The concentration hypothesis: the statistical evidence

The extent to which fish populations can compensate for changes in stock si ze has an important bearing on the capability of stocks to survive fishing and other pressures. The degree of compensation can be linked with the exte nt to which juvenile fish populations concentrate during their early life h istory. 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 po pulation size. In other species, pelagic throughout their juvenile lives, t here is little or no concentration effect. A statistical analysis is descri bed comparing best-fitting stock-recruitment relationships fitted to publis hed data for a large number of stocks. It is shown that in those species th at do not concentrate the relationship is typically close to linear, wherea s in those that concentrate the relationship tends to be sharply curved awa y from a straight line. Theory suggests that in those species that concentr ate the scatter of recruitment around a stock-recruitment relationship is l ess variable than in those that do not concentrate. Analysis of the data su ggests that patterns in variability are broadly in line with this hypothesi s. (C) 2000 International Council for the Exploration of the Sea.