Distribution changes of Atlantic cod (Gadus morhua L.) in the northern Gulf of St Lawrence in relation to an oceanic cooling

Abstract

Research vessel trawl survey data were examined to investigate age-by-age changes in the distribution of Atlantic cod (Gadus morhua L.) with respect to depth, temperature, and latitude in the northern Gulf of St Lawrence (Canada) in both winter (1978-1994) and summer (1984-1995) in relation to a water cooling event. We used a cumulative distribution function method that statistically compares distributions of sampled variables with those occupied by fish. There was no evidence that the ambient temperatures occupied by cod changed with the cooling. On average, cod occupied waters with temperatures of 4.4-5.9°C in winter and of 1.4-3.5°C in summer, with no temporal trend. However, major distribution shifts occurred in winter: the median latitude of the geographic distribution in 1993 was 2°(220 km) south of that in 1985, and cod were also distributed 200 m deeper in the 1990s. Even though stock abundance decreased drastically during the time period considered, abundance in the deepest stratum surveyed (>365 m) increased. The median latitude of distribution in winter was correlated with an index of temperature anomaly in the cold intermediate layer during the previous summer (r=0.85, p<0.05, corrected for autocorrelation). This indicates that the cooling at mid-depth influenced cod distribution and resulted in an earlier wintering migration. Cod responded to a change in their habitat by changing their migration and distribution patterns.