Sea Lice in Farmed Salmon: Biology, Impact and Control
Sea lice (Lepeophtheirus salmonis, Caligus spp.) are parasitic copepods that damage farmed salmon skin. Control combines cleaner fish, mechanical and chemical methods.
Overview
Sea lice are among the most economically and welfare-significant parasites in salmon farming. They are copepods (small crustaceans) of the family Caligidae, order Siphonostomatoida. The most important species on farmed salmonids are Lepeophtheirus salmonis in the Northern Hemisphere and several Caligus species, including Caligus elongatus and Caligus rogercresseyi. They attach to the host and feed on mucus, skin, and tissue, causing physical and enzymatic damage.
Affected hosts and life cycle
Principal hosts include Atlantic salmon (Salmo salar), sea trout (Salmo trutta), Arctic char (Salvelinus alpinus), and Pacific salmon. The life cycle of Lepeophtheirus salmonis proceeds through planktonic nauplius stages, an infective host-seeking copepodid, attached chalimus stages, mobile pre-adult stages, and the adult. Development time is temperature-dependent, ranging from roughly two and a half weeks in warm water to several months in cold water.
Clinical signs and impact
- Abrasion-like skin lesions and erosion at attachment and feeding sites
- Scale loss and, in heavy infestations, deep lesions especially on the head region
- Osmoregulatory stress from disruption of the skin barrier
- Open wounds that allow secondary bacterial and fungal infection
- Suppressed immune response and increased susceptibility to other pathogens
- Reduced growth, poor welfare, and mortality in untreated heavy infestations
Diagnosis
Diagnosis is based on direct visual examination and counting of attached lice on the fish, typically reported as the average number of mobile or adult female lice per fish. Routine monitoring on farms tracks these counts against regulatory thresholds to trigger control actions.
Control and prevention
Because no single method is fully reliable and because parasites can develop resistance to chemicals, integrated management combines several approaches.
Biological control
Cleaner fish that graze lice off salmon, including various wrasse species and lumpfish (Cyclopterus lumpus), are widely deployed. Their effectiveness depends on stocking ratio and timing within the production cycle.
Mechanical and thermal methods
Non-medicinal delicing methods physically remove lice, for example with freshwater or warm-water baths and mechanical flushing systems. Lice skirts and barriers around pens reduce the number of infective stages reaching the fish.
Chemical treatment
- Avermectins such as emamectin benzoate, administered in feed
- Organophosphates such as azamethiphos, used as a bath treatment
- Pyrethroids such as cypermethrin and deltamethrin, used as bath treatments
- Oxidising agents such as hydrogen peroxide, used as a bath treatment