Investment in maturity-at-age and -length in northeast Atlantic cod stocks

The maturation schedule (maturity ogives) for populations differs with geographic location, and cod (Gadus morhua) stocks are no exception. The environment (latitudinal effects) will have an impact on the maturation schedule, with northern (colder water) stocks growing more slowly and maturing at a greater age. This is, however, modified by the impacts of fishing, whereby under severe exploitation, maturation tends towards an earlier age. Cod in the Irish Sea are towards the southern end of the species’ range and the stock is severely depleted, with most males and females maturing between 2 and 3 years of age and at a relatively small size. The North Sea cod stock is considered to be harvested unsustainably, and, in addition, in recent years has shown the classic shift in maturity schedule to most fish maturing at a younger age and smaller size. The Barents Sea (Northeast Arctic) cod stock has undergone major changes in population size, but is currently not overexploited. There have also been shifts in the maturation schedules of Northeast Arctic cod. However, over the past decade or so, there has not been a clear trend in the age- or length-at-50% maturity with time or population size. The relationship between the age- and length-at-50% maturity differs markedly between the three cod stocks (Irish, North and Barents Seas), when considered over years and by year classes, which suggests that there may be stock- or area-specific characteristics. Such stock-specific differences will have consequences for applying relationships between areas where the maturity schedules are dynamic, e.g. through climate change or under severe or changing exploitation patterns.