Repeatable individual variation in migration timing in two anadromous salmonids and ecological consequences

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.