EFFECT OF ALGAL RATION AND SUBSTITUTION OF ALGAE BY MANIPULATED YEAST DIETS ON THE GROWTH OF JUVENILE MERCENARIA-MERCENARIA

The development of a cost-effective algal substitute would greatly reduce the operating costs of bivalve hatcheries. Previous work has resulted in the development of a yeast diet with improved digestibility and nutritional composition. The use of this yeast product as a partial algal substitute for the culture of juvenile hard clams Mercenaria mercenaria was investigated in a series of growth experiments. Juveniles of the hard clam (I mg live weight) were batch cultured in an 18 1 recirculating system for 2 to 3 weeks. The optimal weight-specific daily ration for a mixture (50/50 on dry weight basis) of Isochrysis galbana (clone T-Iso) and Chaetoceros gracilis was found to be 1.5 to 2% dry weight per wet weight of seed. Replacing 50% of the algal ration by yeast did not result in a significant decrease in growth rate relative to the algal-fed controls. The substitution of 80% of the algal diet resulted in growth rates reaching 90% compared with those obtained for the algal-fed controls. The incorporation in, the yeast diet of fat-soluble vitamins, rice starch, or an extract from macro-algae did not improve its nutritional value. However, the addition of kaolin occasionally resulted in a significantly higher growth rate. The present results are compared with previous reports on the use of yeasts for nursery rearing of bivalves.