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Confirmation that sea trout are heavily affected by salmon lice

Photo of a sea trout with salmon lice.

Sea trout with salmon lice.

Photo: Rune Nilsen / IMR

In Western Norway, over half of all sea trout are infested with so many salmon lice that you would expect it to seriously harm their health.

That is the conclusion after analysing 2,937 sea trout from the nationwide monitoring programme for salmon lice (data from 2019).

“Our new results confirm that sea trout are exposed to salmon lice over a much longer period than wild salmon, which makes them more vulnerable”, says Thomas Bøhn.

This information will also play an important role when sea trout are included in the traffic light system.

One thousand farmed salmon for every wild salmon

Sea trout are also affected in other parts of the country, but not to the same extent:

“We can see a clear link between the number of farmed fish in an area and the number of salmon lice on the sea trout”, says senior researcher Thomas Bøhn. He is the lead author of a new scientific article published in the highly respected Journal of Applied Ecology.

The salmon louse is a parasite. In other words, it needs a host to live and reproduce on – and this particular parasite needs to find a salmon, sea trout or Arctic charr in order to survive.

“In Norway, there are around one thousand farmed salmon for every wild one in the sea. The salmon louse therefore has many more hosts now than in the past. They are also available all year around, unlike wild salmonids”, explains Bøhn.

Fish farms generate billions of sea louse larvae

There are rules on how many salmon lice there can be on a farmed salmon. As a result, each individual farmed salmon generally has few lice.

“But each female louse can produce 10,000 offspring per year. And a single fish farm may have one million salmon. Consequently, billions of tiny sea louse larvae drift around on the currents, both in fjords and along the coast, waiting to bump into a salmon, trout or Arctic charr to attach themselves to”, explains Bøhn.

Sea trout live in areas where sea lice are most abundant

It is widely known that salmon lice affect wild salmon, but we know much less about their impact on sea trout.

Salmon and sea trout have different life cycles, and the trout spends long periods of time in areas where there are lots of salmon lice:

“Wild salmon migrate relatively quickly out from the river and into the ocean, where there are few salmon lice. Nevertheless, a proportion of them may die if there are lots of salmon lice in the fjords and coastal waters. In the case of the sea trout, its favourite areas are the ones with most salmon lice”, says Bøhn.

Sea trout face two bad options

Sea trout stay in the fjords and along the coast throughout the summer. It’s where they go to grow big and fat”, says Bøhn.

It should mean they grow faster, produce lots of eggs and make a strong contribution to the next generation when they return to the river, but that is not necessarily the case.

“If there are a lot of lice on a sea trout, it has two options: swim back to the river where the salmon lice gradually fall off, since they cannot tolerate fresh water for extended periods, or remain in the sea where it risks being infested by more lice and suffering serious harm”, he explains.

Neither of these two options are good:

“If the fish stays in the sea, it risks becoming so weak that it dies. But there is also a cost to returning to fresh water. Firstly, the fish may already have so many lice on it that it dies anyway. Secondly, it will lose a lot of the weight that it migrated to the sea to put on. That will have a negative impact on sea trout populations”, explains Bøhn.

Counting lice on sea trout confirms value of models and infection maps

The new findings came out of a review in which field data from the national monitoring programme for salmon lice were compared with modelled data. The model uses louse counts and the number of farmed salmon, temperature, salinity and sea currents to create up-to-date maps showing the risk of salmon louse infection along the whole Norwegian coast. You can monitor these infection maps, which are updated each week and published online (www.lakselus.no).

“The comparisons that we have done between field data and modelled data confirm that the detailed model for the spread of salmon lice on www.lakselus.no closely matches real-world data”, concludes Bøhn.


Bøhn, Thomas, et al. "Salmon louse infestation levels on sea trout can be predicted from a hydrodynamic lice dispersal model." Journal of Applied Ecology (2021).

More about the impact of sea trout migrating upriver early:

Serra-Llinares RM, Freitas C, Nilsen R, Elvik KMS, Albretsen J, Bøhn T, Karlsen Ø, Bjørn PA (2018). Towards direct evidence of the effects of salmon lice (Lepeophtheirus salmonis Krøyer) on sea trout (Salmo trutta L.) in their natural habitat: proof of concept for a new combination of methods. Environmental biology of fishes 101 (12): 1677—1692. doi.org/10.1007/s10641-018-0816-1

Serra-Llinares RM, Bøhn T, Karlsen Ø, Nilsen R, Freitas C, Albretsen J, Haraldstad T, Thorstad EB, Elvik KMS, Bjørn PA (2020). Impacts of salmon lice on mortality, marine migration distance and premature return in sea trout. Marine Ecology Progress Series 635:151-168. doi.org/10.3354/meps13199