Sea trout, Salmo trutta, are found in rivers and coastal seawaters in many regions of the world. Sea trout lay their eggs in rivers and the young fish remain in this freshwater environment. After living for 1–8 years in freshwater rivers, sea trout migrate to the sea. Adult sea trout feed in coastal seawater regions, usually no more than 80 km from their home river. Fish farms that produce Atlantic salmon, Salmo salar, may be located in the coastal seawater regions where wild sea trout are also found. Fig. 2.1 shows one cage at a fish farm. [Figure 2.1] AIR SEAWATER net to stop birds fish cage farmed Atlantic salmon wild sea trout not to scale Fig. 2.1 The increase of fish farming is thought to have caused more infections by salmon lice, Lepeophtheirus salmonis, in wild fish such as sea trout. When sea trout enter coastal seawater regions, they can become infected with salmon lice. Salmon lice larvae live in seawater and can move freely between fish cages and open seawater. Salmon lice attach to the skin of sea trout and feed on their skin and blood. The resulting tissue damage can sometimes cause the death of sea trout. Fig. 2.2 shows a sea trout infected with salmon lice. [Figure 2.2] body of sea trout salmon lice Fig. 2.2