The salmon louse lives on the fish’s mucus, skin and blood. A salmon smolt with more than 10-15 salmon lice is so weakened that it is not likely to survive its sojourn in the sea before returning to the river to spawn (Asplin, L. et al., 2002). Besides the fact that salmon lice can produce directly lethal effects on salmonids, harm has been observed at lower infestation rates. Bite injuries from lice on the fish give pathogens a better foothold and can cause disease in the fish. In addition to the bite injuries, low nonlethal infestations will induce stress responses in the fish. Fish under stress have problems with their salt balance, reduced immunity and are more at risk of infections (Tully, O. et al., 2002).

Before fish farming activities began in earnest along the Norwegian coast, winter was a bottleneck for the salmon louse due to the low number of hosts at that time. The explosive growth of salmonid farming in Norway changed this situation drastically. Today there are large quantities of salmonids in the sea all year round. This makes it possible to sustain a large population of salmon lice and high infestation pressure all year.

In some years we have seen very serious salmon lice infestations on emigrating salmon smolt. The Institute for Marine Research’s counts of lice in Sognefjord in 1999 were unsettling. That year the infestation was 104 lice per fish and a conservative estimate of 86% mortality. The following year the infestation fell to 36 lice per fish and an estimate of at least 65% mortality (Holst J.C. et al., 2001). The fish farmers’ defence has been to focus on this being a year with optimal temperatures for lice and little supply of fresh water due to a winter with little snow. This provided good conditions for lice with subsequent major infestations of wild salmonids. Nevertheless, the situation in 1999 is within natural fluctuations, and there will be more years with low snowmelts and temperatures that suit the salmon louse. These scenarios in which the salmon louse obtains favourable conditions must be decisive for the extent fish farming that may be permitted in an area.

Since 1991 serious infestations of wild stocks of sea trout have been observed. Studies from sixty-three different rivers and streams showed that in fifty-seven of these, the sea trout had problems with salmon lice infestations. An average of 250 salmon lice per fish counted was observed for 1992 (Consulting biologists, 2003). However, the situation has improved since the early 1990s, though the Institute for Marine Research still considers the situation of the sea trout unsatisfactory and probably critical in many places (Holst J.C., 2003). Even though the number of infestations seems to have been reduced, it is still clearly higher than in regions far away from salmon farming, where an infestation level is estimated that is similar to that in western Norway before fish farming was established (Consulting biologists, 2003).

Unless we stop farming salmonids or make all facilities land-based, the likely scenario according to the Institute for Marine Research is for salmon lice, wild salmon and farmed salmon to coexist on the Norwegian coast for the foreseeable future. However, in this scenario the authorities ought to be able to require that salmon lice levels in fish farms be maintained that are sustainable with regard to the salmon and sea trout stocks in the individual fjord system.