Sammendrag
The timing of smolt migration is a key phenological trait with profound implications for
individual survival during both river descent and the subsequent sea sojourn of anadromous
fish. We studied relationships between the time of smolt migration, water temperature
and light intensity for Atlantic salmon (Salmo salar) and sea trout (Salmo trutta).
During 2006–2012, migrating smolts descending the southern Norway River Storelva
were caught in a rotary screw trap located at the river mouth. The date of 50% cumulative
smolt descent correlated significantly with the date when the river temperature
exceeded 8°C for both Atlantic salmon and sea trout smolts. In 2010, smolts of both
species were passive integrated transponder (PIT)-tagged,
and the diel timing of their
migration was precisely documented. The degree of night migration decreased in both
species as the river temperature rose, and at temperatures above 12–13°C, more smolts
migrated during day than during night. A multinomial model was fitted for estimating
temperature and species effects on probabilities of migration during night, daytime,
dusk and dawn. Atlantic salmon smolts preferred migrating under lower light intensities
than sea trout smolts during early, but not late spring when both species migrated during
bright daylight. In accordance with the early-season
tendency to migrate at night, Atlantic
salmon smolts migrated more during darker hours of the day than sea trout. In both
species, smaller smolts migrated under dark conditions than during light conditions.
Most of the findings on thermal, light and temporal effects on the observed smolt migration
pattern can be explained as adaptations to predation avoidance.
migration timing, phenology, temperature influence, light intensities, anadromous fish, predation
avoidance
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