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Topic: Sea lice

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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.

Published: 20.04.2020 Updated: 29.03.2021

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.

Published: 17.12.2018 Updated: 29.03.2021

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.

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Facts about sea lice

Latin name: Lepeophtheirus salmonis. Subspecies, L. salmonis salmonis in the Atlantic and L. salmonis oncorhynchi in the Pacific ocean.
Distribution: Occurs naturally in Norwegian waters. Their numbers have risen significantly in parallel with the growth of the aquaculture industry.
Biology: Sea lice are parasites with eight life stages, the first three are free-swimming, the next two are sessile and the final three are mobile. They attach themselves to salmonid fish in the third life stage. 
Size: Adult female: 12 mm (approx. 29 mm including egg strings); adult male: 6 mm.
Diet: Mucus, skin and blood of salmonid fish. The lice only feed when on a host fish (sessile and mobile stages).
Reproduction: All year round, but reproduce faster in higher temperatures. ​​​​​​​
Dispersal: Free-swimming stages spread with the currents in fjords and along the coast.
Treatment of farmed fish: Preventive skirts or delousing using cleaner fish (Wrasse or Lumpsucker), chemicals or mechanical delousing methods. 
 

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