Why sea lice prevention is better than cure

A new study confirms that preventing sea louse infestations has key advantages over other strategies and identifies the most promising preventative methods.

The study supports the efficacy of preventative methods, such as tubenets, which are designed keep the salmon beneath the level of the water column most frequented by lice. Photo: Arthur Campbell, AKVA

In a new article, Prevention not cure: a review of methods to avoid sea lice infestations in salmon aquaculture, researchers from the University of Melbourne, Norwegian Institute of Marine Research and Nofima argue that effectively preventing infestations before they occur causes less stress for the salmon and fewer production losses, as farmers avoid the need for delousing treatments.

Currently, farmers have options to manage lice by preventing infestations, continuously controlling infestations to keep lice at low levels, or waiting until infestations reach “trigger” levels and then carrying out immediate delousing.

“Many delousing methods subject the salmon and the lice to the same unpleasant experience - they work because the salmon usually survive the experience and the sea lice do not. Preventative methods avoid this situation by targeting lice before they attach to the host, or by helping salmon fight off lice larvae at the moment of attachment,” said lead author, Dr Luke Barrett, from the University of Melbourne.

A diagram showing the lice stages targeted by different lice management strategies. Preventative methods are in green, while the most common lice removal methods are in red and orange. Photo: Barret et al, 2020

What are the most effective methods to prevent sea louse infestations?

The researchers trawled through the data from all published tests of sea lice prevention and identified the most effective methods. They found that the best approach is to keep lice out of sea cages using mesh or tarpaulin barriers: lice skirts prevented 55 percent of infestations, while snorkel cages prevented 76 percent. These methods can make the job of controlling lice much more manageable, while fully enclosing cages is more difficult but can be up to 100 percent effective.

However, lice barriers may not be suitable at all locations, for example where there are strong currents or low oxygen levels. In such cases, other preventative methods are worth trying, such as encouraging salmon to swim deep below the surface waters where sea lice are typically most numerous, improving the immunity of salmon using breeding or functional feed additives, and using repellents or masking scents to stop lice being attracted to salmon.

“Most preventative methods leverage our knowledge of the natural behaviours and physiology of salmon and lice, such as their preferred swimming depths, to reduce the likelihood of salmon encountering lice and becoming infected,” said co-author Prof Tim Dempster from the University of Melbourne.

Different types of preventative methods and their measured effects on lice levels. Each dot is the outcome of a single study testing that method

What does the future hold?

Farming companies, especially in Norway, are beginning to invest more heavily into preventative methods such as skirts and snorkel cages, as well as continuous control methods, to avoid having to delouse. The authors expect this trend to continue.

Crucially, susceptibility to sea lice is a genetic trait that is passed on to offspring. Accordingly, some researchers and farming companies have also made progress on breeding more lice-resistant salmon, while work on a cost-effective vaccine continues. “Selective breeding against sea lice is a long-term strategy that will likely bring significant benefits in the future,” said co-author Dr Nick Robinson, a senior researcher at Nofima.

The research was published this week in Reviews in Aquaculture. A copy of the article can be obtained from the authors upon request.