The exploitation pattern, density-dependent catchability, and growth of cod (Gadus morhua) in the west-central North Sea

Abstract

Age compositions and weights at age of cod landed from the west-central North Sea by UK vessels are used as the basis of an historical assessment of the dynamics of the population between 1959 to 1977, employing Virtual Population Analysis. It is suggested that this treatment, which assumes isolation from the cod of the rest of the North Sea, is justified by the low rate of emigration from or immigration into the west-central North Sea from other areas, a situation which has been enhanced by the locally high fishing mortality rate. Both catchability and the growth rate are shown to have been inversely correlated with stock abundance, which has changed dramatically following a succession of strong year classes between 1963 and 1970. The exploitation pattern is shown to have been generated by independent sections of the exploiting fleet which concentrate upon either young (ages 1 to 4) or old (age 6+) fish. The implications of these results for the management and understanding of the North Sea cod stock are discussed.