Influence of Thermal Challenge on Conditioned Feeding Forays of Juvenile Rainbow Trout

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Abstract
Juvenile rainbow trout (S. gairdneri) conditioned to traverse a 2.4 m long channel to receive a food reward were subjected to in-transit thermal challenges. Conditioning was to a criterion that required 80% of the fish to leave the home area and reach the reward area within 2 min of release. Challenges were at successive 3.degree. C increments above acclimation or the previous challenge temperature. Fish were first observed to delay their entrance into the intervening heated water at challenge temperatures of 12-15.degree. C above acclimation. At each increment above 12-15.degree. C over acclimation temperature, delay in transit increased; complete group inhibition was never achieved. Above their critical thermal maximum (CTM) the reward was achieved even at the expense of deaths among the achievers. Responses were the same whether fish were challenged individually or as groups. Fish exposed to their CTM without prior challenges at less stressful temperatures responded similarly to those receiving progressively greater challenges. [Many fish make movements of varying distances to achieve dispersal of individuals and (or) to reach feeding and spawning grounds. Introductions of thermal discharges across routes of transit may have adverse influences on these vital movements.].