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 to them, 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 the salmon louse is Lepeophtheirus salmonis.

Sea lice are common on adult salmon, and usually don’t cause major physical damage. In contrast, sea lice have always been rare on juvenile salmon—at least, until the advent of salmon aquaculture.

BC’s sea lice epidemic

	Photo: Alexandra Morton

Sea lice from salmon farms are one of the most significant threats facing wild salmon in British Columbia.

Although sea lice occur naturally, lice infestations have only recently put wild salmon populations at risk. In the spring, fish eggs hatch and juvenile salmon emerge from the rivers and make their way to the ocean estuaries and bays. It is primarily when young salmon fry migrate past salmon farms that they encounter large concentrations of sea lice. When these large concentrations of lice attach themselves to juveniles, their bodies may not be able to cope, and they may die.

Stocking hundreds of thousands of fish in small areas (net-pens) makes fish farms ideal and unnatural breeding grounds for lice. This significantly increases the number of lice in surrounding waters and the threat to out-migrating wild juvenile salmon.

• Farmed fish make great hosts for sea lice because they are confined and in high densities.
• 91% of BC’s farmed salmon are the non-native Atlantic species, which are more susceptible to sea lice than many other salmon species¹
• Research experiments have shown that pink and chum salmon fry can die when infected with only a single mature sea louse, and data suggests that up to 95% of migrating juvenile salmon can be infected².
• Current chemical treatment of sea lice on farms may be harmful to other marine species and may not reduce lice levels enough to protect wild salmon

Location of Farms Is Important

Fish farms are typically located in sheltered bays and inlets near rivers on or near the migratory routes juvenile salmon use to reach the open ocean.

Before commercial-scale salmon farming began, sea lice numbers were typically low in the spring because the number of available hosts in coastal areas was also low.

Juvenile pink and chum salmon are smaller than an AAA battery when they migrate by salmon farms. Salmon farms create an unnatural reservoir of sea lice that is particularly perilous for juvenile wild pink and chum salmon heading for the ocean because of their small size and the fact that some may not have fully developed scales yet. Peer-reviewed research has shown that one to three sea lice are enough to kill a juvenile pink salmon newly arrived in saltwater³. Much higher numbers of lice have been observed on juvenile pink salmon near some of BC’s salmon farms.

British Columbia boasts one of the greatest diversities of wild salmon on the planet. Over 9,000 “runs” of wild Pacific salmon still survive in BC’s rivers. Sea lice from fish farms can have serious consequences on wild salmon which, in turn, can affect overall ecosystem health and function.

How sea lice harm wild salmon

A growing body of peer-reviewed research published in prestigious science journals—indicates that sea lice are dangerous to juvenile wild salmon.

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.^4,5 An outbreak of ISA on salmon farms in Chile in late fall, 2007 has 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.⁶

The Science on Sea Lice is Clear

Cutting edge research published in the prestigious journal Science in December, 2007 was the first study (link to abstract) to calculate the impact individual wild salmon mortalities from sea lice infestation can have on the population of a whole run of salmon.² The recurrent louse infestations associated with salmon farms in BC’s Broughton Archipelago, have depressed wild pink salmon populations there and placed them on a trajectory toward rapid local extinction. The Science study shows louse-induced mortality of pink salmon is commonly over 80% and exceeds previous fishing mortality. The study concludes:

• If outbreaks continue, a 99% collapse in pink salmon population abundance is expected within two salmon generations (four years) from the study’s publication date and local extinction is predicted.

• A 99% population collapse means, in just four short years, the pink salmon runs in the area will disappear, impacting the bears, orcas (killer whales), eagles, seals, sea lions and fish species that they sustain.

• The decaying bodies of salmon also fertilize riparian (stream-side) forests, contribute to nutrients and feed coastal food webs. The loss of pink salmon populations will erode the entire coastal ecosystem, threatening the survival of not only the flora and fauna but also the communities and economies that depend on these resources.

References
¹ BC Seafood Industry Year in Review (2006) BC Ministry of Agriculture and Lands. (pdf)
²Krkošek, 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.
³ 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
⁴Dannevig, 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.
⁵APHIS Veterinary Services, Infectious Salmon Anemia Tech Note. 2002, US Department of Agriculture.
⁶Johson, 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.