Spatial and temporal variation in the density of northern cod and a review of hypotheses for the stock's collapse

The collapse of northern cod, Gadux morhua, off Newfoundland and Labrador was associated with clearly defined spatial and temporal changes in density and biomass. Between 1981 and 1992, low density research survey tows (<100 kg/tow) increased gradually from 76 to 97% concomitant with a gradual decline in medium density tows (100 500 kg/tow) from 22 to 2%. By contrast, high density tows (>500 kg/tow) remained proportionately constant (~1.5%) until 1992, whereafter they declined to zero. Southward, spatio-temporal changes in stock biomass were unaccompanied by a shift in cod distribution. A simple density composition model provides a biological basis for observed changes in mobile and fixed-gear catch rates, increased catchability of cod with declining stock biomass, and rapid increases in fishing mortality. A nested aggregation model of a small, constant number of dense cod aggregations, each encompassed by, and recruited from, lower density areas, explains how cod vulnerability to fishing can increase with declining stock biomass. A review of recent research identifies excessive fishing mortality as the sole significant cause of northern cod's collapse. Prevention of fishery collapses arguably rests on the dominant question to emerge from this review: what are the effects of fishing on the behaviour, life history, and population biology of exploited fishes?.