Studies on fisheries and life history of Euphausia pacifica HANSEN off northeastern Japan

Euphausia pacifica is the dominant euphausiid in the North Pacific Ocean. In the Japanese waters of the Pacific Ocean, it occurs as far south as Suruga Bay, 34˚50'N and extends northwards as far as the southwestern area of the Okhotsk Sea. E. pacifica is commercially exploited in Sanriku （38˚－41˚N） and Joban （36˚－38˚N） waters off northeastern Japan from late winter to early summer. The average annual catch and average annual value on landing is 66,000 ton and 3.1 billion yen, respectively, during the years 1992 and 2001. Because the annual catch, fishing period and landing value of E. pacifica varies between regions and years, accurate assessment of the fishery resource and prediction of fishing conditions for the next season have been demanded to maintain the stability of this fishery. On the other hand, E. pacifica is considered a key species because many endemic and migrant predators depend on this species as prey. In addition, the feeding impact by E. pacifica on the lower trophic levels in the ecosystem are significantly high considering its high biomass. Considering the important role of E. pacifica to transport organic matter from the lower trophic levels to higher trophic levels in the North Pacific, qualitative and quantitative analyses of distribution, life history and carbon budgets of E. pacifica have great importance for understanding the structure of marine ecosystems in this area. From the above mentioned reasons, the fishery, horizontal and vertical distribution, life history, biomass, carbon budgets （production and metabolism） and feeding of E. pacifica off northeastern Japan were examined in this study. To examine how oceanographic conditions have affected fishing conditions, annual variations of fishing conditions such as the strength of catch and CPUE （catch per unit effort） were investigated in relation to the oceanographic conditions such as the first branch of the Oyashio Current as well as the history of development of fishing methods and regulations in E. pacifica fishery. The relationship between the fishing conditions and oceanographic conditions is as follows. When the first branch of the Oyashio Current is weak, fishing conditions in northernmost Iwate Prefecture are rather stable but fishing condition in Miyagi Prefecture and southernmost Joban area is unstable, showing low CPUE and delayed formation of the fishing ground. On the other hand, when the first branch of the Oyashio Current is strong, fishing conditions in Miyagi Prefecture and Joban area are favorable as well as in Iwate Prefecture, showing high CPUE and early formation of the fishing ground. To examine the distribution and life history of E. pacifica, seasonal changes in spawning, distribution and growth patterns and life span of this species were investigated over a broad geographical area （mainly 36˚－43˚00' N and west of 145˚E） using Norpac net and cylindrical-conical net samples. Mating and spawning occurred throughout the year in the Oyashio area （T100≦5℃） and colder waters of the transitional area （5℃＜ T100≦10℃ , with a peak in spring but less in late fall-winter. Apparent growth was recognized from March to June for the small-sized group of adults. The life span of male and female E. pacifica off northeastern Japan was estimated to be 24 months and 28 months, respectively. E. pacifica gradually adapts to colder areas as the developmental stage progresses from furcilia to adults, and advanced-age adults （large-sized group of adult） rarely occur in warmer areas. Geographical distributional pattern of spawning and each stage was generally closely related to the distribution of the water masses. To examine physical and biological factors which determine the vertical distribution of E. pacifica, seasonal variations of vertical distribution of E. pacifica were investigated in the coastal and offshore waters （36˚-42˚00' N and west of 145˚E）, using ORI, beam-trawl and MOCNESS-I net samples. In the Sanriku coastal waters, benthopelagic E. pacifica occurred on the upper slope from early summer to fall where strong thermocline occurred in the middle layer above colder temperatures under 8℃ in the low layer. In spring, adult E. pacifica forms dense pelagic aggregations throughout the day on the continental shelf where water with 7-8℃ dominates in the total water column. In the offshore area, annual average median depth of each developmental stage of E. pacifica at night tended to increase with the developmental stages from middle-furcilia to large adults, while the median depth in the daytime tended to increase with developmental stages from first calyptopis to sixth furcilia. Median depth at night of immature and adult stages tended to be deeper in the warmer areas in summer and fall, especially large adults hardly reached beyond the thermocline in the mid layer. In spring, the median depth during daytime of immature and adult stages was significantly shallower than that in other months and thus the distance of diurnal ...