Sea Lice

Sea lice are small marine parasites that occur naturally on many different species of wild fish including wild adult salmon. Sea lice are planktonic and are transported on the tide. When they encounter marine fish they attach themselves, usually on the skin, fins and/or gills and feed off the mucous or skin.

There are 13 known species of sea lice in the marine waters of British Columbia, but the common ‘salmon louse’ is the one we hear the most about. The Latin name for this salmon louse is Lepeophtheirus salmonis.

Sea Lice & Salmon Farms

Sea lice from salmon farms are one of the most significant threats facing wild salmon in British Columbia. Stocked year round with hundreds of thousands of fish in small areas (net-cages) fish farms are ideal, and unnatural breeding grounds for lice. Infestations on farms significantly increase the number of lice in surrounding waters, far beyond what would occur naturally.

In the spring, when fish eggs hatch and juvenile salmon emerge from the rivers and make their way to the ocean many are exposed to sea lice during their journey because fish farms are typically located in sheltered waters along wild salmon migration routes. Juvenile pink and chum salmon are smaller than an AAA battery when they migrate by salmon farms and some may not have fully developed scales yet. When lice attach themselves to juveniles, their bodies may not be able to cope, and they may die.

Peer-reviewed research has shown that one to three sea lice are enough to kill a juvenile pink salmon newly arrived in saltwater.1

To understand the link between salmon farms, sea lice and wild salmon – watch this short video:

Sea Lice Infestation & Disease

Sea lice feed on the mucous, blood and skin of salmon. While a few lice on a large salmon may not cause serious damage, large numbers of lice on that same fish, or just a couple of lice on a juvenile salmon, can be harmful or fatal. The feeding activity of sea lice can cause serious fin damage, skin erosion, constant bleeding, and deep open wounds creating a pathway for other pathogens.

It is also possible for sea lice to carry diseases between farmed and wild salmon. This disease “vector” has already been shown for Infectious Salmon Anemia (ISA) on the Atlantic coast.2,3 An outbreak of ISA on salmon farms in Chile in 2007 spread rapidly from one farm to the next, leading to whole pens and in one case an entire farm’s worth of fish having to be destroyed. Sea lice have been identified as a possible factor in the rapid spread of the disease.

The furunculosis bacterium has also been found on the bodies of sea lice, making it likely that sea lice spread this disease as well.4

The Science on Sea Lice is Clear

Research published in the prestigious journal Science in December, 2007 was the first study to calculate the impact individual wild salmon mortalities from sea lice infestation can have on the population of a whole run of salmon.5

A growing body of peer-reviewed research indicates that sea lice are dangerous to juvenile wild salmon. Check out our sea lice research page for the latest science on sea lice.


1 Morton, A. and R. D. Routledge (2005). Mortality rates for Juvenile Pink Oncorhynchus gorbushca and Chum O. keta salmon infested with Sea Lice Lepeophtheirus salmonis in the Broughton Archipelago. The Alaska Fisheries Research Bulletin. 11(2): 146-152

2 Dannevig, B.H. and K.E. Thorud, Other viral diseases and agents of coldwater fish: infectious salmon anemia, pancreas disease and viral erythrocytinecrosis, in Fish Diseases and Disorders, Volume 3, Viral, Bacterial and Infections, P.T.K. Woo and D.W. Bruno, Eds. 1999, CAB International: Wallingford and New York p. 149-175.

3 APHIS Veterinary Services, Infectious Salmon Anemia Tech Note. 2002, US Department of Agriculture.

4 Johnson, S.C., Crustacean Parasites, in Diseases of Seawater Net pen-reared Salmonid Fishes, M.L. Kent and T.T. Poppe, Editors. 1998, Fisheries and Oceans Canada: Nanaimo, BC. P. 80-90.

5 Krkošek, M., Ford, J. S., Morton, A., Lele, S., Myers, R. A. and Lewis, M. A. (2007). Declining wild salmon populations in relation to parasites from farm salmon. Science 318: 1772-1775.