Effects of ice on behaviour of juvenile Atlantic salmon (Salmo salar)

The behaviour of Atlantic salmon (Salmo salar) parr was compared among three periods of winter (pre-ice period, PI; period of subsurface ice, SSI; period of surface ice, SI). Salmon parr remained nocturnal regardless of the ice conditions. The level of nocturnal activity was similar during PI and SI periods but was significantly reduced during the SSI period. Immobility was also highest during the SSI period, but was only partly attributable to salmon parr being trapped under ice. No differences in daytime activity among ice periods were observed. Two nocturnal movement tactics were observed: (i) emerge-settle-return and (ii) emerge-move-return. The tactics were used similarly during PI and SSI periods, but the move tactic was predominant during the SI period. Atlantic salmon parr showed a strong tendency to return to their home stone after a period of activity. Homing was reduced during the SSI period, but the distance moved to a new home stone was typically <10 m. In general, salmon parr were able to cope well with subsurface and surface ice.