17:15 〜 18:30
[AOS13-P05] A state-space production model to analyze the impact of stock enhancement on population dynamics: a case study of Japanese flounder in the Seto Inland Sea
キーワード：放流効果、状態空間モデル、ヒラメ
More than 26 billion juveniles of 180 marine species have been released annually into the wild in over 20 countries. Because it is not easy to separate the wild fish and hatchery fish in the natural condition and clarify how released fish influence the biomass of the wild stock, the usefulness of stock enhancement remains unclear (Kitada, 2020). Recapture rate has been adopted as an important index to measure the success of the stock enhancement, and the corresponding cohort models were used to analyze the biomass of the hatchery stock separated by the recapture ratio. Lorenzen (2005) added the density-dependent parameters and depicted the growth of the hatchery cohort more rationally. However, those models require a huge amount of information, and generally, the dimensions and quality of data are not enough in the fishery. Thus, new models should be developed to cope with those situations. We developed state-space production models to answer the question about “Do hatcheries produce extra fish?”, and applied it in the fishery of Japanese flounder Paralichthys olivaceus in the Seto Inland Sea to shed some light on the effect of stock enhancement.
First, we developed the state-space production model (SSPM) to elucidate the fishery with stock enhancement. Considering the data with the large process and observation errors in the real fishery might vague the hidden information, maximum likelihood augmented with prior information (Maximum a posterior or MAP) was used in the parameter estimation. Time-trajectories of biomass and catch per unit effort (CPUE) data were simulated to test the reliability of MAP. Then, we used the SSPM to evaluate key parameter values that influence the biomass variation. The biomass increment by the stock enhancement can be extracted from the difference between the estimated current biomass and the hypothesized biomass trajectories without hatchery releasement. Furthermore, we used the Monte Calo Markov chain to calculate the distribution of process error, and projected future biomass under the different fishing pressures.
As a result, although the whole biomass resulting from the stock enhancement in the Seto Inland Sea has been increased until 2005, the extra biomass showed more moderate growth. It suggested the hatchery stock did not contribute to increasing much extra biomass, instead, it tended to “replace” the niche of the wild fish. In the future projections, the biomass increment by the stock enhancement was increased under higher fishing pressure, but the total biomass was reduced simultaneously. Therefore, the balance between the stock enhancement and the exploitation is important.
First, we developed the state-space production model (SSPM) to elucidate the fishery with stock enhancement. Considering the data with the large process and observation errors in the real fishery might vague the hidden information, maximum likelihood augmented with prior information (Maximum a posterior or MAP) was used in the parameter estimation. Time-trajectories of biomass and catch per unit effort (CPUE) data were simulated to test the reliability of MAP. Then, we used the SSPM to evaluate key parameter values that influence the biomass variation. The biomass increment by the stock enhancement can be extracted from the difference between the estimated current biomass and the hypothesized biomass trajectories without hatchery releasement. Furthermore, we used the Monte Calo Markov chain to calculate the distribution of process error, and projected future biomass under the different fishing pressures.
As a result, although the whole biomass resulting from the stock enhancement in the Seto Inland Sea has been increased until 2005, the extra biomass showed more moderate growth. It suggested the hatchery stock did not contribute to increasing much extra biomass, instead, it tended to “replace” the niche of the wild fish. In the future projections, the biomass increment by the stock enhancement was increased under higher fishing pressure, but the total biomass was reduced simultaneously. Therefore, the balance between the stock enhancement and the exploitation is important.