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Quantitative Evaluation of Post-Release Mortality Using Salt Pond Mesocosms: Case Studies of Hatchery and Wild Juvenile Tiger Puffer

Quantitative approaches in release trials are typically used to evaluate fish quality for release. However, release trials under natural conditions include masking factors in their experimental designs. To streamline the experimental design of release, we used a semi-natural environment: a salt pond mesocosm (5,300 m2). This article reviews recent findings related to comparison of post-release mortality, behavior, and physiological conditions between wild and hatchery-reared juveniles of the tiger puffer (Takifugu rubripes), which were released into the mesocosm with or without predators (sea bass, Lateolabrax sp.). Results of release trials in the mesocosm suggested that hatchery-reared juveniles were more easily preyed upon than wild juveniles, and that predation occurred shortly after release, decreasing thereafter. Results revealed differences in swimming depth after release and different accumulations of tetrodotoxin (TTX) in reared and wild fish. Predation shortly after release and behavioral defects in hatchery-reared juveniles, such as changes in swimming behavior and feeding behavior, might be the main causes of mortality in this species' stock enhancement programs.