北太平洋狭鳕资源声学评估调查研究

１９９３年夏季使用“北斗”号渔业调查船对北太平洋的狭鳕进行资源声学评估和渔场环境调查，调查覆盖面积达７６万平方公里。调查结果表明：夏季白令海阿留申海盆区５０－２００ｍ为冷水团占据，水温为１．３－４．９℃。狭鳕成鱼主要分布于海盆区东南部及公海区，主要栖息在１７５－２２５ｍ层。在公海区东北部与陆架之间，首次发现大量狭鳕当生幼鱼，主要栖息在８０－１００ｍ层。成鱼和幼鱼均有明显昼夜垂直移动。海盆区狭鳕平均     

Temperature-dependent stock-recruitment model for walleye pollock (Theragra chalcogramma) around northern Japan

Changes in fish year‐class strength have been attributed to year‐to‐year variability in environmental conditions and spawning stock biomass (SSB). In particular, sea temperature has been shown to be linked to fish recruitment. In the present study, I examined the relationship between sea surface temperature (SST), SSB and recruitment for two stocks of walleye pollock (Theragra chalcogramma) around northern Japan [Japanese Pacific stock (JPS) and northern Japan Sea stock (JSS)] using a temperature‐dependent stock‐recruitment model (TDSRM). The recruitment fluctuation of JPS was successfully reproduced by the TDSRM with February and April SSTs, and February SST was a better environmental predictor than April SST. In addition, the JPS recruitment was positively related to February SST and negatively to April SST. The JSS recruitment modeled by the TDSRM incorporating February SST was also consistent with the observation, whereas the relationship between recruitment and February SST was negative, that is the opposite trend to JPS. These findings suggest that SST in February is important as a predictor of recruitment for both stocks, and that higher and lower SSTs in February act favorably on the recruitment of JPS and JSS respectively. Furthermore, Ricker‐type TDSRM was not selected for either of the stocks, suggesting that the strong density‐dependent effect as in the Ricker model does not exist for JPS and JSS. I formulate hypotheses to explain the links between SST and recruitment, and note that these relationships should be considered in any future attempts to understand the recruitment dynamics of JPS and JSS.     

Evaluation of stock management procedures for walleye pollock in northern waters of the Sea of Japan using a simulation

To evaluate walleye pollock stock management procedures in the northern waters of the Sea of Japan, 30-year population dynamics, including uncertainties, were forecast. Errors in current stock size estimation, variability in future recruitment and changes in future fishing mortalities were incorporated. Results of virtual population analysis (VPA) from resampled catch-at-age data with bootstrap methods was used as the current stock size estimation with uncertainty. Performances of each scenario were evaluated using conservation, utilization, stability and reliability factors. Twenty-two management scenarios and continuing the current fishing mortality were evaluated. Scenarios with minor regulation changes and continuing the current fishing mortality showed poor stock conservation performances. Scenarios with minor regulation changes produced good short-term but poor long-term utilization. Stabilities were poor in continuing the current fishing mortality and fishing ban scenarios. Reliability in all scenarios after 30 years was smaller than in continuing the current fishing mortality; however, small differences among scenarios were observed. The simulation results indicated that multilateral assessment is needed to evaluate the management candidates. Uncertainty caused by recruitment variability mostly affected future population dynamics. The role of simulations in the production of effective scientific advice is discussed.     

Market and consumption of walleye pollock in Korea

Walleye pollock Theragra chalcogramma is the most popular fish in Korea, often being used in a variety of food delicacies. However, since 2000, production of walleye pollock from distant-water fishing has been rapidly decreasing, and its domestic production is slowly dying out. Therefore, supply of walleye pollock to the Korean market has become heavily dependent on imports. Based on the supply–demand situation of walleye pollock in the Korean market and analyses of its production and markets, it can be undoubtedly predicted that there is a large potential for consumption of walleye pollock. From past records of walleye pollock consumption, Korea’s potential walleye pollock consumption can be estimated to be around 500,000 tons. However, it seems that the prospect of supplying such a high demand is not very bright.     

