No.79 (March,2011)
No.79 (March,2011)
A－461 Osamu SHIDA
Studies on influence of oceanographic conditions in the spawning season on interannual fluctuations in recruitment of the Japanese Pacific walleye pollock (Theragra chalcogramma) stock
The Japanese Pacific walleye pollock (Theragra chalcogramma) stock (JPS) which is widely distributed off the Pacific coast of northern Japan is one of the most important target species for local fisheries. Resources of this species have been managed under the TAC rules that require estimation of stock abundance. Therefore, it is necessary to have a clear grasp of the relationship between the ocean environment and the variation in the stock abundance of walleye pollock, and to device a suitable method of fisheries management based on pertinent biological information. I have proposed a conceptual model of the mechanisms that determine JPS year-class strength, which appear to be the major factor that drives changes in stock biomass. In this model, successive controls are believed to be important for determining year-class strength. These controls or “switches” are factors related to survival in each stage from spawning adults to recruits. This study focused upon these switches in the spawning adult stage, as adult fish are the most important target for commercial fisheries. The objectives of this study were to investigate the interannual variability of the timing and location of spawning, and the influence of oceanographic conditions on them. Furthermore, I addressed the relationship between this variability and landings of commercial fisheries. The distribution and abundance of spawning adults in Hidaka Bay, the main spawning ground, were observed from 1998 to 2008 using a quantitative echo sounding system, and the data were analyzed together with oceanographic observation data using the Geographic Information System (GIS) method. The results showed that adult pollock migrate from the Doto area, the major feeding ground, to Hidaka Bay along the Cold Lower-layer water. They then moved to the shallower continental shelf region covered with Oyashio water (OW) in the spawning period. After spawning, they migrated to the Doto area along the OW. The timing of spawning migration to Hidaka Bay showed interannual vatiation that was affected by water temperature and abundance of prey zooplankton during the feeding period. The spawning location in Hidaka Bay was affected by distribution of the cold Coastal Oyashio water. Furthermore, these interannual variations in the timing of spawning migration and spawning location affected the landings of commercial Danish seine and gillnet fisheries. These findings are of value for understanding the mechanism of stock variation, and provide important guidance for walleye pollock stock management.
The Japanese Pacific walleye pollock (Theragra chalcogramma) stock (JPS) which is widely distributed off the Pacific coast of northern Japan is one of the most important target species for local fisheries. Resources of this species have been managed under the TAC rules that require estimation of stock abundance. Therefore, it is necessary to have a clear grasp of the relationship between the ocean environment and the variation in the stock abundance of walleye pollock, and to device a suitable method of fisheries management based on pertinent biological information. I have proposed a conceptual model of the mechanisms that determine JPS year-class strength, which appear to be the major factor that drives changes in stock biomass. In this model, successive controls are believed to be important for determining year-class strength. These controls or “switches” are factors related to survival in each stage from spawning adults to recruits. This study focused upon these switches in the spawning adult stage, as adult fish are the most important target for commercial fisheries. The objectives of this study were to investigate the interannual variability of the timing and location of spawning, and the influence of oceanographic conditions on them. Furthermore, I addressed the relationship between this variability and landings of commercial fisheries. The distribution and abundance of spawning adults in Hidaka Bay, the main spawning ground, were observed from 1998 to 2008 using a quantitative echo sounding system, and the data were analyzed together with oceanographic observation data using the Geographic Information System (GIS) method. The results showed that adult pollock migrate from the Doto area, the major feeding ground, to Hidaka Bay along the Cold Lower-layer water. They then moved to the shallower continental shelf region covered with Oyashio water (OW) in the spawning period. After spawning, they migrated to the Doto area along the OW. The timing of spawning migration to Hidaka Bay showed interannual vatiation that was affected by water temperature and abundance of prey zooplankton during the feeding period. The spawning location in Hidaka Bay was affected by distribution of the cold Coastal Oyashio water. Furthermore, these interannual variations in the timing of spawning migration and spawning location affected the landings of commercial Danish seine and gillnet fisheries. These findings are of value for understanding the mechanism of stock variation, and provide important guidance for walleye pollock stock management.
A－462 Motohito YAMAGUCHI，Hiroya MIYAKE and Shiro TAKAYANAGI
Prediction of the Ishikari Bay herring, Clupea pallasii，fishery based on virtual population analysis and a trawl survey by research vessels.
A technique to predict the stock and fishing condition of the Ishikari Bay herring Clupea pallasii, was developed based on virtual population analysis (VPA) and a trawl survey by research vessels. The stock size of fish aged 3 years or over, in terms of the number of individuals, was derived using VPA. And then that of 2-year-old fish was estimated from the product of the stock, derived as above, and the ratio of 2-year-old fish relative to the older fish obtained from the trawl survey. The predicted catch tended to be underestimated while the resource was on the increase. Therefore, the catch forecast in the present study is considered to be a target value for sustainable use of the Ishikari Bay herring resource.
A technique to predict the stock and fishing condition of the Ishikari Bay herring Clupea pallasii, was developed based on virtual population analysis (VPA) and a trawl survey by research vessels. The stock size of fish aged 3 years or over, in terms of the number of individuals, was derived using VPA. And then that of 2-year-old fish was estimated from the product of the stock, derived as above, and the ratio of 2-year-old fish relative to the older fish obtained from the trawl survey. The predicted catch tended to be underestimated while the resource was on the increase. Therefore, the catch forecast in the present study is considered to be a target value for sustainable use of the Ishikari Bay herring resource.