The Institute of Marine Research has monitored the levels of sea lice in salmon smolt for many years, using several different methods. Wild smolt have been caught using a floating trawl, small cages containing smolt have been placed in fjords in the spring and hatchery-reared smolt have been treated against sea lice before being tagged and released. By comparing the recapture rates for adult fish that have and have not been treated against sea lice, we can estimate the importance of sea lice during the smolt migration. All of these methods have shown that sea lice are an unknowable factor. Sea lice larvae spread with the currents, and simulations of water movement in fjords and coastal areas have attempted to explain why sea lice levels vary significantly both over time and in space.
Half of all of the smolt in Daleelv have been treated against sea lice. In many of the groups that we have released, we have not observed the treatment to have any effect. However, in some cases it has been clear that more treated than untreated smolt have survived and returned. One year the grilse that had not been treated when they were smolt were also smaller than those that had been treated, probably because their growth in the sea was stunted due to them being infected with sea lice. The true impact of sea lice may, however, be bigger than observed in these tagging experiments. Studies have revealed that some of the fish may have received relatively low doses of the substance that protects against the sea lice, in addition to which the substance breaks down fairly quickly. This means that many of the fish may have been inadequately protected if they spent some time getting out of the river and fjord before meeting the sea lice further out by the coast. Unlike in fjords with a less marked fresh water layer on the surface, such as Hardangerfjorden, we have assumed that in Osterøy the probability of the smolt being infected by sea lice is much greater outside than inside the fjord system.
