9. Temporal Changes in Distribution of Walleye Pollock Eggs South of Hokkaido, Japan

Ⅰ. Physical, Chemical Environment, Primary Production, Zooplankton and Their Coupling Model Studie

Temporal change in the benthic biota and the demersal fish composition in the heavily exploited fishing ground on the continental slope off the Pacific coast of northern Japan, between the 1950's and recent years

We examined the physical oceanographic condition, the macro-benthos composition, and the demersal fish composition in the continental slope (300-800m in depth) off the Pacific coast of northern Japan in summer of 2002 and 2003, and compared with the data collected and published in the 1950's. Macro-benthos was collected by the Niino's dredge and the Smith-McIntyre bottom sampler, and fish was caught by the bottom trawl. No clear difference in the physical condition was observed between the 1950's and recent years. In 1955, Annelida (mainly Polychaeta) and Mollusca (mainly Bivalvia) frequently appeared in the macro-benthos composition. In 2002, however, only Annelida was much and appearance frequency of Mollusca became low. Demersal fish composition was remarkably different between the 1950's and recent years. Fishing target species, namely, Scorpaenidae (Sebastes and Sebastolobus) and Pleuronectidae (mainly Clidoderma) frequently appeared in the 1950's. But, in recent years, fishing target species decreased, and Zoarcidae (mainly Bothrocara and Zestichthys), Macrouridae (mainly Albatrossia and Coryphaenoides) and Synaphobranchus increased alternately. This difference in the demersal fish composition must result from high fishing intensity in 50 years. And, change in the fish composition might influence the benthic fauna through their feeding habits. Our results suggest that direct and/or indirect impact of fishing should be taken into consideration for conservation of the marine environment and ecosystem.International Symposium on Long-term variations in the coastal environments and ecosystems （国際シンポジウム「世界の沿岸海域における環境と生態系の長期変動」）. 27-28 September 2004. Matsuyama, Japan

An illustrated and annotated checklist of fishes on Kitami-Yamato Bank, southern Sea of Okhotsk

An annotated checklist of benthic fishes on the western part of Kitami-Yamato Bank off northeastern Hokkaido, Japan in the southern Sea of Okhotsk, is presented based on voucher specimens with accompanying color photographs. All material was collected in late April, 2013 and 2014 by the bottom trawl net, a total of 450 specimens being classified into 92 species representing 21 families. These include Bathyraja trachouros, the first record from the Sea of Okhotsk, and Careproctus segaliensis, only the second confirmed record of the species

Interannual fluctuations in recruitment of walleye pollock in the Oyashio region related to environmental changes

The Japanese Pacific walleye pollock (Theragra chalcogramma) stock is the largest stock of this species in Japanese waters. It is a key component of the Oyashio ecosystem. In southern Hokkaido waters, these fish spawn mainly during January and February near the mouth of Funka Bay (FB), and most eggs and larvae are transported into FB. During midsummer juvenile pollock migrate along the southern coast of Hokkaido to a nursery ground on the continental shelf off eastern Hokkaido (Doto area). However, some eggs and larvae are transported southward to the Tohoku region (TR). Transport depends largely on the Oyashio, which generally flows southward along the eastern coasts of Hokkaido and Tohoku. Thus, this stock has two different recruitment routes: FB–Doto and FB–TR. In the 1980s, when the southward flow of the Oyashio was strong, the number of age-2 pollock estimated from a virtual population analysis (VPA) indicated that recruitment to the entire stock remained at a medium level. In the 1990s, when the Oyashio weakened, strong year-classes occurred in 1991, 1994, and 1995, but not in the latter half of the 1990s. Juvenile catches in the TR by commercial fisheries, which can be taken as indices of recruitment level via FB-TR, were high during the 1980s and decreased in the 1990s. Although there was no significant difference in the average number of recruits between the 1980s and the 1990s as estimated from a VPA, the recruitment patterns differed between the two decades. Here, we propose that recruitment routes of this stock shifted in response to environmental changes