The sticky problem of sea lice – and what's being done to stop them

Since its inception in the 1970s, the global salmon farming industry has struggled to manage the severe economic, animal welfare, and ecological impacts of sea lice infestation, which are preventing salmon farming from reaching its true potential. The economic impact has been estimated at between USD 400 to USD 600 million (EUR 366 million to EUR 549 million) per year. 

Sea lice (Lepeophtheirus salmonis and members of the Caligus genus), settle on their host as free-swimming larvae, attaching firmly to and feeding from the salmon. They cause physical damage and stress in the fish, and adversely affecting growth and performance. Severe infestations can lead to secondary infections and mass mortalities. 

Sea lice are not harmful to humans, but the lesions caused by even a minor infestation can make salmon unmarketable.

Unfortunately for industry, sea lice have been around for millions of years and have adapted well to living on salmon. They are difficult to control, and can quickly build up resistance to chemical treatments used to eradicate them. 

According to Geoff Boxshall, a professor at the Natural History Museum in London, United Kingdom, and a copepod crustacean expert, finding a host in the wild is a difficult part of the life cycle for sea lice. However, it is much easier for the parasites to find hosts when fish are kept together in aquaculture.

In Norway, which is the leading producer of Atlantic salmon (Salmo salar), responsible for 47 percent of global production, the Norwegian Institute of Marine Research (IMR) has implemented a sophisticated digital model system, in conjunction with field monitoring, to track and advise on the level of sea lice infestation. The results are published online every week, showing the number of free-swimming lice larvae added over the previous 10 days. 

In Chile, the world’s second-largest producer of salmon, responsible for around 30 percent of global salmon production, sea lice are also a major problem. According to Salmones Camanchaca CEO Manuel Arriagada, Chile’s salmon farms face mandatory harvests if the number of adult female sea lice rise above three per fish. 

To fight the pests, for the past few decades, salmon farmers have used a range of chemotherapeutants in the battle against sea lice outbreaks. However, these can have negative effects on the fish, reducing appetite and growth, and preventing the salmon from being sold for several weeks post-treatment.  As a result, there has been a recent shift away from chemotherapeutants such as azamethiphos, cypermethrin, deltamethrin, and hydrogen peroxide, which are used in water bath treatments, and avermectins, which are used in medicated feeds. 

In their place, mechanical and thermal treatments have been introduced. Cleaner fish including wrasse and lumpfish are also widely used, resulting in an increasing number of hatcheries dedicated to producing these species specifically for industry. Special diets have also been produced to maintain wrasse health.

Optical underwater counting systems and lasers, such as the Stingray, are gaining in popularity, along with freshwater treatment in well-boats. Mechanical delousing systems include the Hydrolicer and Optimar's Optiliser, which require fish to be pumped through a low-pressure spray system that removes the lice.

Thermal options such as the Thermolicer rely on exposing fish to lukewarm seawater, which causes the sea lice to detach from the fish, after which they are collected and destroyed. A 2018 report by the Norwegian Veterinary Institute confirmed that the preferred method of non-medicinal lice treatment is currently heated water. Of a recent 21 percent increase in non-medicinal treatment, 68 percent was thermal delousing.

Twelve salmon-farming companies are now collaborating to test new treatment systems and share results in a bid to improve the effectiveness of sea lice eradication.  Big players including Mowi, Salmonis Austral, and Cermaq are reviewing the current treatment methods, which include the Optilicer, Hydrolicer and the FLS Delouse system. They will also test a number of new tools.

The basket of chemotherapeutants used for salmon in the first five months at sea include Lufenuron, followed by Azamethiphos for the next five months. However, industry reports hint that Azamethiphos is losing its efficacy. A new treatment, Alfaflux, is being evaluated for this stage. However, Camanchaca’s Arriagada warned that it takes considerable time to properly evaluate new treatments.

For the final stage of production, Azamethiphos remains the industry’s weapon of choice, but a study of treatment with hydrogen peroxide is starting in October and its efficacy will be evaluated over the next year.

Salmoclinic, a continuous swim-through bath treatment that offers precise dose control and a low-stress environment for the fish, will be also be trialed, along with freshwater treatments in well-boats.

In both Norway and Salmon, other approaches include use of feed formulas that can promote enhanced immunity in salmon and significantly affect the infection intensity of sea lice and the use of permaskirts as physical barriers in cages to slow the settlement of sea lice larvae.

But the most effective method of fighting sea lice over the long-term is most likely selective breeding, according to Nina Santi, the chief executive of salmon and trout egg breeding company AquaGen in Norway. Santi told SeafoodSource that her company has been using powerful genomic tools to select for sea lice resistance since 2013, and is involved in ongoing collaborations with the Sea Lice Research Centre at the University of Bergen and the University of Life Sciences to study the issue. Results have been promising thus far, she said.

“In one experimental line heavily selected for the resistance trait, we have already observed a reduction of sea lice infestation of 40 to 50 percent in both experimental challenge and under commercial farming conditions,” she said.  

While selective breeding won’t be of help to salmon-farming companies looking for a solution by the time their next quarterly financial report is due, it does show promise for companies planning for their long-term futures.

“With a three- to four-year cycle for salmon, it takes time before the full effect of breeding has the anticipated impact on the industry, but in one of the lines of selection, we hope to see a 60 to 70 percent reduction next year compared to unselected lines,’ Santi said. "This clearly demonstrates the great potential of breeding as a long-term solution to the sea lice problem.”

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