development of the oysters, and optimizes their growth performance in commercial scale farming operations.