Sex-specific temperature distribution in four populations of American plaice Hippoglossoides platessoides

In the American plaice Hippoglossoides platessoides, the sexes differ in si ze at age in most populations, with females being larger. Because of the li nks between growth, ration and temperature, differences in temperature sele ction might be expected between the sexes in species with dimorphic growth, with the sex with the higher growth rate selecting higher temperatures. Te mperature selection has also been predicted to be density-dependent in fish es, with fish occupying colder temperatures at higher levels of abundance. We examined these aspects of temperature selection in 4 populations of Amer ican plaice: the southern Gulf of St. Lawrence population and 3 populations off Newfoundland (Labrador and NE Newfoundland, Grand Bank and St. Pierre Bank). Throughout the 1970s and most of the 1980s plaice occupied cold wate r relative to that available in all of the areas surveyed. In all 4 populat ions, females tended to occupy warmer water than males. Differences between the sexes in temperature selection and in length at age both tended to be greatest in the southern Gulf population during this period. However, there was no correspondence among populations in these differences in the early 1990s, when distribution of the Grand Bank and St. Pierre Bank populations shifted sharply into warm water and the difference in temperature distribut ion between the sexes increased dramatically in these populations. These sh ifts to warmer water may be related to density-dependent effects on tempera ture preference. There was a strong negative relationship between the tempe ratures occupied by plaice and their relative abundance in the 3 Newfoundla nd populations but not in the southern Gulf population. Densities appear to have remained higher in the southern Gulf population than in the Newfoundl and populations, suggesting that density-dependent effects on temperature p reference only occur when densities fall below some threshold level.