Pacific oyster ( Crassostrea gigas) feeding responses to a fish-farm effluent

Abstract

Bivalves have often been used in integrated fish-farming to enhance the economical value of by-products and/or to improve water quality. However, no physiological studies have dealt with the contribution of the two main sources of organic matter potentially present in a fish-farm effluent: living cells of phytoplankton and detritical matter (fish-faeces and uneaten feed). This study evaluated feeding responses of the Pacific oyster ( Crassostrea gigas) to a land-based fish-farm effluent comprised mainly of fish-faeces ( Dicentrarchus labrax) and compared them with those obtained with a diatom ( Skeletonema costatum). A particular distinction of the main sources of organic matter was made in the experiments, the two diets being evaluated separately and mixed. Feeding responses were evaluated using the biodeposit method with special attention being paid to pre-ingestive processes and absorption efficiency (AE). Experiments were carried out between May and July 1998 in the laboratory at constant water temperature (20°C). Results showed that food quality (nature of particulate organic matter, POM, organic content of the total suspended matter, TSM) had no influence on clearance rates. However, clearance rates were significantly reduced for oysters in advanced stage of gametogenesis. S. costatum was preferentially ingested compared to the fish-faeces when offered in a mixed diet. Absorption efficiency and energy content (56% and 15 J mg −1, respectively) of the fish-faeces were lower than those estimated for S. costatum (66–70%, 20 J mg −1). Nevertheless, these values were high for such a detritic type of food. This study confirms that both concepts of biomechanical filter and economical value improvement of fish-farming waste using oysters are of interest.