POTENTIAL ADVANTAGES OF FROZEN ALGAE (NANNOCHLOROPSIS SP) FOR ROTIFER (BRACHIONUS-PLICATILIS) CULTURE

The eustigmatophyte Nannochloropsis is widely used in many aquaculture hatcheries to establish the initial step of an artificial food chain. The advantage of Nannochloropsis over other unicellular algae is primarily its unique fatty acid composition. Rotifers which consume the algae carry these fatty acids to the fish larvae. Cultivation of large quantities of algal biomass to support this food chain is a heavy burden in many hatcheries, and in many other locations it cannot be carried out all year round. In this study we examined the possibility of substituting frozen biomass for fresh Nannochloropsis as a sole food source for rotifer cultures or as an enrichment treatment prior to feeding the rotifers to the larvae. Relatively high reproductive rates were found in rotifers of three strains which were fed frozen Nannochloropsis biomass. Total fatty acid content of these rotifers and fatty acid distribution were related to the chemical composition of the algae. Although seasonal variations in biochemical composition and fatty acid distribution were found in the algal biomass, the quality of the long-term frozen algae was adequate to provide the rotifers with the essential fatty acids almost all year round. No differences were found in the fatty acid composition of rotifers fed with algae stored at - 20 degrees C or -70 degrees C. The thawed algal biomass could be kept at 4 degrees C for 7 days and be used for rotifer feeding without a significant adverse effect on the fatty acid content and composition in the rotifers. The results of this study suggest that application of frozen Nannochloropsis biomass may promote easier management in biomass production of lipid-enriched rotifers. This provides the artificial food chain with essential fatty acids, which are crucial for the development and cultivation of fish larvae, with a relatively limited effort for algae production on the hatchery site.