Publication Abstract

Distribution-abundance relationships for North Sea cod: observation versus theory

J.L. Blanchard, C. Mills, S. Jennings, C.J. Fox, B.D. Rackham, P. Eastwood and C.M. O’Brien

Density dependent habitat selection has implications for fisheries management and for the recovery of depleted fish populations. According to ideal free distribution (IFD) theory, populations contract into areas of highest habitat suitability as their size decreases. This can increase their vulnerability to fishing and predation. We detected density dependent habitat selection by juvenile cod (ages 1 and 2) in the North Sea and compared the observed distribution- abundance relationships with those predicted from a model based on IFD theory and knowledge of optimal temperatures for growth, where temperature was used as a measure of suitability. As predicted by the model, in years when stock size was low, the catches were largely confined to regions with near- optimal bottom temperatures. Conversely, when population size was high, catches were spread across a larger area including regions with sub-optimal temperatures. The spatial extent of optimal habitat appears to have decreased from 1977 to 2002, reflecting a gradual warming of the North Sea. The combined negative effects of increased temperature on recruitment rates and the reduced availability of optimal habitat may have increased the vulnerability of the cod population to fishing mortality.

Reference:

J.L. Blanchard, C. Mills, S. Jennings, C.J. Fox, B.D. Rackham, P. Eastwood and C.M. O’Brien (2005)  Distribution-abundance relationships for North Sea cod: observation versus theory. Canadian Journal of Fisheries and Aquatic Sciences, 62: 2001-2009.