Seasonal, ontogenetic and bathymetric variations of diet were examined for walleye pollock Theragra chalcogramma based on a total of 6666 fish collected off the southeastern coast of Hokkaido Island, northern Japan (Doto area) during 1989 to 2000. Walleye pollock depended exclusively (> 99%) on pelagic prey and showed a clear ontogenetic dietary shift: smaller fish depended mainly upon mesozooplankton such as Neocalanus cristatus and Euphausia pacifica, and larger fish preyed upon pollock (cannibalism), myctophids Diaphus theta and firefly squid Watasenia scintillans. Seasonal variation was also evident. Smaller pollock depended mainly upon N. cristatus during spring then shifted gradually to E. pacifica during other seasons. For larger pollock, major prey shifted from pollock (cannibalism) during spring to micronekton during other seasons. Bathymetric variation was less pronounced, with cannibalism and Themisto spp. being more important in the shallow area (≤150 m). Feeding intensity, measured as stomach content index, was generally higher during spring and summer than autumn and winter, but showed incidentally high values during winter, suggesting sporadic but intense feeding opportunities. Fish condition fell during winter and then recovered rapidly during May and June. Recovery coincided with superabundance of N. cristatus during blooming, so that this species is essential for the recovery of pollock from wintering and spawning. KEY WORDS: Theragra chalcogramma · Food habits · Seasonality · Feeding intensity · ConditionResale or republication not permitted without written consent of the publisher

Juvenile walleye pollock of the Japanese Pacific population were collected from the Funka Bay [spawning ground; 16-64 mm fork length (FL)] in spring and the Doto area (nursery ground; 70-146 mm FL) in summer. Hatch dates were estimated by subtracting the number of otolith daily increments from sampling dates, and their early growth was back-calculated using otolith radius-somatic length relationships.

We review published/unpublished information on the early life stage of walleye pollock along the Pacific coast of Hokkaido. Evidence of the recruitment fluctuations was suggested by cohort analysis, and the 1995 year class was recognized as the strongest year class in the last two decades. The highest distribution of eggs was observed in the vicinity of Funka Bay, the southwestern part of Hokkaido. Egg transport and match/mismatch processes are pointed out as possible causes of recruitment fluctuations during the larval stages. In August 2000, we observed a high-density juvenile aggregation in the eastern Pacific Hokkaido. The estimated hatch dates of juveniles ranged between early-January and mid-March, suggesting they originated from Funka Bay. We suggest that the juveniles migrate from Funka Bay to the eastern Pacific Hokkaido during their first summer and that they use this area as for a nursery ground. Larval transport, migration mechanism and environmental conditions in the nursery area may also be important in establishing year-class strength.

No abstract

We examined stock fecundity, abundance of eggs and larvae, and catch per haul of age 0 walleye pollock, Theragra chalcogramma, and considered the relationship between reproduction, recruitment and the ocean environment off the Pacific coast of northern Japan for the period between 1985 and 1996. To calculate stock fecundity, we estimated age at maturity and fecundity, and expanded estimates to the expected age composition of the population as estimated by Virtual Population Analysis. Eggs and larvae were collected by research cruises in the main spawning ground in and around Funka Bay. Catch in number of age 0 per haul were obtained from catch statistics. Annual stock fecundity decreased in the late 1980s, from 97 billion eggs in 1986 to 74 billion in 1990, and increased in the 1990s, to 137 billion in 1995. Abundance of eggs and larvae, and catch of age 0 per haul, were high in the late 1980s; however, they were low or intermediate in the 1990s. The steepness of the spawner recruitment relationship in the 1990s was different from the late 1980s, based on positive relationships between stock fecundity and catch of age 0 per haul in each decade. Sea surface temperature in the main spawning ground around Funka Bay was low in the late 1980s, but high in 1990s. We conclude that the decadal-scale change in the ocean environment around the main spawning ground between the 1980s and the 1990s caused a change in the reproduction and recruitment of walleye pollock.