Annual variation in temperature selection by Atlantic cod Gadus mortiua 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 southern Gulf of St. Lawrence, Canada, we examined variation in the temperature distribution of Atlantic cod Gadus morhua from 1971 to 1991. Distribution and temperature selection were descnbed using the cumulative distribution functions of temperature and of catch of each age class in relation to temperature. Temperature distribution varied widely among years. For example, the median temperature occupied by Age 5 cod ranged from 0.3 to 7SC and the 97.5th percenhle of occupied temperatures ranged from 4.6 to 16.6OC. Median occupied temperature decreased as age increased. Temperature selection was highly variable in both extent and direction, varying from cases of no selection to statistically sigmhcant positive or negative selection, depending on year and age class. Cod tended to select warm temperatures relative to those available m 1972 to 1976, 1978 and 1991 and relatively cool temperatures in 1980 to 1990. We used multiple regression to test for an effect of cod abundance on temperature distribution and selection, controlling for variation in available temperatures. When abundance was high, cod tended to occupy colder water, both in absolute terms (the 97.5th, 50.0th and 2.5th percentiles of cod temperature distribution) and in relative terms (the measure of temperature selection). The 9725th and 2.5th percentiles of occupied temperature increased with increasing available temperatures; conversely, temperature selection was inversely related to available temperature. These patterns provide the first field support for the hypothesis that a density-dependent decrease in food availability should result in increased preference for habitats with lower density-independent costs and provide a possible explanation for density-dependent shifts m spatial and bathyrnetnc patterns in this population KEY WORDS.