CARBON AND NITROGEN SCOPE FOR GROWTH AS A FUNCTION OF DIET IN THE SEA SCALLOP PLACOPECTEN-MAGELLANICUS

Laboratory feeding experiments with the sea scallop Placopecten magellanicus were carried out to compare scope for growth (SFG) to measured growth and determine the effect of diet on carbon and nitrogen SFG. Diets consisting of cultured phytoplankton, kelp detritus, and resuspended sediment were provided daily for 52 days (October-December). Measurements of clearance rate, absorption efficiency, respiration, O/N ratio, and carbon and nitrogen content of diets and scallop tissue were used to construct carbon and nitrogen budgets for each diet. Growth coefficients were calculated from change in tissue weight during the study period. Scope for growth tended to overestimate actual growth for phytoplankton diets, but assessment of variance in energy budget terms suggested closer agreement between observed and predicted growth. Individuals fed kelp and sediment diets showed depressed respiration rates and greatly reduced growth compared to phytoplankton, a result verified by SFG estimates. Phytoplankton provided at least six times greater carbon and nitrogen balance than detrital sources. Sediment allowed a relatively favourable nitrogen balance due to low C/N, but unfavourable carbon balance due to high inorganic content. In contrast, kelp provided comparatively better carbon than nitrogen nutrition. Tissue C/N did not differ between scallops fed kelp and microalgal diets. Depending on the nutritional state of Placopecten magellanicus and its subsequent O/N ratio, sediment and kelp could enhance phytoplankton diets, but could not act as sole food sources. These results indicate that carbon or energy based scope for growth measurements may overestimate secondary production under nitrogen-limited conditions.