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Like many other countries, the economy and society of Japan have been deeply affected by the Covid-19 outbreak, and the fishery sector is no exception. This study takes an interdisciplinary approach to analyze the economic and social impacts of the pandemic on Japanese fisheries, gauging the extent and nature of the damages incurred from Covid-19 while helping to provide tailored post-recovery recommendations for the industry. Using results from an online survey questionnaire (N = 429) and compiling additional economic information from public sources, this study revealed the overwhelmingly negative changes in sales figures and overall financial security that survey participants experienced when compared to a year earlier. High-value and fresh fish species were also significantly affected in 2020 across Japan, in line with similar trends across the developed world. Aquaculture businesses were shown to be more vulnerable to the spread of Covid-19 than small-scale fishing operations, which tend to be more diverse and flexible. Bonding social capital was also shown to be important for mutual help and human well-being, especially among small-scale fishers. This "customary nature" of Japanese fisheries, at the same time, can be seen as a barrier to the transformation of the industry. Given these results, several policy implications are discussed to help fisheries stakeholders and their communities build back better from the Covid-19 pandemic.

Declines in landings of the hairtail Trichiurus japonicas indicate the need for more effective management of this species. Hairtail spawning peaks occur twice yearly in the Bungo Channel, in spring and autumn. Relationships between hairtail stock and brood seasonality were examined to determine if an association between either and a decline in landings existed. Stock assessments show that the biomass of both spring and autumn hairtail broods from within and around the Bungo Channel are decreasing, with a rapid reduction in spring-brood stock abundance after 2007 largely responsible for decreased landings. Yield and spawning per recruitment analyses indicate current fishing pressure to be higher than several reference points. We suggest that fishing pressure needs to be reduced by at least 20% of the current level for this fishery to remain sustainable, as the projected stock abundance and catch demonstrate that the current fishing pressure is unsustainable. Analysis of time-series data of recruits per spawning revealed spring-brood recruitment to have been strong in year classes 2003 and 2005. Of various options available for improved management of this fishery, we propose that fishing pressure should be reduced in the years following the appearance of strong year classes to increase future biomasses and landings.

Under the coastal fisheries co-management regime in Japan, local fishers play important roles in deciding on and implementing local management measures. Therefore, using the case study of a hairtail trolling line fishery, we conducted a transdisciplinary research with fishers, processors, government officers, etc. Taking social and biological factors into account, we defined management criteria and targets (resource, economic, human community, and local food culture) in collaboration with local fishers. We chartered local fishing vessels to determine parameters that were easily understood by local fishers. We established a local consultative committee comprising fishers, processors, and distributors, and developed a fish distribution strategy to nurture the local hairtail food culture. The integrated effects of various management scenarios were evaluated by the operating model. We found that co-ordinated management by both the trolling line fishery and purse seiners in the neighbouring prefecture was required to achieve targets. Two realistic scenarios are recommended: a conservative scenario that could achieve targets even with few strong year classes, and an adaptive scenario that protects strong year classes, but relies on a higher frequency of these. We shared these results with the local fishers and government officers, which led to actual improvements in management measures.

Overfishing has reduced the stock of hairtail Trichiurus japonicus around the Bungo Channel, Japan. To determine whether using larger bait in the trolling line fishery could avoid catching small/undersize hairtail, we developed and trialled a new large artificial bait (a soft plastic bait, 6 inches long). A traditional lure with natural bait (type-1), a new lure of the new artificial bait hook without natural bait (type-2), and a new lure with natural bait (type-3) were tested in fishing operations around the Bungo Channel. Compared with type-1, type-2 and type-3 caught fewer undersized and immature female individuals. The number of fish caught, yield, and composition of commercial size grades per recruitment were calculated from field data for each lure and compared. The catch sizes (number of fish per recruitment) were smaller for type-2 and type-3 than for type-1, but yield per recruitment was higher for type-3 than for type-1. Compared with type-1, type-2 and type-3 caught more large individuals, which are more valuable. The newly developed artificial bait conserves hairtail stocks by targeting larger fish, which is economically beneficial for the hairtail trolling line fishery.

