Sammendrag
Sea trout (Salmo trutta) return to the river for spawning and often also for overwintering, and unsuccessful river migration can threaten the population viability. The present thesis investigated the river migration of 44 veteran sea trout in River Beiarelva in Northern Norway. The sea trout were tracked for one year (August 2020 -
August 2021), with main focus on the autumn migration in 2020. Acoustic telemetry was used to assess individual migration patterns between the tidal influenced stretch and the upstream freshwater stretch of the river. Individual’s total body length, body condition factor and sex were linked to behavioural metrics to evaluate if these characteristics could explain the observed behaviour. Upstream migrants entered the freshwater stretch above the tidal zone between 19 August and 28 September, consistent with the peak of upstream migrants in other northern sea trout populations. From August to December, the sea trout spent on average significant less time in the upstream freshwater zone of the river (mean = 41 days) than in the tidal influenced zone (mean = 55 days). The majority of the tagged sea trout (75%) entered the upstream freshwater stretch, probably for spawning. Larger sea trout, sea trout in better body condition and males, in particular, were more likely to enter freshwater than smaller individuals, individuals in poorer body condition and females. This could likely be explained by a higher proportion of immature individuals among the smaller sea trout, sea trout in poorer body condition and, particularly, among females.
The downstream migration from the upstream freshwater stretch to the tidal influenced
waters occurred mainly between 2 October and 21 November (n = 22), although six sea
trout descended over winter (December – April). The relatively shorter freshwater
residence suggest that the tidal zone is a favourable habitat to reside in between August
and December, likely due to unfavourable conditions in the upstream freshwater stretch
and/or better growth opportunities in the tidal influenced stretch. Sea trout were
detected on all receivers deployed in the anadromous stretch of the river, except the
receiver deployed farthest upstream in the tributary Tollåga. Total body length, body
condition factor and sex had limited effects on timing of upstream migration, migratory
distance in the river, timing of downstream migration and freshwater residence time,
suggesting that other intrinsic or environmental factors, not included in this thesis, could
be important for the autumn river migrations of sea trout. This thesis highlights the
importance of tidal influenced parts of rivers for sea trout populations, and implies that
the species may be especially susceptible to habitat alternations and fishing pressure in
the lower parts of rivers.
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