Variability in cannibalism in Northeast Arctic cod (Gadus morhua) during the period 1947–2006

Cannibalism is probably the most important and also the most variable cause of natural mortality for age 1-4 Northeast Arctic cod. Also, the proportion of cod in the diet of Northeast Arctic cod increases with increasing size of the predatory cod. For this stock, long time series describing diet composition are available (qualitative stomach content data for 1947-2005 and quantitative data for 1984-2006). These data were analysed together with survey indices for young cod and abundance of capelin, the most important prey item for cod. The abundance of age 1-3 cod indicated by the survey indices and the abundance of young cod in cod stomachs are well correlated. An inverse relationship between mortality induced by cannibalism and capelin abundance was found. Cannibalism was at a high level from 1947 to 1965 and then again in the mid-1990s. The high level of cannibalism in the mid-1990s was comparable to that observed in the 1950s. It is outlined how the qualitative and quantitative stomach content data could be combined to include cannibalism in the estimates of cod abundance back to 1947.