Consistent individual differences in behaviour has been demonstrated for many animals, but there are few studies of consequences of such repeated behaviour in the wild. We tested consistency in migration timing to and from the sea among anadromous Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta), using data from a study period of about 25 years, including more than 27,000 uniquely Carlin-tagged individuals that migrated to sea for feeding in the spring and returned to the river in late summer for up to 13 successive years. Consistency was found between individuals across time in timing of the seaward migration. Individuals migrating early during their first migration tended to migrate early the following years, and late migrants tended to migrate late. The same pattern was found also at ascent to freshwater. Hence, this study demonstrated that individual fish in nature can differ in behaviour related to migration timing, and that these differences can be consistent during their life time. Early migrants increased their mass more than late migrants, and had a higher specific growth rate. Early migrating Arctic char, but not brown trout, experienced a longer life after the first migration to sea than late migrants. In both species, maturity occurred earlier in individuals that migrated early. For brown trout, but not for Arctic char, fecundity was significantly correlated to the timing of smolt migration. Hence, the repeatable individual variation in migration timing seemed to have ecological and fitness consequences in terms of growth, longevity, timing of maturity, and life-time fecundity.

During 1987–2012, Arctic char and brown trout were sampled via permanent fish traps installed across the river 200 m upstream from the sea. All fish larger than 10 cm were trapped with a Wolf trap (apertures 10 mm, inclination 1:10) for descending fish and a fixed box trap for ascending fish. The traps were operated during the ice-free period (April through October) and were emptied twice per day (at 0800 and 2000 h). Body length (L in mm, measured as total length of the fish with the tail fin in its natural position) and mass (M, in g) were recorded for all fish. Smolts of brown trout and Arctic char > 18 cm were tagged with individually numbered Carlin tags. These individuals were recorded each time they passed the fish traps during their annual migration to sea and back to the river for the rest of their lives.