Winter behaviour of juvenile Atlantic salmon Salmo salar L. in experimental stream channels: effect of substratum size and full ice cover on spatial distribution and activity pattern

• Published in: Journal of fish biology 2008-06, Vol.72 (10), p.2518-2533
• Main Authors: Linnansaari, T., Cunjak, R. A., Newbury, R.
• Format: Article
• Language: English
• Subjects: activity; cover; Freshwater; ice; juvenile Atlantic salmon; Marine; Salmo salar; survival; winter
• ISSN: 0022-1112; 1095-8649
• DOI: 10.1111/j.1095-8649.2008.01857.x

Activity and choice of areas offering different cover (substratum or surface ice) for juvenile Atlantic salmon Salmo salar were studied in experimental stream channels during winter. Channels were completely ice covered between December and March. During this period, the ice thickness increased from 50 to 300 mm after which 50% of the ice was experimentally removed and followed by c. 2·5‐fold increase in discharge to simulate the effects of spring flood. Large substrata provided preferred habitats but areas with small substratum sizes were also used when full surface ice provided above‐stream cover and the stream discharge was relatively low. The fish remained nocturnal throughout the study but the level of day activity significantly increased as the surface ice became thicker. Maximum movement distance during a 24 h period and homing‐at‐dawn behaviour remained at a constant level throughout the main winter, but significantly changed during the simulated spring flood (mean ±s.e. maximum extent of movements within 24 h increased from 1·1 ± 0·1 to 3·0 ± 0·5 m; homing behaviour decreased from the highest level of 89·3 to 34·6% during spring flood). Overwinter survival was high (92·9%). Relative mass increase during the study ranged from –8·3 to 28·5%, and 84% of the juvenile Atlantic salmon gained mass. The highest rates of mass increase were associated with frequent movements between areas of different substratum size. The results indicate that during winter: (1) Atlantic salmon parr preferred large substratum cover compared with surface ice cover at the fish densities studied here, (2) juvenile Atlantic salmon were predominantly nocturnal but diurnal activity increased as surface ice became thicker and (3) increase in water discharge during spring altered the behaviour of juvenile Atlantic salmon and may have caused additional habitat shifts.