ANNUAL VARIATION IN TEMPERATURE SELECTION BY ATLANTIC COD GADUS-MORHUA IN THE SOUTHERN GULF OF ST-LAWRENCE, CANADA, AND ITS RELATION TO POPULATION-SIZE

Using data from bottom trawl surveys conducted each September in the s outhern Gulf of St. Lawrence, Canada, we examined variation in the tem perature distribution of Atlantic cod Gadus morhua from 1971 to 1991. Distribution and temperature selection were described using the cumula tive distribution functions of temperature and of catch of each age cl ass in relation to temperature. Temperature distribution varied widely among years. For example, the median temperature occupied by Age 5 co d ranged from 0.3 to 7.5 degrees C, and the 97.5th percentile of occup ied temperatures ranged from 4.6 to 16.6 degrees C. Median occupied te mperature decreased as age increased. Temperature selection was highly variable in both extent and direction, varying from cases of no selec tion to statistically significant positive or negative selection, depe nding on year and age class. Cod tended to select warm temperatures re lative to those available in 1972 to 1976, 1978 and 1998 and relativel y cool temperatures in 1980 to 1990. We used multiple regression to te st for an effect of cod abundance on temperature distribution and sele ction, controlling for variation in available temperatures. When abund ance was high, cod tended to occupy colder water, both in absolute ter ms (the 97.5th, 50.0th and 2.5th percentiles of cod temperature distri bution) and in relative terms (the measure of temperature selection). The 97.5th and 2.5th percentiles of occupied temperature increased wit h increasing available temperatures; conversely, temperature selection was inversely related to available temperature. These patterns provid e the first field support for the hypothesis that a density-dependent decrease in food availability should result in increased preference fo r habitats with lower density-independent costs and provide a possible explanation for density-dependent shifts in spatial and bathymetric p atterns in this population.