A sustainability criterion for the exploitation of North Sea cod

The spawning stock biomass of cod, Gadus morhua, in the North Sea has been in decline for many years. There is evidence that at low stock sizes recruitment is reduced. Using non-parametric methods, equilibrium spawning stock biomass and yield curves are constructed, which are consistent with annual observations that show the continuous decline in the spawning stock can be explained by high fishing mortality rates. A new reference point, G(loss), is defined, which corresponds to the stock replacement line at the lowest observed spawning stock size. Recent fishing mortality rates yield replacement lines with a high probability of exceeding G(loss) and therefore indicate there is a high probability of further stock decline and a possibility of collapse. The replacement line is very sensitive to fishing mortality rate which makes managing the stock at high rates of exploitation risky, A reduction in fishing mortality rate by approximately 30-40% is required to ensure the probability of collapse is small.