Distribution changes and abundance of northern cod (Gadus morhua), 1981-1993

Research vessel survey data for cod off Newfoundland (NAFO divisions 2J3KL) were used to examine changes in distribution of the northern cod (Gadus morhua) population from 1981 to 1993. The area occupied by northern cod was positively correlated with stock abundance. That is, when more abundant, the stock occupied a larger area, and the observed stock decline was associated with a shrinkage of occupied range. This relationship was demonstrated using two different techniques: area occupied by 90% of the estimated biomass and a statistical ellipse technique describing the bivariate central tendency of the catch data. This latter technique is novel and allows straightforward quantification and visualization of distribution data. The declines in occupied area preceded declines in estimated biomass, confirming previous work which indicated a breakdown in population structure beginning in 1989. The analyses also confirm distribution changes both southward and offshore since 1988. We hypothesize that the cod concentrated in the southern range of the stock beginning in 1990, perhaps as a result of environmental factors, and resultant increases in local density led to increased catchability and fishing mortality for offshore fleets such that catch rates remained high as the stock declined.