MEPS prepress abstract  -  DOI: https://doi.org/10.3354/meps12403

Migration of Atlantic salmon post-smolts in a fjord with a high infestation pressure of salmon lice

Elina Halttunen*, Karl-Øystein Gjelland*, Kevin A. Glover, Ingrid Askeland Johnsen, Rosa-Maria Serra-Llinares, Øystein Skaala, Rune Nilsen, Pål-Arne Bjørn, Ørjan Karlsen, Bengt Finstad, Ove T. Skilbrei

*Email: elina.halttunen@gmail.com

ABSTRACT: Understanding Atlantic salmon post-smolt coastal migration behaviour is crucial for predicting their exposure to ecological challenges including the parasite salmon lice. 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 timing of release or treating post-smolts with an anti-parasitic drug (prophylaxis) altered their migratory behaviour. We found no significant differences in mean progression rates between the three release locations, genetic groups or treatments (range 11.5–16.9 km d-1). However, individual variation in progression rate and migratory routes resulted in large differences in fjord residence times (range 2–39 days). 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 intensity growth rates from smolts held in sentinel cages throughout the fjord, indicated that individuals taking more than 10 days 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 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 the potential migration routes and in the full potential migratory period.