Temperature selection by brown trout residing in a thermal discharge area

Abstract

Brown trout (Salmo trutta) are common occupants of thermal discharge areas and are frequently caught by sport fishermen at shoreline discharge sites on Lake Michigan. Since 1972, three independent methods have been developed and used to estimate temperature selection by fish occupying heated discharges: a temperature-integrating fish tag; measurement of fish body temperatures immediately after capture from a thermal plume; and underwater telemetric monitoring of fish responses to plume temperatures. The results obtained by each method are summarized and compared. Temperature-integrating fish tags were placed on 376 brown trout collected at the Point Beach Nuclear Power Plant between 1973 and 1974, and 20 recaptured tags were analyzed to determine mean exposure temperatures for each fish. Body temperatures of 339 brown trout were measured immediately after their capture from the thermal discharge area at Point Beach between August 1972 and October 1973. Occupied water temperatures and internal body temperatures of 44 plume-resident brown trout were monitored using radiotelemetry between October 1975 and October 1977. Results indicated that brown trout select intermediate plume temperatures during each season and only select their final preferred temperature (17 to 18/sup 0/C) in summer when maximum ambient temperatures occur. However, the seasonal selected temperatures (mean or mode) consistently exceed mean intake (T/sub i/) temperatures by 3 to 10/sup 0/C, indicating elevated acclimation by plume-resident fish. Maximum increases in body temperature (T/sub b/ to T/sub i/) occur during seasons with low ambient temperatures, i.e., behaviorally-controlled increases in body temperature are inversely related to natural water temperatures.