Background levels of sea lice parasites occur naturally in the sea. However, the arrival and exponential expansion of salmon farming, particularly in fjordic or largely enclosed sea lochs, has caused to a fundamental change in the density and occurrence of sea lice in the coastal waters of the west Highlands and Islands. Even one or two mature female sea lice per fish within a set of cages housing hundreds of thousands of farmed salmon amount to a very large breeding reservoir producing huge numbers of mobile juvenile sea lice that move out into the local marine environment.

The consequences for wild salmon and sea trout smolts, the metamorphosing fragile skin of which is not adapted to cope with more than a few lice, as they migrate from local rivers to the sea, can be devastating. Lice feed by grazing on the surface of the fish and eating the mucous and skin. Large numbers of lice soon cause the loss of fins, severe scarring, secondary infections and, in time, death. Badly infested wild salmon smolts disappear out to sea, never to be seen again. In contrast, afflicted sea trout smolts remain within the locality and they, together with the impact of the deadly burdens they carry, are more easily monitored.