Sea lice management is a stressful task for salmon and salmon farmers. In a new article titled "Prevention Not Cure: A Review of Methods to Avoid Sea Lice Infestations in Salmon Aquaculture," researchers from the University of Melbourne, the Norwegian Institute of Marine Research and Nofima argue that methods to prevent sea louse infestations have some key advantages over other strategies, and they identify the most promising preventative methods, according to an announcement from Nofima, a Norway-based institute focused on fisheries, aquaculture and foods research.
Currently, salmon 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, Nofima said.
The report authors explained 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.
“The major 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 in Australia.
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, Nofima said, reporting that 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 but can be up to 100% effective.
However, lice barriers are not suitable at all locations, Nofima said, such as where there are strong currents or low oxygen levels. In such cases, other more widespread preventative methods are worth trying, such as encouraging salmon to swim below the most common depth for sea lice, improving the immunity of salmon using breeding or functional feed additives and using repellents or masking scents to stop lice from being attracted to salmon.
“Most preventative methods leverage our knowledge of the natural behaviors 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 Tim Dempster from the University of Melbourne.
What does the future hold?
Fish farming companies, especially in Norway, are beginning to invest more heavily into preventive methods such as skirts and snorkel cages as well as continuous control methods to avoid having to delouse. The authors said they expect this trend to continue.
Crucially, susceptibility to sea lice is a genetic trait that is passed on to offspring, Nofima said. 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 in Reviews in Aquaculture.