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

¹ University of Toronto, Toronto 5, Canada

1. The distribution of groups of five speckled trout in a longitudinal gradient of temperature was studied under a variety of conditions of illumination.

2. The modes of the distributions, i.e., the selected temperatures, did not change with successive determinations separated by a week or more. However the selection of temperature became more precise as the fish acquired experience in the apparatus.

3. Selection of temperature was apparently more precise at low light intensities than at high intensities although the actual temperature selected did not vary with the light intensity.

4. Some experiments were done in which only part of the experimental environment was illuminated. When temperature was constant trout would not remain in the lighted part unless the intensity of illumination was very low. When, in a gradient of temperature, the illuminated part was made to coincide 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, it was the response to temperature which prevailed and in spite of the illumination the organisms selected temperature in a normal way.