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

• Published in: Journal of fish biology 2001-09, Vol.59 (3), p.645-656
• Main Authors: Yamamura, O, Yabuki, K, Shida, O, Watanabe, K, Honda, S
• Format: Article
• Language: English
• Subjects: Theragra chalcogramma
• ISSN: 0022-1112; 1095-8649
• DOI: 10.1006/jfbi.2001.1669

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 400 mm L sub(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 super(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. Copyright 2001 The Fisheries Society of the British Isles