The effects of unialgal and mixed diets on growth of hatchery-produced spat of the sea scallop, Placopecten magellanicus (Gmelin)

Abstract

Sea scallop (Placopecten magellanicus) juveniles have been reared in a hatchery at the Ocean Sciences Centre, Memorial University of Newfoundland, for several years and transplanted to scallop farms located in three Newfoundland bays. This study was undertaken to investigate the effect of diet on growth of spat to help optimize hatchery production. Four experiments were performed to determine which available algal diet(s) resulted in greatest growth (in terms of shell height, dry weight and organic weight) of juvenile P. magellanicus ranging in size from 1-4 mm shell height. It was found that of the diets tested, a ternary mixed diet consisting of the three species Isochrysis galbana, Isochrysis aff. galbana (T-Iso) and chaetoceros spp. (muelleri or calcitrans), fed in a 1:1:1 ratio at a final concentration of 50 cells/μl, resulted in the greatest growth for juveniles sized 2-4 mm. A unialgal Isochrysis galbana diet supported as much growth as the ternary mixed diet in scallops sized 1-2 mm. The unialgal Chaetoceros diets did not support any appreciable growth. ALGAL 161, a commercially available, spray dried, heterotrophically grown Tetraselmis suecica product, was found to be a poor food for P. magellanicus juveniles. Gross growth efficiencies were determined for scallops for the different diets tested. No significant differences in growth efficiencies among the diets were found -- A short-term experiment was done to study the effect of ration on filtration and ingestion rates for three size classes of juvenile Placopecten magellanicus (2-4 mm, 4-6 mm, > 6 mm). It was foudn that as ration increased, filtration rates decreased and ingestion rates increased. Values for filtration and ingestion rates (F) were related to growth rate (W) using the general allometric equation F=aWb. The physiological rates were found to be increasing faster than the growth rates of these young juvenile scallops with values of the exponent (b) ranging from 1.47-2.24.