Spring cannibalism on 1 year walleye pollock in the Doto area, northern Japan: is it density dependent?

Abstract

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

Diel changes in vertical distribution patterns of zooplankton and walleye pollock (Theragra chalcogramma) off the Pacific coast of eastern Hokkaido, Japan, estimated by the volume back scattering strength (Sv) difference method

“…There is a reason why the CF of small walleye pollock increased more rapidly than for large walleye pollock. In this survey area, because the distribution of small and large walleye pollock overlap during spring, cannibalism of walleye pollock occurred, and the minimum ratio cannibal/prey size was 1.74 (Yamamura et al. , 2001).…”

Contributions of the VENFISH program: meso‐zooplankton, Pacific saury (Cololabis saira) and walleye pollock (Theragra chalcogramma) in the northwestern Pacific

“…Virtually all ages of pollock are cannibalistic. Juveniles, especially age‐0 pollock, thus need to segregate from adults to avoid predation; the wide continental shelves along the Doto coast offer refuges protected by thermal barriers (Yamamura et al. , 2001).…”

Interannual variation in diets of walleye pollock in the Doto area, in relation to climate variation

“…In the 1990s, the incidence of heavy cannibalism was limited to the spring period in the Doto area (Yamamura et al 2001) -because strong stratification during summer and autumn offered juveniles thermal barriers against cannibalistic adults, and mesopelagic fishes and squids were important prey during winter (Yamamura et al 2002). The increased winter cannibalism observed in this study compensated for the lack of mesopelagic fishes and squids, which were absent from diets between 2001 and 2005.…”

“…8). Thus, summer cannibalism was more prevalent after a year with high recruitment, implying the potential to dampen interannual variations in recruitment (Yamamura et al 2001), whereas winter cannibalism showed only a non-significant negative correlation with recruitment. Reproductive success index (RPS) %Proportion of pollock in diets Fig.…”