Migratory behaviour in relation to smolt development and releasing strategies in Atlantic salmon (Salmo salar L.) smolts

The timing of smoltification appears to be important for survival and homing in Atlantic salmon. For smolt produced as part of population enhancement strategies, it is important to produce smolts that smoltify at the same time as the wild smolts in the area. Further, both transportation and releasing methods induce stress in smolts which can affect migratory behaviour. Therefore, strategies to reduce stress will improve smolt survival. In the present three-year study (2001, 2002 and 2003), smolt production experiments were carried out at a hatchery located in the northern part of Norway (70°N). Prior to release 1 km above a permanent fish trap in the nearby River Halselva, smolts were tested according to standardized seawater challenge tests. Pairs of groups of individually Carlin tagged smolts were either released directly into the river, or given a 7-day recovery in net pens before they were released. The release experiments were performed during the two last weeks of June to the first week of July. Seawater tolerance in the beginning of July was better than 2 weeks earlier, and smolts released in the beginning of July also descended the river sooner after release than did fish released 2 weeks earlier. There was no difference in the proportion that migrated among groups released at different times. In the recovery experiment, a significantly higher proportion of the fish given 1-week recovery migrated compared to fish released directly after transport, in two of the 3 years examined.Thus, the hatchery-reared fish showed the same pattern as wild smolts. If they were released before they were fully smoltified, they delayed their migratory onset in the river and migrated when ready. Further, our results suggest that a period of recovery after handling and transport and before release caused more fish to migrate.