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
Cannibalism in walleye pollock off the eastern coast of the Hokkaido Island, Japan was important only during spring (April to June), and its importance increased from 0% in dry mass for <200 mm L S fish to 48·9% for >400 mm L S fish. Most of the prey was represented by age 1 year fish, showing a unimodal body size distribution with a mode at 121-130 mm. Although cannibal body size was larger in deeper (>150 m) water, there was no difference in prey size by depth, suggesting impingement of the predators inhabiting deeper water into the shallow areas to cannibalize 1 year fish. The minimum ratio cannibal: prey size was 1·74. There was a positive but non-significant correlation between the contribution of cannibalism to a potenital predator's (>300 mm) diet and an estimate of the previous year's recruitment. This was due to an extremely high contribution of cannibalism during 1992, when a distinctly larger size of predators seemed to bias the contribution. When the 1992 data were removed from the analysis, a significant correlation was obtained (r 2 =0·77, P<0·01), showing that pollock cannibalism is rather density dependent. Based on the results, it is hypothesized that the ' overflow ' of 1 year fish from the shelf waters due to their high abundance and the weak stratification in the spring water column results in increased co-occurrence with adult fish and consequent cannibalism. 2001 The Fisheries Society of the British Isles
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.
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
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