Processing technology for walleye pollock: surimi innovation

Imitation crab meat, first developed in Japan, is very popular in restaurants in Paris, appearing as “surimi” on the menu. Today, more than 300,000 tons of imitation crab meat are produced in modern imitation crab meat plants in North America, Europe, and Russia, and the term “surimi” has gained acceptance internationally. The term “surimi” originally referred to frozen surimi, an intermediate material for fish paste products. Frozen surimi was developed from walleye pollock, in Japan, in 1959, marking the beginning of the use of intermediate material for aquatic food products. As intermediate food materials such as wheat flour and starch paved the way for the modernization of the agricultural food industry and the industrialized production of agricultural food, so the development of frozen surimi made worldwide production of imitation crab meat possible, and paved the way for the modernization of the aquatic food industry and the industrialized production of aquatic food based on the results of chemical and technical research. Two major developments, namely frozen surimi and imitation crab meat, promoted the use of walleye pollock and led to innovation in aquatic food production technology.     

Causes of walleye pollock (Theragra chalcogramma) recruitment decline in the northern Sea of Japan: implications for stock management

Recruitment of the northern Japan Sea stock (JSS) of walleye pollock has been decreasing since around 1990. In this study, I analyzed the factors causing this decrease in recruitment by investigating the relationship between recruitment, spawning stock biomass (SSB) and environmental factors using a generalized additive model (GAM). GAM fit to the data showed the importance of SSB, sea surface temperature (SST), ocean current strength (Tsushima Warm Current) and wind intensity (Asian monsoon) in determining the recruitment. Of these, the relationship between SSB and recruitment was positive and not negatively density‐dependent. On the other hand, the recruitment was negatively related to SST and ocean current strength, and a dome‐shaped relationship was observed between wind intensity and recruitment. Since around 1990, the values of SST and ocean current strength have mostly been high and that of wind intensity mostly low. In addition, SSB has been decreasing since the late 1990s. It is likely that the recruitment decline of JSS after approximately 1990 has been caused by warm water temperature, strong Tsushima Warm Current and weak Asian monsoon, and that the recent decrease in SSB has amplified this recruitment decline. According to the model’s estimation, a recruitment recovery due to environmental improvement will be highly restricted as long as SSB remains at its current low level. Significant recovery of SSB is urgently needed for JSS.     

Relationship between sea-surface temperature and catch fluctuations in the Pacific stock of walleye pollock in Japan

ABSTRACT:   This paper investigates the relationship between sea-surface temperature (SST) and catch fluctuations in the Pacific stock of walleye pollock Theragra chalcogramma in Japan. Incorporating time lags between years of birth and harvest, the correlation coefficients between the catch and SST in two regions off the east coast of Hokkaido were calculated. The catch in year t had a high negative correlation with the SST during January–April and November–December of the years t- 2 and t- 3 in the spawning area. These results coincided well with the correlation observed in the northern 'Sea of Japan' stock. Both analyses suggested that the long-term catch fluctuations of the two stocks could be explained by the same mechanism, that is, the fluctuations would be explained by the SST in their spawning area during the spawning season using 2–3 or 3–5 years time lags, which corresponded to the dominant age of the catch within these two stocks.     

Variations in larval walleye pollock feeding and condition: a synthesis

There was a strong association among concentrations of microzooplankton prey sampled from the walleye pollock, Theragra chalcogramma , larval habitat, gut contents of larvae, and their nutritional condition. Subsequently, hypothesized survival potentials linked to food availability were validated by independently determined mortality rates. We present evidence that a significant number of walleye pollock larvae were starving in 1991 but that fewer were starving in 1992. At some stations where prey levels were anomalously low in 1991, up to 40% of the larvae were in poor condition. There appears to be a 2-week period after first feeding when walleye pollock are vulnerable to starvation.     

Effects of sampling errors on abundance estimates from virtual population analysis for walleye pollock in northern waters of Sea of Japan

ABSTRACT:   Effects of sampling errors on abundance estimates from virtual population analysis (VPA) were quantified with the bootstrap method for stock of walleye pollock in the the Sea of Japan. In the bootstrap method, individual fish measurements were resampled. A total of 1000 bootstrap samples were produced for each year from 1991 to 2001. The coefficients of variation (CV) of catch at age in 2001 ranged 6.1–33.1%. The CV of an abundance estimate in 2001 ranged 9.0–35.7%. Abundance estimates of the oldest age and the latest year, which had larger CVs than the other estimates, were sensitive to sampling errors. Effects of sampling errors became smaller when the catch at age had been accumulated over a few years. Although VPA includes various types of errors, only the sampling errors have room for improvement in reality. Quantifying the effect of sampling error on VPA estimates is essential for sound and efficient stock management, and is emphasized in this study.     

