The concentration hypothesis: the statistical evidence

The extent to which fish populations can compensate for changes in stock size has an important bearing on the capability of stocks to survive fishing and other pressures. The degree of compensation can be linked with the extent to which juvenile fish populations concentrate during their early life history. Populations that concentrate into nursery grounds during the first year of their lives may approach the carrying capacity of their habitat in years when settlement is high, and this limitation may then moderate the population size. In other species, pelagic throughout their juvenile lives, there is little or no concentration effect. A statistical analysis is described comparing best-fitting stock-recruitment relationships fitted to published data for a large number of stocks. It is shown that in those species that do not concentrate the relationship is typically close to linear, whereas in those that concentrate the relationship tends to be sharply curved away from a straight line. Theory suggests that in those species that concentrate the scatter of recruitment around a stock-recruitment relationship is less variable than in those that do not concentrate. Analysis of the data suggests that patterns in variability are broadly in line with this hypothesis.