Reduction in size-at-age of black sea bream (Acanthopagrus schlegelii) following intensive releases of cultured juveniles in Hiroshima Bay, Japan

More than 20 million black sea bream (Acanthopagrus schlegelii) juveniles have been released in Hiroshima Bay since stock enhancement of the fishery began in the early 1980s. The majority (90%) of the black sea bream harvest occurs between April and June. Catch per unit effort (CPUE) increased substantially following the most intensive releases (1985-1996). However, the size-at-age in 2000 was significantly lower than in 1983. In addition, by 2000 many of the released fish had matured as females, changing the sex ratio of this protandrous hermaphrodite. These changes are thought to be related to the intensive release of hatchery-reared juveniles in the bay, although there is also evidence to suggest that a lower market price for the species, resulting in reduced fishing effort, also contributed to these effects. Our results highlight the potential negative impact of intensive stock enhancement on the population structure of a local fishery. They also emphasise the importance of assessing the status of the stock, and monitoring the carrying capacity of the habitat for the target species, to determine how many, if any, cultured juveniles should be released to achieve management objectives.