Diets of walleye pollock Theragra chalcogramma in the Doto area, northern Japan: ontogenetic and seasonal variations

Abstract

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

Measurements of density contrast and sound-speed contrast for target strength estimation of Neocalanus copepods (Neocalanus cristatus and Neocalanus plumchrus) in the North Pacific Ocean

“…pacifica even if that inhabit in the same Oyashio region [27] from that in Neocalanus copepods. Target strength estimated by using g and h in C. finmarchicus (open circles numbered 4 and 5) and C. hyperboreus (open circles numbered 6 and 7)…”

Summer primary productivity and phytoplankton community composition driven by different hydrographic structures in the East/Japan Sea and the Western Subarctic Pacific

“…In this respect, the strikingly high phytoplankton biomass observed at Station YB-1, where a high concentration of prymnesiophytes and high water temperature (>10 C) were observed in the euphotic zone, might have arisen from low grazing pressure (Figures 2 and 7). Several studies have reported that phytoplankton size composition and biomass are controlled by zooplankton grazing [Malone and Chervin, 1979;Burkill et al, 1995;Han and Furuya, 2000;Chiba and Saino, 2002;Yamamura et al, 2002]. Hiromi [1996] also found that grazing by zooplankton heavily depressed the phytoplankton biomass and productivity in Tokyo Bay.…”

Lipid and fatty acid/alcohol compositions of the subarctic copepods Neocalanus cristatus and Eucalanus bungii from various depths in the Oyashio region, western North Pacific

“…In the ecosystem of the Oyashio region, large calanoid copepods are important constituents of zooplankton comprising 80-90% of the zooplankton biomass (Vinogradov, 1970;Ikeda et al, 2008). These calanoid copepods are major prey for pelagic fishes (Takeuchi, 1972;Taka et al, 1982;Odate, 1994;Yamamura et al, 2002), marine mammals (Nemoto, 1963) and sea birds (Hunt et al, 1998), and are therefore regarded as key organisms in this region. In terms of biomass in the Oyashio region, Neocalanus cristatus is the most abundant copepod, followed by Eucalanus bungii (Ikeda et al, 2008).…”