Adaptation to ice-cover conditions in Atlantic salmon, Salmo salar L.

Hypothesis: Salmonid fishes adapt to ice-cover conditions in their home river. Organism: Atlantic salmon (Salmo salar) originating from six Norwegian populations along a climatic gradient (59-70 degrees N) ranging from insignificant natural ice cover (southern populations) to several months of extensive ice cover (northern populations). Site of experiments: NINA Research Station, Ims, south-western Norway (59 degrees N, 6 degrees E). Methods: Common environment experiments in tanks (manipulating light) comparing growth, food consumption, and growth efficiency under simulated ice cover (darkness) and no ice (6 h daylight), and comparison of energy-loss rates in semi-natural stream channels with or without simulated ice cover. Results: Mass standardized growth (Q) of the three southern populations from ice-free rivers was significantly higher in tanks with light (mean = 0.13) than in darkness (mean = -0.21), whereas the northern populations originating from rivers with ice cover showed smaller or insignificant differences in growth between light (mean = -0.01) and darkness (mean = 0.08). A similar pattern was found for food consumption. In stream channels, the differences in energy loss with and without simulated ice cover increased significantly with increasing latitude of population origin. Fish from the northern populations naturally experiencing extensive ice cover lost more energy without ice than fish from the southern populations, and are thus more susceptible to ice-cover removal.