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.

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.

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.