To compare the eŠects of predation on the post-release mortality of wild and hatchery-reared juveniles of the tiger puŠer Takifugu rubripes we conducted release experiments in a semi-natural environment using a salt pond mesocosm (5,300 m 2). We released hatchery-reared juveniles together with wild juveniles into a pond with predators. Survival of the hatchery-reared juveniles (56.0) was signiˆcantly 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 diŠerences 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
The occurrence of vertebrate-type steroid hormones in the hemolymph and ovaries has been reported in several crustacean species; however, their physiological roles are not sufficiently understood. In the present study, correlations between hemolymph levels of the vertebrate-type steroid hormones and ovarian development in female kuruma prawn Marsupenaeus japonicus were examined. In the first experiment, hemolymph samples were taken in November (non-reproductive period) and June (maturing and spawning period) and the hemolymph levels of estradiol-17 b , estriol, progesterone, testosterone, and 11-ketotestosterone were found not to be significantly related to the ovarian development (gonadosomatic index, 0.80-9.39). In the second experiment, ovarian development was induced by bilateral eyestalk ablation in the non-reproductive period. The hemolymph levels of estradiol-17 b , estriol, and progesterone did not significantly differ between the control female prawn and the eyestalk-ablated female prawn, which underwent vitellogenesis. If the examined steroid hormones were vitellogenesis-stimulating hormones as in fish, their hemolymph levels would be expected to show significant correlations with ovarian development. However, the present study found no significant correlations between hormone levels and ovarian development. This suggests that based on the available data, the examined vertebrate-type steroid hormones do not play an important role in ovarian development in M. japonicus .
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 m 2 ). 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.
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