Elevated spring temperature stimulates growth, but not smolt development, in anadromous Arctic charr

Parr-smolt transformation and growth were studied in captive offspring of anadromous Arctic charr (Salvelinus alpinus) from the Hals watercourse in northern Norway (70 degrees N), held either at a natural temperature (<1 degrees C until May) or at a temperature elevated to 6 degrees C in late March. In mid-May, 5 weeks after the increase in photoperiod from 8:16 h light:dark to continuous light, gill Na+, K(+)-ATPase activity started to increase in both temperature groups, concurrent with the final development of full seawater tolerance. Temperature had no effect on the development of gill Na+, K(+)-ATPase activity, or on hypoosmoregulatory ability. The fish in both treatments resumed growth in mid-May, but from then on growth was faster in the elevated than in the ambient temperature group. In the former group, fish mass doubled in 6 weeks (from 65 to 137 g), and growth ceased at the time when the fish were about to complete their parr-smolt transformation. These findings show that an early vernal temperature increase advances the seasonal growth cycle, but not the parr-smolt transformation, in anadromous Arctic charr.