Obtaining high-quality larvae for mass production of glass eels: Can we refine our approach?

Because the post-larval (leptocephalus) survival rates of batches of Japanese eels vary widely, a useful indicator is needed to identify good-quality larval batches before the time of first feeding. This study therefore evaluated the effects on post-larval survival of early ontogenetic variables, including rates of fertilization, abnormal cleavage, hatching, survival during the larval stage until 5 days post hatching (dph), and deformities of individual larvae reared in wells of 48-well microplates. In addition, the survival rate of larvae reared in a 180-l mass rearing tank with flowing seawater was measured. Both simple and multiple linear regression analyses showed that none of the variables assessed by the microplate method correlated with post-larval survival, although the survival in the mass rearing tank at 5 dph was significantly associated with post-larval survival. These results suggest that early ontogenetic variables were likely strongly influenced by incubation methods, with the optimal method of identifying good-quality larvae being to measure the survival rate of mass reared rather than individually reared larvae. We suggest that larvae reared by the microplate method were likely subjected to excessively reduced environmental stress, due to the presence of antibiotics and the lack of interference between individuals, resulting in an overestimation of larval quality.