The preferred temperature of fish and their midsummer distribution in temperate lakes and streams

Laboratory studies of preferred temperature with yellow perch (Perca flavescens) are compared with results from 21 other species. These show that temperature, if acting alone, can determine the distribution of fish in laboratory apparatus. Factors such as light, conditioned responses related to feeding routines, and social behaviour can interfere with the expression of the response to temperature. Subdued lighting conditions were necessary in the experiments with Oncorhynchus, Salvelinus and Coregonus, whereas full daylight was required in experiments with Perca flavescens.The level of thermal acclimation influences the range of temperature preferred. In general the preferred temperature is considerably higher than the acclimation temperature at low thermal acclimations, but this difference decreases up to the final preferendum, where both coincide. The final preferendum and the relation between acclimation and preferred temperature is characteristic for the species. The shape of the resulting curve may have some value in interpreting observations of fish mortalities and distribution in nature. The final preferendum of the yellow perch from the present work was 24.2 °C., from other work using older fish it was 21.0 °C.Summer field observations of yellow perch in Lake Nipissing, Costello Lake and Opeongo Lake in Ontario, showed average thermal distribution of 19.7 °C., 21.0 °C. and 21.2 °C. respectively. This agrees well with 20.8 °C. observed for four Wisconsin lakes. Oxygen depletion reported for Tennessee Valley reservoirs, distribution of primary prey species of lake trout in New York waters, and other factors, have been shown to modify the thermal distribution in nature. Differential sex response to temperature may be important in the perch. Field observations of thermal distributions for other species are also presented.A comparison of the laboratory and field data shows good agreement with fish having colder final preferenda: Salvelinus fontinalis, Salvelinus namaycush, Salvelinus hybrid and Coregonus clupeaformis. Fish with warmer final preferenda, such as Micropterus salmoides, Micropterus dolomieu and Lota lota lacustris, showed higher temperatures in the laboratory than was shown by field observations. Young Perca flavescens showed similar results, but experiments with older perch showed excellent agreement between laboratory results and held observations. The lack of agreement between laboratory results and field observations is attributed to age differences; laboratory experiments being performed with young fish and held observations being made on older fish.