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 7S°C 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

I examined variation in the temperature distribution of American plaice (Hippoglossoides platessoides) in the southern Gulf of St. Lawrence using data from bottom trawl surveys conducted each September since 1971. Plaice tended to select cold temperatures relative to those available. Temperature distribution was unrelated to age over the range 3-12+ years. Females tended to occupy warmer water than males, although the extent of this difference between the sexes varied among years. Annual variation in plaice temperature distribution was positively correlated with variation in the temperatures available. After accounting for annual variation in available temperatures, female temperature distribution was density independent but males tended to occupy colder water at higher levels of abundance. The median temperature occupied by both sexes tended to be warmer at higher levels of abundance of Atlantic cod (Gadus morhua), a competitor of plaice. The difference between the sexes in temperature distribution increased as cod abundance decreased or as plaice abundance increased (depending on the measure of temperature distribution used). The observed difference between the sexes in temperature distribution and its density dependence may reflect differences in foraging rate associated with differences between the sexes in the fitness benefits of growth.

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