developed gastrointestinal tract and feed on host mucus and tissues within range of their attachment. Preadult and adult sea lice, especially gravid females, are aggressive feeders, in some cases feeding on blood in addition to tissue and mucus. Blood is often seen in the digestive tract, especially of adult females. L. salmonis is known to secrete large amounts of trypsin into its host's mucus, which may assist in feeding and digestion. Other compounds such as, prostaglandin E2, have also been identified in L. salmonis secretions and may assist in feeding and/or serve the parasite in avoiding the immune response of the host by regulating it at the feeding site. Whether sea lice are vectors of disease is unknown, but they can be carriers of bacteria and viruses likely obtained from their attachment to and feeding on tissues of contaminated fish. Disease Pathology Sea lice cause physical and enzymatic damage at their sites of attachment and feeding, which results in abrasion-like lesions that vary in their nature and severity depending upon a number of factors, including host species, age, and general health of the fish. Whether stressed fish are particularly prone to infestation is unclear. Sea-lice infection causes a generalized chronic stress response in fish since feeding and attachment cause changes in the mucus consistency and damage the epithelium resulting in loss of blood and fluids, electrolyte changes, and cortisol release. This can decrease salmon immune responses and make them susceptible to other diseases and reduce growth and performance. The degree of damage is also dependent on the species of sea lice, the developmental stages that are present, and the number of sea lice on a fish. Little evidence exists of host tissue responses in Atlantic salmon at the sites of feeding and attachment, regardless of the development stage. In contrast, coho and pink salmon show strong tissue responses to L. salmonis characterized by epithelial hyperplasia and inflammation. This results in rejection of the parasite within the first week of infection in these species of salmonids. Heavy infections of farmed Atlantic salmon and wild sockeye salmon (Oncorhynchus nerka) by L. salmonis can lead to deep lesions, particularly on the head region, even exposing the skull. Interactions between wild and farmed fish Some evidence indicates that sea lice flourishing on salmon farms can spread to nearby wild juvenile salmon and devastate these populations. Sea lice, particularly L. salmonis and various Caligus species,