Momoko Ichinokawa

Spatiotemporal catch distribution of age-0 Pacific bluefin tuna Thunnus orientalis caught by the Japanese troll fishery in relation to surface sea temperature and seasonal migration

This study used a delta-lognormal model to analyze monthly catches of age-0 Pacific bluefin tuna by the troll fishery. The model included fixed effects of month, area, and month–area interaction, and random effects of port, year and port–year interaction. The catch patterns by month and area predicted by the statistical model (standardized catch) revealed that main fishing grounds along the Tsushima Warm Current generally shifted from north to south as the season turned from autumn to winter. In contrast, the standardized catch along the Kuroshio Current did not show such clear spatiotemporal patterns. The standardized catch along the Tsushima Warm Current is significantly associated with average monthly sea surface temperatures in the fishing grounds and consistent with migration routes revealed by tagging experiments in previous studies. These associations indicate the spatiotemporal catch pattern in the Tsushima Warm Current region partly reflects seasonal migration. Knowledge of the possible associations among fish migration, environmental factors and spatiotemporal distribution of the catch will contribute to future management of this species.

The status of Japanese fisheries relative to fisheries around the world

The status of Japanese fisheries relative to fisheries around the world Momoko Ichinokawa*, Hiroshi Okamura, and Hiroyuki Kurota Research Center for Fisheries Management, National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, 2-124 Fukuura, Kanazawa, Yokohama, Kanagawa, 236-8648, Japan Fisheries Management and Oceanography division, Seikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 1551-8, Taira-machi, Nagasaki-shi, Nagasaki, 851-2213, Japan *Corresponding author: tel: þ81 45 788 7645; fax: þ81 45 788 5001; e-mail: ichimomo@affrc.go.jp.

Ridge virtual population analysis to reduce the instability of fishing mortalities in the terminal year

Ridge virtual population analysis to reduce the instability of fishing mortalities in the terminal year Hiroshi Okamura*, Yuuho Yamashita, and Momoko Ichinokawa Research Center for Fisheries Management, National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, 2-124 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan Fisheries Management Department, Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 116 Katsurakoi, Kushiro, Hokkaido 085-0802, Japan *Corresponding author: tel: þ81 45 788 7514; fax: þ81 45 788 5004; e-mail: okamura@fra.affrc.go.jp

Impacts of the nonlinear relationship between abundance and its index in a tuned virtual population analysis

The abundance index used in a tuned virtual population analysis (VPA) is usually assumed to be proportional to actual abundance. However, the actual abundance and abundance index do not always have a linear relationship. Such nonlinearity can cause biases in abundance estimates as well as retrospective biases arising from systematic differences in abundance estimates when more data are successively added. Severe retrospective biases can damage the reliability of stock assessments. In this study, we use an approach to estimate an additional parameter that controls the nonlinearity in a tuned VPA. A performance test using simulated data revealed that the tuned VPA was able to accurately estimate the nonlinearity parameter and thus yielded less biased abundance estimates and smaller retrospective biases. We also found that estimating the nonlinearity parameters was effective even under other model misspecification scenarios, such as disregarding historical increases in catchability and time-varying natural mortality. Moreover, we applied this approach to some Japanese fish stocks and evaluated its validity. We found that estimating the nonlinearity parameters in the tuned VPA enhances the reliability of fisheries stock assessments.

Ecosystem Model in Data-Poor Situations

Ecosystem assessment is one of the most interesting topics in ecology and fisheries science; modeling is an essential and indispensable part of ecosystem assessment. We briefly review existing ecosystem models that are employed around the world and present a new ecosystem model that can be applied in data-poor situations, e.g., when diet-composition data are unavailable. The new model is based on a multivariate state-space model with an allometric relationship between the biological parameters and body mass. The model generally does not require unrealistic assumptions, such as equilibrium prior to fishing and mass balance during a certain period. The simulation study demonstrated that the model outperformed a single-species assessment in terms of the inference of biological reference points. As an illustration, we applied the model to environmental index data and three species in the western North Pacific, which are known to show conspicuous species replacement (chub mackerel, sardine, and anchovy). The model can be extensively applied to various multispecies data in data-poor situations.

Comparison of the performance of age-structured models with few survey indices

Comparison of the performance of age-structured models with few survey indices Hiroshi Okamura*, Yuuho Yamashita, Momoko Ichinokawa, and Shota Nishijima Research Center for Fisheries Management, National Research Institute of Fisheries Science, Fisheries Research and Education Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan Fisheries Management Department, Hokkaido National Fisheries Research Institute, Fisheries Research and Education Agency, 116 Katsurakoi, Kushiro, Hokkaido 085-0802, Japan *Corresponding author: tel: þ 81 45 788 7514; fax: þ 81 45 788 5004; e-mail: okamura@fra.affrc.go.jp.

Incorporating accessibility limitation into the surplus production model

Limited access to aquatic populations hinders estimation of their status and establishment of effective management measures. We propose a modified surplus production model to cope with this problem. The model provides population parameters and biological reference points from a time series of annual accessible abundance data. Simulation tests showed that the model provided biological reference point estimates with little bias when sufficiently long time series were available. Even for short time series, we could obtain nearly unbiased estimates by providing information on the exploitation rate at the maximum sustainable yield (FMSY). As an application, we fit the modified surplus production model to 7-year accessible biomass estimates of a local population of Japanese spiky sea cucumber Apostichopus japonicus using a Bayesian approach. The results indicated that the stock in the area studied was likely to have experienced recent overfishing and had a high probability of being overfished in the future.