For parasites in aquaculture, prevention is better than cure

Managing sea lice, one of the worst parasites in salmon aquaculture, is stressful for both salmon and salmon farmers. In a new article titled Prevention not cure: a review of methods to avoid sea lice infestations in salmon aquaculture, we argue that methods to prevent sea louse infestations have some key advantages over treatments, and identify the most promising preventative methods.

Currently, farmers can manage lice by either preventing infestations, continuously controlling infestations to keep lice at low levels, or waiting until infestations reach ‘trigger’ levels and then carrying out immediate delousing. We argue that effectively preventing infestations before they occur is likely to cause less stress for the salmon and fewer production losses, as farmers avoid the need for delousing treatments.

“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”, says lead author Dr Luke Barrett.

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

We trawled through the data from all published tests of sea lice prevention and identified the most effective methods. So far, the best approach is to keep lice out of sea cages using mesh or tarpaulin barriers: lice skirts prevented 55% of infestations, while snorkel cages prevented 76%. These methods can make the job of controlling lice much more manageable, while fully enclosing cages is more difficult and expensive but can be 100% 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”, says co-author Prof Tim Dempster.

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. We 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” says co-author Dr Nick Robinson.

The research was published recently at Reviews in Aquaculture.