Comparison of factors affecting recruitment variability of walleye pollock Theragra chalcogramma in the Pacific Ocean and the Sea of Japan off northern Japan

The Japanese Pacific stock (JPS) and the northern Japan Sea stock (JSS) of walleye pollock Theragra chalcogramma are mainly distributed in the Pacific Ocean and the Sea of Japan off northern Japan, respectively. This paper summarizes and compares the factors affecting the recruitment variability of these two stocks. Spawning season is from December to March for both stocks. JPS recruitment has a positive relationship with the water temperature in January and February, whereas that of JSS has a negative relationship with the water temperature in January, February, and April. One possible reason for this is that pollock larvae have an optimum growth temperature of approximately 5 °C in the field. Drift of early life stages also appears to be an important influence on the recruitment of both stocks. Because the current generated by the northwest wind carries eggs of JPS into the main larval nursery ground, JPS recruitment is enhanced in years when the northwest wind is predominant in February. On the other hand, early life stages of JSS are transported into the nursery ground by the Tsushima Warm Current. However, this current also carries early life stages into the Sea of Okhotsk and offshore, resulting in poor JSS recruitment in years when this current is strong in March. In contrast to JPS, the recruitment of which is significantly impacted by cannibalism, young pollock have not been found in the stomachs of adult JSS. Warm temperatures in the Sea of Japan seem to induce the separation of young and adult pollock, and the shape of the stock–recruitment relationship also suggests that cannibalism is not important for JSS. Based on this knowledge, and on the hatch date distributions of larvae and juveniles, we propose mechanisms that can explain the recruitment fluctuations for JPS and JSS pollock.     

Marketing and distribution of walleye pollock: past,current and future developments

In this paper, we examine the structure and change of the walleye pollock industry from a marketing and distribution perspective. Our focus is not only on industrial internationalisation, but also on local industry and community utilisation of walleye pollock. This is a subject of interest because its diverse utilisation has the potential to promote industrial management as well as the livelihood of local communities. We reason that Japan’s weakness in marketing, distribution and price setting of walleye pollock is largely due to its lack of competitiveness in the world market. Given the rapid decline of walleye pollock utilisation by local industry and society, it is of critical importance to take steps towards a long-term solution considering the rapid decline of walleye pollock use by our local industry and society. Our analysis indicates that Japan’s walleye pollock industry is inherently resilient, with diverse commodity chains, business flexibility and traditional food culture. However, as local industry and community have been reducing use of walleye pollock over time, there is a need to restore and reactivate the local society.     

Annual variation in otolith increment widths of walleye pollock (Gadus chalcogrammus) larvae in Funka Bay,Hokkaido, Japan

To clarify relationships between year‐class strength and larval growth of walleye pollock (Gadus chalcogrammus), and oceanographic conditions in the Pacific stock off Hokkaido and Tohoku, Japan, we undertook conductivity/temperature/depth (CTD) observations and investigated larval densities, larval otolith increment widths and larval prey densities (of copepod nauplii) of the 2008, 2009, 2010 and 2011 yr classes in Funka Bay. Oyashio Coastal Water (OCW) flowed into the bay in late February in 2008, 2010 and 2011, and the mean water temperatures decreased to 1.9–3.1 °C in March. OCW was not observed in 2009, and it was warm in late February (≥3.4 °C). Increment widths of lapillar otoliths during the yolk‐sac stage were wide in 2009 and 2011, medium in 2010 and narrow in 2008. Increment widths during the first‐feeding stage tended to become wider as the hatch month progressed, and the annual variation during the first‐feeding stage was larger than that of the yolk‐sac stage. The densities of the primary food for the larvae were high in 2008 when larval increment widths were narrowest, so the effect of prey abundance on larval growth appeared to be small. The ranking of the larval abundance in March was nearly coincident with that of the increment width during the larval stage. We, therefore, suggest that the larval growth rate is associated with the mortality rate and that the growth–mortality hypothesis may be applicable to walleye pollock in Funka Bay. Feeding success under warm water conditions may be an important factor that contributes towards high growth rates.     

