Patchiness formation and development of schooling behavior in pacific threadfin Polydactylus sexfilis reared with different dietary highly unsaturated fatty acid contents

Nutritional condition in larval fish culture has generally been evaluated b ased on growth and survival, and stress tolerance by air exposure. Here we made direct observations of fish in rearing tanks to estimate the quality o f the fish. Pacific threadfin Polydactylus sexfilis larvae Were reared with rotifers and Artemia nauplii enriched with three levels of highly unsatura ted fatty acids (HUFA). The three levels were defined as no-, medium-, or h igh-HUFA groups, in which the fatty acids emulsions contained 0, 30, or 50% respectively, of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA ). The formation of patchiness (dense aggregations of fish on the surface) and development of schooling behavior were observed in four replicate reari ng tanks for each feed treatment. Patchiness was formed in every tank with the highest densities of 60-250 fish/L on day 7-8 posthatch. Both medium-HU FA and high-HUFA groups showed schooling behavior at 19 d after hatching, w hereas fish in the no-HUFA group did not form schools even at day 25. The b ody lengths of fish from each group were not significantly different, altho ugh the no-HUFA group suffered high mortality after day 20. Our results sug gest that HUFA deficiency has a serious negative impact on P. sexfilis duri ng metamorphosis rather than during the early larval stage. We also conside r that direct observation rather than stress tests by air exposure may be a n effective tool to measure the quality of juvenile fish.