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

Bivalves have often been used in integrated fish-farming to enhance the eco nomical value of by-products and/or to improve water quality. However, no p hysiological studies have dealt with the contribution of the two main sourc es of organic matter potentially present in a fish-farm effluent: living ce lls of phytoplankton and detritical matter (fish-faeces and uneaten feed). This study evaluated feeding responses of the Pacific oyster (Crassostrea g igas) to a land-based fish-farm effluent comprised mainly of fish-faeces (D icentrarchus labrax) and compared them with those obtained with a diatom (S keletonema costatum). A particular distinction of the main sources of organ ic matter was made in the experiments, the two diets being evaluated separa tely and mixed. Feeding responses were evaluated using the biodeposit metho d with special attention being paid to pre-ingestive processes and absorpti on efficiency (AE). Experiments were carried out between May and July 1998 in the laboratory at constant water temperature (20 degrees C), Results sho wed that food quality (nature of particulate organic matter, POM, organic c ontent of the total suspended matter, TSM) had no influence on clearance ra tes. However, clearance rates were significantly reduced for oysters in adv anced stage of gametogenesis. S, costatum was preferentially ingested compa red 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 w ere lower than those estimated for S. costatum (66-70%, 20 J mg(-1)). Never theless, these values were high for such a detritic type of food. This stud y confirms that both concepts of biomechanical filter and economical value improvement of fish-farming waste using oysters are of interest. (C) 2000 E lsevier Science B.V, All rights reserved.