A Comparison of Temperatures Selected by Rainbow Trout (Salmo gairdneri) in Spatial and Temporal Gradients

The selected temperatures of yearling (13-month-old) rainbow trout, Salmo gairdneri, acclimated to 10℃, were determined in three commonly used laboratory devices—horizontal and vertical spatial gradients, and a temporal-gradient shuttlebox apparatus. There was no significant difference between the mean temperatures selected by the fish in the horizontal spatial tank and the shuttlebox. The mean selected temperature of trout tested in the vertical-gradient tank (11.4℃) was significantly (p＜0.05) lower than those in the other two devices. We suggest that the most likely reasons for this discrepancy lie in the geometric nature of the gradients being compared. Negatively phototactic behaviour in the vertical-gradient tank may have resulted in the fish swimming lower in the water column in response to the overhead light source, thus depressing the observed preferred temperatures.