Evaluation of the effects of stock enhancement on population dynamics using a state-space production model: A case study of Japanese flounder in the Seto Inland Sea

The effect of stock enhancement on increasing biomass is not well understood and is highly controversial because released fish might have a competitive impact on wild stocks. Quantitative assessments exploring the possible impacts of stock enhancement on population dynamics are rare, especially in data-limited fisheries. We developed a state-space production model incorporating stock enhancement to evaluate the impact of stock enhancement at a population scale. Since observation and process errors impair the accuracy of parameter estimation and biomass prediction, we investigated whether prior biomass reference information could improve analytical performance of the state-space production model. We compared the performance of maximum likelihood (ML) and maximum a posterior (MAP) estimation in a special case of a state-space Pella-Tomlinson model or in a state-space Schaefer model, for three simulated levels of process and observation errors. We found that the MAP parameter estimation with valid prior biomass reference data was generally superior to the ML parameter estimation. As a case study, we applied the extended state-space Pella-Tomlinson model to disentangle the impacts of stock enhancement for the Japanese flounder (Paralichthys olivaceus) in the Seto Inland Sea using long-term fishery data. The results showed a limited contribution of stock enhancement to Japanese flounder in the Seto Inland Sea, suggesting that most released flounder replaced the wild fish niche. Given the number of fish released in 2018, future projections for the next ten years under three different fishing pressures showed that stock enhancement was more effective at augmenting biomass when fishing pressure was higher, but that total biomass decreased simultaneously. Catch increase (decrease) in 30% resulted in a 9% decrease (8% increase) in the biomass and a 28% increase (15% decrease) in the net biomass augmentation from stocking. Therefore, a balance between stock enhancement and exploitation is important to obtain sustainable production and efficient stocking, especially facing global climate change which possibly influences the carrying capacity. Our results suggest that a state-space production model can be used to evaluate the impact of stock enhancement and implement better fishery management.