Successful stocking of a depleted species, spotted halibut Verasper variegatus, in Miyako Bay, Japan: evaluation from post-release surveys and landings

Post-release adaptation and stocking effectiveness of a highly depleted pleuronectid flatfish, spotted halibut Verasper variegatus, were assessed in Miyako Bay, Japan. In early July 2004 and 2005, 22 000 and 5000 hatchery-reared juveniles (mean 81.2 and 89.8 mm total length, respectively) were released in shallow waters in the inner bay area. Results from surveys with a 2 m beam-trawl conducted over a 2 mo post-release period, laboratory predation experiments using potential predators (age-1 Japanese flounder), and a market census continued until December 2008, suggested that released juveniles, which had a size refuge from predators, could quickly adapt to brackish water habitats where there were suitable physical conditions (temperature, salinity and sediments) and abundant prey organisms. Juveniles selectively preyed on epibenthic gammarids and cumaceans, and had full stomachs 1 wk after release. Some released fish grew to >40 cm by age-1+, feeding exclusively on larger crustaceans (e. g. isopods, carideans and brachyurans). Market census revealed that released fish showed >99% contribution to catches, and recapture rates for fish released in 2004 (7.6%) and 2005 (15.7%) were higher than for releases in 2000 to 2003 (0.7 to 5.0%), possibly due to the development and application of improved release strategies (i.e. release habitat, size and season was appropriate in 2004 and 2005). Our study demonstrates that (1) local depleted stocks can be effectively restored by hatchery releases and that (2) development of release strategies based on the analysis of ecological interactions between released juveniles and the target ecosystem can largely contribute to the successful restocking of depleted species.