Inter-annual variation in the surplus prey production for stocking of Japanese flounder Paralichthys olivaceus

Stocking hatchery fish into the wild should only be implemented at a level that can be sustained within the available food-based carrying capacity to avoid food resource competition with wild conspecifics. We constructed a simulation model to assess the growth of wild conspecifics under different magnitudes of hatchery fish releases, including no releases and actual release levels, and under different wild fish abundancies. Stocking of juvenile Japanese flounder Paralichthys olivaceus (approximately 70-110 mm TL) in Fukushima from 2003 to 2005 was analyzed. We monitored post-release changes in abundance of both cultured and wild juveniles and their prey (mysids). Our survey and modelled simulations revealed that the surplus prey production was sufficient to support the consumption of stocked and wild Japanese flounder (up to 150,000 individuals per site in 2003 and 2004), during which wild juveniles achieved nearly maximum growth rates. However, in 2005, during which a dominant year class of wild Japanese flounder occurred, the body size of wild juveniles was 49% smaller than the predicted size based on the maximum growth rates. This suggests that the food-based carrying capacity was reached in that year. Thus, it is important to evaluate the existing prey and wild juveniles levels and adjust the release accordingly every year.