The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.