Sea lice are naturally-occurring tiny, translucent ectoparasitic copepods, which affect both wild and farmed fish and do not affect humans. Sea lice attach to the surface of a fish's skin, usually behind sheltered areas such as the fins and gills.

Once attached they will feed off the mucus, skin and blood which can cause the following pathogenic effects in salmon: skin damage, bleeding, a decrease in carcass quality at harvest, reduced growth rates, a loss of their physical and microbial protective function, and increased susceptibility to secondary infections due to the disruption of the epithelium (tissues) at the point of attachment.

Aside from the direct damage to the fish’s skin, the potential for secondary infections to arise is a major concern for Scottish salmon farmers. Treating salmon for these parasites is paramount to maintaining fish health and welfare.

In all farming situations animals are held together in a group. As with any disease or parasite issue this makes it easier for the problem to rapidly increase to high densities within the population.

Keeping fish healthy is of primary importance to Scottish salmon farmers. The sector has invested heavily in recent years to develop a diverse and effective array of management methods, including the use of cleaner fish, that enable it to effectively control the numbers of wild sea lice (Figure 1).

In order to maintain trust in the salmon farming sector and offer increased levels of transparency the Scottish Salmon Producers Organisation (SSPO) published sea lice data for all active farms on a monthly basis. Responsibility for publishing such data has now passed to the Scottish Government and Marine Scotland.

Below you can see that numbers of sea lice have been decreasing steadily over the last few years, and for 2018 were at their lowest ever.

How planktonic stages of sea lice disperse and find new hosts is still not completely known but marine temperature, light, and currents are all major factors.

Watch: Are salmon troubled by parasites?