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

Cannibalism in walleye pollock off the eastern coast of the Hokkaido Island , Japan was important only during spring (April to June), and its importanc e increased from 0% in dry mass for < 200 mm L-S fish to 48(.)9% for > 400 mm Ls fish. Most of the prey was represented by age I year fish, showing a unimodal body size distribution with a mode at 121-130 mm. Although canniba l body size was larger in deeper (> 150 m) water, there was no difference i n prey size by depth, suggesting impingement of the predators inhabiting de eper water into the shallow areas to cannibalize I year fish. The minimum r atio cannibal: prey size was 1(.)74. There was a positive but non-significa nt correlation between the contribution of cannibalism to a potenital preda tor's (> 300 nun) 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 correlati on was obtained (r(2) = 0(.)77, P < 0(.)01), showing that pollock cannibali sm is rather density dependent. Based on the results, it is hypothesized th at the 'overflow' of 1 year fish from the shelf waters due to their high ab undance and the weak stratification in the spring water column results in i ncreased co-occurrence with adult fish and consequent cannibalism. (C) 2001 The Fisheries Society of the British Isles.