Sammendrag
Understanding Atlantic salmon Salmo salar post-smolt coastal migration behaviour is
crucial for predicting their exposure to ecological challenges such as the parasite salmon louse
Lepeophtheirus salmonis. We compared the migration of acoustically tagged, hatchery-reared
Atlantic salmon post-smolts of wild and domesticated origins from the inner, middle and outer part
of a 172 km long aquaculture-intensive fjord in western Norway. Additionally, we examined if the
timing of the release or treatment with an anti-parasitic drug (prophylaxis) altered migratory
behaviour. We found no significant differences in mean progression rates among the 3 release
locations, among genetic groups or between treatments (range: 11.5−16.9 km d−1). However, individual
variation in progression rates and migratory routes resulted in large differences in fjord residence
times (range: 2−39 d). Ocean-current directions during and after release affected swimming
speed, progression rate and route choice, and for most post-smolts, swimming speeds were
much higher than their progression rates out of the fjord. The predicted lice loads based on lice
intensity growth rates on smolts held in sentinel cages throughout the fjord indicated that individuals
taking >10 d to exit the fjord in periods with high infestation pressure are likely to get lethally
high sea-lice infestations. We conclude that, as migratory routes of S. salar post-smolts are hard to
predict and migration times can stretch up to over a month, it is important to develop aquaculture
management that keeps salmon lice levels down along all potential migration routes and during
the full potential migratory period.
Migratory behaviour · Salmo salar · Lepeophtheirus salmonis · Acoustic telemetry ·
Management · Fish farming · Parasite
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