1Momoshima Station, National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Onomichi, Hiroshima 722-0061, 2Faculty of Fisheries, Nagasaki University, Nagasaki 852-8521, Japan
To compare the effects of predation on the post-release mortality of wild and hatchery-reared juveniles of the tiger puffer Takifugu rubripes we conducted release experiments in a semi-natural environment using a salt pond mesocosm (5,300 m2). We released hatchery-reared juveniles together with wild juveniles into a pond with predators. Survival of the hatchery-reared juveniles (56.0%) was significantly lower than that of the wild juveniles (86.0%). These results indicate that predation is a major cause of post-release mortality of this species. We also compared swimming depths of wild and hatchery-reared juveniles immediately after release into an experimental tank (33.5 cm in depth). Wild juveniles swam near the tank bottom and showed bottom-dwelling behavior, but hatchery-reared juveniles swam mostly in the water column. Similar behavioral differences were observed in release experiments in a mesocosm. We also compared body compositions of tetrodotoxin (TTX), which was detectable in the wild juveniles, but hatchery-reared juveniles had no detectable TTX. We concluded that predation shortly after release and behavioral defects in hatchery-reared juveniles, such as changes in swimming behavior and feeding behavior, might be among the main causes of mortality in the stock enhancement program of this species.