Manipulated yeast diets as a partial algal substitute for rearing bivalve molluscs: laboratory trials with <i>Tapes semidecussata</i>

The development of a cost-effective algal substitute would greatly reduce operating costs of bivalve hatcheries and has therefore been the subject of many studies. Previous work has resulted in the development of a yeast diet with improved digestibility and nutritional composition. Preliminary trials showed that this diet can substitute up to 80% of the algal diet in the seed culture of the Manila clam Tapes semidecussata. In this communication we propose a culture test diet which will be used to study further dietary improvements of the yeast product.Juveniles of the Manila clam (1 to 5 mg unit weight) were batch cultured in a 51 recirculating system. Optimal daily ration for the algal control diet Chaetoceros gracilis was found to be 1% dry weight per wet rates of 88% and 64%, respectively, compared with the algal control. The incorporation of kaolin in the yeast diet significantly improved growth. The results for the yeast diets in this system are compared with those obtained under other culture conditions, and with preliminary results from recent tests with Mercenaria mercenaria