Predation mortality of hatchery-reared juvenile Japanese flounder Paralichthys olivaceus released in Mano Bay, Sado Island [Japan]
2006
Sudo, H.(Japan Sea National Fisheries Research Inst., Niigata) | Kajihara, N. | Fujii, T.
We estimated the impact of predation by crabs and fishes on hatcheryreared juvenile Japanese flounder Paralichthys olivaceus released into a flounder nursery habitat in Mano Bay, Sado Island. In 2001 and 2002, nearly 50,000 reared juvenile (ca. 6.1 cm total length) were released each year and 68% and 83%, respectively, of them were estimated to suffer mortality in the first week after release. Foregut content analysis of crabs caught in the nursery, using a PCR-based technique, revealed that the swimming crab Charybdis japonica is a significant predator on released flounder. This finding was supported by laboratory observations that the crabs readily preyed on live reared flounder. In addition, through microscopic observations of stomach contents of piscivorous fishes, 4 species (devil stinger, 1-age Japanese flounder, spotted flathead and bartailed flathead) were identified as predators. According to the calculations based on gut content analyses and abundances of these predators, predation by swimming crabs is estimated to account for 41% and 46% of the loss of released flounder during the first week after release in 2001 and 2002, respectively, whereas predation by 4 piscivorous fishes accounted for 19% and 21% of the loss in 2001 and 2002, respectively. Thus, we conclude that predation is the major factor of heavy mortality of released flounder shortly after release.
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