Thermal preference of Arctic charr, Salvelinus alpinus, and brown trout, Salmo trutta – implications for their niche segregation

Abstract

This study tested a new experimental apparatus to estimate thermal preferences of fish. The apparatus was designed to minimise the effect of the thermal history of the fish and allow for easy feeding. The set-up consisted of two connected sections of an aquarium, both receiving an excess of food, with slightly different water temperatures. Initially, the fish spent most of its time in one of the sections, but when the temperatures were slowly increased (or decreased), the fish spent increasingly more time in the other. The temperature at which the fish spent equal time in both sections was defined as the preferred temperature. Brown trout, Salmo trutta, preferred the reported optimal temperature for growth of the species. However, Arctic charr, Salvelinus alpinus, selected a significantly lower temperature than its optimal temperature for growth and thus contradicted the general view of good correlation between the optimal temperature for growth and preferred temperature of fish. The reason for this may be that charr is optimising its growth efficiency instead of its growth rate. Individuals that utilise a limited resource in an optimal way, by selecting a temperature where the growth efficiency is maximised, will possibly be favoured. Several factors affect the distribution of fish in lakes, but the difference in thermal preference between charr and trout might partly explain the frequently observed niche segregation of these two species in Scandinavian lakes.