The Preadult II stage and the Adult stage are free to move around on the fish's body, but they tend to congregate in certain areas, such as behind the dorsal fin, behind the anus, and on the top of the head.
This picture shows a common congregation
spot for sea lice on Salmon. This is an area just
behind the dorsal fin (on the right).
The sea lice found on fish eat the epidermis (skin) along with mucus, blood and cells, and if given enough time, can eat right through the fish's protective layer. The exposure of delicate underlying tissues can cause death due to bacterial infections, stress, and osmotic regulation problems. In some cases, sea lice have even eaten through the entire skin covering the head, and the brain became exposed!
In this photograph, a congregation of females
with egg strings has eaten through the skin and
into the delicate underlying tissue (pink).
This depicts a salmon who had a severe sea lice infestation. They
have eaten through all of the skin protecting the brain leaving it
Some species are more suceptible to sea lice infestation than others. Atlantic Salmon do not have enough of a response to reject the parasite, and often get lesions and severe cell damage if not treated. However, a related species - the Coho salmon - mounts enough of a response to kill the lice.
In addition to the cellular damage sea lice cause to the fish, there are other important effects. Fish affected with lice can get extremely stressed, and this stress can even kill the fish. In addition, fish which have infections are often less active and are smaller than normal fish, and their lesion-marked bodies cause them to be downgraded in value in the market.