LAUSR

In mass culture of Pacific bluefin tuna Thunnus orientalis, yolk-sac larvae of other species are fed as a major prey item to tuna larvae from 7 to 8 mm in total length. Marked growth variations in tuna larvae are frequently observed after feeding of yolk-sac larvae, and this variation in the growth of tuna larvae is subsequently a factor leading to the prevalence of cannibalistic attacks. To elucidate details of the mortality process of hatchery-reared tuna larvae after the initiation of yolk-sac larvae feeding, we compared the nutritional and growth histories of the surviving (live) tuna larvae to those of the dead fish, found dead on the bottom of the tank, as direct evidence of their mortality processes. Cause of mortality of tuna larvae 3 and 5 days after the initiation of feeding of yolk-sac larvae was assessed from nitrogen stable isotope and otolith microstructure analyses. Stable isotope analysis revealed that the live fish rapidly utilized prey fish larvae, but the dead fish had depended more on rotifers relative to the live fish 3 and 5 days after the initiation of feeding of yolk-sac larvae. The growth histories based on otolith increments were compared between the live and dead tuna larvae and indicated that the live fish showed significantly faster growth histories than dead fish. Our results suggest that fast-growing larvae at the onset of piscivory could survive in the mass culture tank of Pacific bluefin tuna and were characterized by growth-selective mortality.