Dependence of Recruitment on Parent Stock

Observations on the relation of recruitment to stock suggest that there is a lightly convex curve for herring and a dome-shaped one for gadoid fishes, in contrast to flatfishes where a flat-topped curve is well established for several species. Theoretical examination of potential mechanisms lead to a suggestion that natural mortality rate decreases with age down to a minimum, then increases among the older, senescent, fish. This has been incorporated into yield curves for the North Sea plaice and Arcto-Norwegian cod, which have maxima at rates of fishing (F) of 0.22 and 0.26, respectively.