Conservation of marine ecosystems is important for the sustainable utilization of marine resources under environmental changes and anthropogenic impacts. In Japan, fishery management has been based on single-species stock assessments and little attempts have been made to assess and manage the impacts of multi-species multi-gear fisheries except for a few local case studies. In this study, we analyzed two local Ecopath food-web models constructed for Japanese waters (Northeastern Japan and Seto Inland Sea). We adopted a comparative approach to assess the ecosystem characteristics and fishery impacts through a comparison of these models with other published Ecopath models for various periods and regions of the world ocean. Among the metrics derived from Ecopath outputs, total biomass and total productivity represented the gross metabolic activities of the systems, and trophic level fractionation of biomass and catch reflected the trophic structure of the food webs and fishery removals. Loss of food for top predators, primary production required, and mean and variance of trophic level of catch were good indicators of the impacts and patterns of fishery removals. Our results illustrate the characteristics of food webs and fisheries in Japanese local ecosystems: The Northeastern Japan model showed the moderate productivity of the ecosystem, and fishery removals from lower and wider trophic levels. The Seto Inland Sea model indicated large impacts of fisheries on the low productivity neritic system. Such profiling of ecosystems and fisheries provides good overviews that will help initiate stakeholder discussion toward the set-up phase of ecosystem-based fishery management in Japan.

Age determination and growth using otolith rings in Muraenesox cinereus was re-examined in the western Seto Inland Sea, Japan. Previous study in this area indicated that new rings were formed annually from March to May, and then from September to October once individuals had achieved four or five rings. In this study, monthly changes in marginal growth rate indicated that the first ring was formed before November in the year following hatching, and from then on another ring was formed annually in July or August. The birth month was determined to be August based on a peak in monthly change in the gonad somatic index. Estimated von Bertalanffy growth functions were L = 806.6{1 − exp[− 0.33(t + 0.06)]} and L = 1264.0{1 − exp[− 0.19(t + 0.15)]} for males and females, respectively. Lengths after 3 years of age in this study were 100 mm longer than those in a previous study for both sexes.

瀬戸内海豊前海のシャコについて，2007～2009 年に小型底曳網で試験操業を実施し，資源評価を行った。2007 年級群を用い，加入以降半年ごとの漁獲係数を推定したところ，加入直後から投棄による死亡がみられた。加入後 1 年間の生残率は 13.4% で，水揚対象サイズに達するまでの減耗が大きかった。加入あたり漁獲量と加入あたり産卵量の解析の結果，漁獲量と産卵量の双方を増大させるには，着底から 2 年間の漁獲圧を削減することが効果的であり，特に投棄による死亡の多い，着底翌年の夏の漁獲圧削減の重要性が示唆された。

Age and growth of the yellowstriped butterfish, Labracoglossa argentiventris, around Izu Oshima Island were studied using a total of 1450 fish. Age was determined by counting the edge of the opaque zones as a ring mark on sectioned sagittal otoliths. Formation of the first ring was observed during spring or summer, corresponding to 1.5 years after hatching. Thereafter, one ring was formed each year in the same season as the previous year. The growth of the butterfish was rapid until 2 years of age. The maximum likelihood method was applied to the age and length data for estimating parameters in von Bertalanffy, Gompertz, and Logistic growth models. The selected model, based on the Akaike Information Criterion, was the von Bertalanffy growth model, which indicated differential asymptotic length and variance by sex.

Design of practical structures can involve difficulties when it is based only on analytical models. Experimental data are therefore often made use of for such cases. There have been many studies on identification of statistical and dynamical characteristics of structural systems, making use of experimental data. In this study, we propose a method to identify the characteristics of structures with proper accuracy by using a hierarchical neural network (HNN) that has excellent projection ability even for systems which have nonlinearities. HNN has been generally used to learn dynamic characteristics with minimum number of experimental data. We use HNN as the solver of an optimum design, that is to estimate the dynamic characteristics and sensitivities. The effectiveness of this method is demonstrated through several numerical examples and experiments. Then we utilize the derived sensitivities for gradient-based optimization techniques and to solve the optimum design problems. It is found that the proposed method is valid and effective for such optimum design problems.

In performance assessment of geological disposal systems the phenomenon of dispersion in geological media is one of the most important processes to be modeled and is dependent on the heterogeneity of the media. In order to understand the dispersion process in a well defined heterogeneous field, laboratory experimental apparatus, named MACRO, was constructed to study fluid flow and mass transport. A synthetic heterogeneous field was constructed in a flow-bed using six kinds of glass beads with diameters of 0.1, 0.15, 0.2, 0.4, 0.6 and 0.8 mm. Both dye (brilliant blue) and NaCI solution were used as tracers in the experiment. A particle tracking method, considering advection, was used in numerical simulations of the experiment, and the dispersion effect was assumed to be expressed by the variability of the local advective velocity in a high resolution numerical grid. The simulated results obtained by the particle tracking method agreed with the measurement, confirming the applicability of modelling approach used in this study.