Density-Dependent Geographic Distribution of Atlantic Cod (Gadus morhua) in the Southern Gulf of St. Lawrence

We tested effects of population size on the geographic distribution of Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence for age groups 3–8+ over the period 1971–90. Distribution was density dependent for all six age groups. Geographic range or stock area increased with abundance for each age group. Variation in abundance explained 63–94% of the variation in stock area. During periods of low abundance, the highest cod densities occurred in western regions of the southern Gulf for all age groups. Geographic distribution during these periods was more restricted for younger cod than for older cod. Density of age-3 cod was relatively high in only one of three western regions whereas densities of older cod were relatively high in two or all of these regions. As abundance increased, density increased slowly in the regions where cod were concentrated at low population sizes and more rapidly in surrounding regions. The greatest densities were found in the same area at low and high levels of abundance for younger cod but in different areas at these two abundance levels for older cod. We discuss this variation in geographic distribution in terms of density-dependent habitat selection and note its implications for stock assessment.