Cod (Gadus morhua L.) cannibalism in the Central Baltic: interannual variability and influence of recruit abundance and distribution

Abstract

Baltic cod feed mainly on clupeoids and invertebrates. Other fish, including cod, form only a small part of their diet. To examine the frequency of cannibalism in the Central Baltic, stomach data were analysed for 1963–1990. During this period, cod in cod stomachs in the Gotland Basin constituted 2.8% by weight of the total food content, while the frequency of occurrence was 1.1%. Cannibalism was recorded mainly in cod >35 cm, and the size range of cod found in stomachs was 5–15 cm. Cannibalism was observed in restricted areas only and varied strongly between years. The intensity of predation on cod was significantly different between the periods 1969–1975, 1976–1979, and 1980–1990. Until 1980, the population in the eastern part mainly inhabited near-bottom layers. Higher abundance of recruits and their wide distribution raised the level of cannibalism. Development of stagnation during the 1980s resulted in the reduction of recruitment and stock size. The distribution of the adult population changed from near-bottom to mid-water during the first half of the year. The low abundance of small cod and the differences in distribution between small and large fish appear to have been the major factors determining very low predation on cod during 1980–90. Overall, the influence of cod cannibalism on recruitment success is negligible in the Central Baltic. Stock dynamics in the area are determined by the combined effects of variable oceanographic conditions and the fishery.