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Topic: Sea lice

The salmon louse is the most common parasite on farmed salmon, and the biggest disease problem in the aquaculture industry in Norway. Monitoring programs of salmon lice shows that the magnitude of the problem is increasing and that in many cases the lice have become resistant to frequently used treatments such as oral and bath treatments.

The salmon louse is a marine parasitic crustacean on salmonids (Atlantic salmon, sea trout and Arctic char), occurring naturally in the northern hemisphere.

Salmon lice does not tolerate low salinity and will eventually fall off when the fish returns to the river. Hence, the presence of salmon lice on the host was considered a sign that the fish had recently returned from the sea, but laboratory experiments have shown that salmon lice can survive on the fish for up to 14 days in fresh water.

The louse damages the host fish by eating mucus, skin and blood. This makes the fish more susceptible to other infections such as bacteria, viruses and fungi and also affects the salt balance of the fish. Large numbers of salmon lice can cause the fish to die, but also indirect damage, reduced growth and a reduction in suitable habitats (as the fish avoids areas with many lice) weaken the reproductive potential of wild salmonids.

Farming of Atlantic salmon (Salmo salar) in Norwegian fjords and along the coast has increased in recent decades, and it is a goal for both industry and the authorities that production should be further intensified. Since salmon farming takes place in open cages, parasites such as salmon lice can freely spread from farmed to wild fish. The large increase in the number of available hosts along the coast has led to an increased abundance of salmon lice on wild salmonids. Salmon lice are therefore a challenge for the sustainable growth in the aquaculture industry. Consequently, to reduce the amount of lice on wild fish, the government has introduced an limit on the number of allowed salmon lice on farmed fish  

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The salmon lice infection pressure, estimated with the Institute of Marine Researchs’ bio-hydrodynamic model, show the number of infective salmon lice larvae (copepods) per square meter during the time frame of interest.

Salmon lice begin life with larval stages hatched directly into the water masses and develop through three non-feeding planktonic stages: two nauplius stages (non-infective) and the infective copepodid stage. If the lice larvae cannot locate a host fish in time they will die from starvation or predation. During these early life stages the larvae can position themselves vertically in the water column, but otherwise they drift freely horizontally with the water currents.

Information on the number of hatched eggs from the salmon farms is crucial for the quality of model estimated salmon lice infection pressure. Based on the temperature and number of adult female lice per fish (reported weekly by the fish farms), and the estimated number of fish in the farms (reported monthly), the number of nauplii released into the water masses per hour from each farm is calculated. This information is fed into a dispersion model that estimates the position of the salmon lice larvae as a function of the oceanographic conditions (currents, temperature and salinity). 

The salmon lice infection pressure shown at lakselus.no is estimated in real time, thus the number of fish at the farms might be inaccurate in cases where fish are slaughtered but have not yet been registered in the database. The model estimates are therefore calculated in retrospect when more accurate registrations from the farms are available, before the estimated salmon lice infection pressure are use in management assessments.

Derivative product

The results from the salmon lice dispersion model are calibrated against observations on 1) salmon lice on fish in sentinel cages and 2) lice on migration postsmolt of Atlantic salmon catch by trawl. Thus, information on when and where there are unacceptable amounts of salmon lice larvae in the water masses can be provided – also in areas where we do not have observations.

How do we use the results?

The results from the bio-hydrodynamic model serve as a status report on the level of lice in the fjords and along the Norwegian coast. These model data are included as important parts in 1) planning of field activity, 2) report to the Norwegian Food Safety Authority, 3) risk assessment of Norwegian fish farming and 4) basic data for the traffic light system.

Sea lice: Caligus curtus and Caligus elongatus

Three fish lice commonly found in Norwegian waters: Cod lice (Caligus curtus) on the left (female and male), salmon lice (female and male) and Caligus elongatus (female with eggs and male). In the case of cod lice, the male is the largest, while in the other two species female specimen are larger.

Photo: Lars Hamre

The salmon louse has two "relatives" which can be found on non-salmonid fish.

Many fish can be infested with lice, which as a group can be called fish lice. Some fish lice are species-specific and can only be found on one or a few fish species. An example of this is the salmon louse that only infects fish from the salmon family (salmonids). In Norway, this includes Atlantic salmon, sea trout and Arctic char, plus farmed rainbow trout.

Looks like a salmon louse

Similar to salmon lice which are only found on salmonid fish, cod lice can only be found on fish from the cod family.

Cod lice on cod. Adult female with eggs. The photo also contains another much smaller louse just below the cod louse. This louse is a smaller stage and has not been identified to species. Photographer Bjørnar Skjold / IMR.  

These lice look relatively similar (compare this image with the image of all three lice species at the top), and looking at the fish host can often guide you to determine the louse species. If you want to be sure of what species of louse you have, you should take high quality pictures, preferably of adult individuals of both sexes. Remember to include something to indicate the size. Alternatively, the animal must be collected on alcohol and species determined using molecular methods. 

Caligus elongatus is not a picky eater

But there are also fish lice that are not so picky in their choice of fish host. An example of this is the the fish louse, Caligus elongatus (in Norwegian known as “the Scottish louse”) that has been found on many different fish species including lumpsuckers, herring, capelin but also on fish from the cod and salmon families.

Caligus elongatus are smaller than both the salmon louse and the cod louse and is more common in northern Norway. Caligus elongatus also has a different behaviour than the salmon louse and can often be observed to transfer between fish. 

For example, farmed fish may experience large transfers of Caligus elongatus from wild fish when schools of herring or capelin pass by farm sites.