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[AOS12-17] 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
Keywords:stock enhancement evaluation, state-space production model
In coastal marine fisheries, stock enhancement has been viewed as an important management tool to replenish or increase wild stocks by releasing hatchery-reared juveniles into the wild. Theoretically, released fish can augment overall biomass by overcoming recruitment limitations. However, released fish may fail to increase overall biomass for various reasons. One important reason is that hatchery fish can have competitive impact on the wild fish given limited carrying capacity. Therefore, it is important to consider the competitive impact from released fish when evaluating the benefit of stocking.
Surplus production models are important methods for conducting stock assessments in data-limited fisheries. Recent advancements have cast surplus production models into state-space production models that differentiates two distinct sources of variability: process error in the surplus production model and measurement error between the underlying biomass states and observation data. State-space production models has been increasingly used in fishery management but not in the evaluation of stock enhancement effects. Therefore, in this study, we aimed to develop a state-space production model to evaluate the stock enhancement effect using Japanese flounder (Paralichthys olivaceus) in the Seto Inland Sea as an example.
Stock enhancement of flounder in the Seto Inland Sea has been conducted since 1978, with the release of over two million fish since 1990. Records of age-specific catch information and corresponding analyses of the efficiency with which released fish were successfully recruited date back to 1994. According to the sampling data from commercial landings, the ratio of released fish in the total recruitments was, on average, over one quarter from 1994 to 2017, indicating a substantial contribution from released fish. However, it is still unclear whether stock enhancement purely increased the stock level or if released fish replaced the niche of natural stock.
We compared the performance of maximum likelihood (ML) and maximum a posterior (MAP) estimation in state-space production model in the twin experiments and 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 in the Seto Inland Sea using catch per unit effort data (1968-2018), catch data (1968-2017), number of released fish (1979-2017). Empirical biomass estimated by the tuned virtual population analysis was used in the MAP estimation. We calculated the net biomass augmentation from the difference between cases with and without stock enhancement. The results showed a limited contribution of stock enhancement (1979-2018) 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, we further studied the benefit of stocking for the next ten years under three different fishing pressures. In the future prediction (2019-2028), stock enhancement was more effective at bringing augmented biomass when fishing pressure was higher, but that total biomass decreased simultaneously. Therefore, a balance between stock enhancement and exploitation is important to obtain sustainable production and efficient stocking.
The content of this study was submitted to Fisheries Research and is now on review. This research was supported by Kakenhi JP21H04735.
Surplus production models are important methods for conducting stock assessments in data-limited fisheries. Recent advancements have cast surplus production models into state-space production models that differentiates two distinct sources of variability: process error in the surplus production model and measurement error between the underlying biomass states and observation data. State-space production models has been increasingly used in fishery management but not in the evaluation of stock enhancement effects. Therefore, in this study, we aimed to develop a state-space production model to evaluate the stock enhancement effect using Japanese flounder (Paralichthys olivaceus) in the Seto Inland Sea as an example.
Stock enhancement of flounder in the Seto Inland Sea has been conducted since 1978, with the release of over two million fish since 1990. Records of age-specific catch information and corresponding analyses of the efficiency with which released fish were successfully recruited date back to 1994. According to the sampling data from commercial landings, the ratio of released fish in the total recruitments was, on average, over one quarter from 1994 to 2017, indicating a substantial contribution from released fish. However, it is still unclear whether stock enhancement purely increased the stock level or if released fish replaced the niche of natural stock.
We compared the performance of maximum likelihood (ML) and maximum a posterior (MAP) estimation in state-space production model in the twin experiments and 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 in the Seto Inland Sea using catch per unit effort data (1968-2018), catch data (1968-2017), number of released fish (1979-2017). Empirical biomass estimated by the tuned virtual population analysis was used in the MAP estimation. We calculated the net biomass augmentation from the difference between cases with and without stock enhancement. The results showed a limited contribution of stock enhancement (1979-2018) 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, we further studied the benefit of stocking for the next ten years under three different fishing pressures. In the future prediction (2019-2028), stock enhancement was more effective at bringing augmented biomass when fishing pressure was higher, but that total biomass decreased simultaneously. Therefore, a balance between stock enhancement and exploitation is important to obtain sustainable production and efficient stocking.
The content of this study was submitted to Fisheries Research and is now on review. This research was supported by Kakenhi JP21H04735.