Life-history traits of walleye pollock, Theragra chalcogramma, in the northeastern Japan Sea during early to mid 1990s

I examined the age, growth, maturity, mortality, and body condition of walleye pollock, Theragra chalcogramma, in the northeastern Japan Sea (northern Japan Sea population) and evaluated their resilience to exploitation. Walleye pollock were collected in pre-spawning (October 1991-1995) and post-spawning (April 1990-1996) seasons. Estimated ages ranged from 3 to 18 years for both sexes. A von Bertalanffy growth model showed that females had longer asymptotic fork length (460 mm) than males (425 mm). Fifty percent of females and males were mature at 348 mm (4.6 years) and 322 mm (3.9 years), respectively. The instantaneous natural mortality rate was estimated to be 0.22. These life-history traits in the northern Japan Sea population were compared to those in the Bering Sea, the Gulf of Alaska, and the Japan Pacific populations. As a result, female walleye pollock in this population matured at small body sizes, grew rapidly toward small maximum sizes, and had short reproductive lifespans with low size-specific fecundity and poor body condition. Low prey availability and habitat temperatures are considered as a possible mechanism for the small maximum sizes in this population. The potential rate of population increase of both the northern Japan Sea population and other pollock populations tended to be lower than other exploited populations of non-viviparous marine fishes, suggesting potentially lower resilience to exploitation in this population and walleye pollock populations in general.     

Effects of spawning temperature on the reproductive characteristics of walleye pollock Gadus chalcogrammus

Fisheries Science - Reproductive characteristics under different water temperature regimes were examined in hatchery-reared walleye pollock Gadus chalcogrammus. The fish had been cultured for...     

Abiotic and biotic factors affecting recruitment variability of walleye pollock (Theragra chalcogramma) off the Pacific coast of Hokkaido,Japan

Abiotic and biotic factors affecting the recruitment variability of the Japanese Pacific stock (JPS) of walleye pollock (Theragra chalcogramma) were examined using a bivariate regression and multivariate combined model. Of the abiotic variables around Funka Bay (spawning ground), February sea surface temperature (SST) and wind direction index showed significant bivariate relationships with recruitment. February SST was positively related to recruitment, suggesting that warmer water temperature in February favors JPS recruitment. On the other hand, the relationship between February wind direction index and recruitment predicts high JPS recruitment under predominant northwest winds in February. For the biotic variables in the Doto area (nursery ground), significant and negative bivariate relationships with recruitment were observed for catch per unit effort of Kamchatka flounder (Atheresthes evermanni), Pacific cod (Gadus macrocephalus), and walleye pollock, implying an important impact of predation by these groundfishes on JPS recruitment. The overall model incorporating these abiotic and biotic factors successfully reproduced the variability in JPS recruitment. Temperature and wind conditions around the spawning ground along with predator condition in the nursery ground appear to play a dominant role in the recruitment dynamics of JPS. Based on these results and prior knowledge, we propose a new hypothesis to explain the processes controlling JPS recruitment.     

Anomalous transport of walleye pollock larvae linked to ocean and atmospheric patterns in May 1996

Larvae from a large aggregation of walleye pollock spawning in early spring in Shelikof Strait, Gulf of Alaska, are normally transported to the south-west in the vigorous Alaska Coastal Current. In the spring of 1996, anomalous winds resulted in unusually weak transport in the Shelikof Strait sea valley. The main aggregation of larval pollock in the Shelikof region was surveyed four times in 1996 over a period of about 40 days, including finer-scale sampling of the leading south-western edge of the larval distribution. The south-western edge of the larval distribution showed weak transport up the sea valley for a period of about 10 days, corresponding to the observations of currents, after which many larvae were transported over the shelf region to the west. These observations are unique in over 15 years of monitoring larval transport patterns and demonstrate how anomalous weather, and hence current patterns, influence variability in larval transport.     

Introduction: from the birth to the table of walleye pollock Theragra chalcogramma

Fisheries provide food, feedstuff, and materials which are reliant on ecosystem services provided by marine and freshwater systems. Fish spawn and mature in aquatic systems, from which they are harvested by fishers, distributed and processed in households, restaurants, or processing plants, and either eaten by consumers, used in aquaculture, horticulture or agriculture, or used in manufacturing. The movement of fish from sea to plate follows distinct sequential stages. We call this the “fisheries system”. While each stage of the fisheries system has been the focus of specific research activities, these activities have generally proceeded from the perspective of individual disciplines, e.g., oceanography, chemistry, biology, ecology, resource dynamics, economics, business management, food science, processing, nutrition studies, etc. As a result, their objectives, data, models, discussions, etc., do not present an effectively integrated perspective of the fisheries system as a whole. This Special Feature brings together research from across disciplines to provide a broad perspective of the walleye pollock Theragra chalcogramma fisheries system. While this falls short of being a fully integrated model, it highlights key interfaces among disciplines and illustrates fundamental research questions for each stage. By highlighting these questions and interfaces, we hope to foster an intellectual environment that will lead to true “integrated research” that can best be pursued by multi-disciplinary teams rather than by individual scientists who limit their research activities to the narrow scope of their specific disciplines.