THE EFFECTS OF LIGHT ON TEMPERATURE SELECTION IN SPECKLED TROUT SALVELINUS FONTINALIS (MITCHILL)

Abstract

Distribution of groups of 5 speckled trout in a longitidunal gradient of temperature was studied under a variety of conditions of illumination. The selected temperatures did not change with successive determinations separated by a week or more. However the selection became more precise as the fish acquired experience in the apparatus. It was more precise at low light intensities than at high intensities, although the actual temperature selected did not vary with the light intensity. When only part of the experimental environment was illuminated, and the temperature constant, trout did not remain in the lighted part unless the intensity of illumination was very low. When, in a gradient of temperature, the illuminated part coincided with the region normally selected by virtue of the temperature, it was found that at high light intensities the organisms did not appear in the illuminated region. Clearly a response to light prevailed. At intermediate and low light intensities, on the other hand, the response to temperature prevailed in spite of the illumination.