Relations between metabolic rate, migration and behaviour in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

ABSTRACT Migration is common among populations of brown trout (Salmo trutta) and Atlantic salmon (Salmo salar). However, not all individuals in the same population migrate, a phenomenon referred to as partial migration. The aim of this thesis was to investigate if an individual’s behaviour and metabolic rate influences its decision to migrate and how such knowledge may be used when trying to produce hatchery-raised smolts with as high a proportion of migrating individuals as possible. In paper I the influence of reduced food ration on the proportion and swimming speed of migrating brown trout and Atlantic salmon smolts was investigated. Furthermore, the standard metabolic rate (SMR) of migrating and non-migrating individuals was compared. In paper II, a laboratory experiment, SMR was correlated to the behaviour of individual brown trout and Atlantic salmon. Dominant fish of both species had a higher SMR than subordinates (paper II). In addition, migrant brown trout had a higher SMR than non-migrant trout when given a normal food ration, whereas no difference in SMR between migrating and non-migrating salmon could be seen (paper I). When administered low food rations, smolts of both species migrated faster than smolts given a normal food ration, and the proportion of migrating smolts was higher for salmon given less food when the size difference for smolts from the two feeding regimes was large (paper I). Other factors that influenced migration speed were the degree of smolt development and water temperature (paper I). SMR was not correlated with aggressiveness, or with different measurements of boldness. Moreover, aggression and boldness were not correlated with each other (paper II). Trout showed a higher level of aggressiveness and acclimated more rapidly to laboratory conditions than salmon (paper II). In summary, there was no support for the existence of coping styles in migratory Atlantic salmon and brown trout. Instead, metabolic rates were related to both migratory behaviour and social status. Furthermore, an individual’s decision to migrate was influenced by ration size.