Know your enemy: why we need a better understanding of sea lice

Salmon lice, Lepeophtheirus salmonis, may be small, but they are the cause of the greatest economic losses on the global Atlantic salmon aquaculture industry and are one of the key restrictions – in both biological and regulatory terms – on much sought-after production increases. 

They have negative effects on the host fish by grazing on their skin, leading to wounds, stress – ultimately, under heavy infestation – mortalities. Lice also have an indirect effect on fish welfare as a result of the stressful delousing procedures required to remove the parasites. 

Understandably, they are treated as parasites that should simply be killed. However, a research group in the Norwegian fjords has been tackling the problem using an original angle – through trying to understand their behaviour.

‘’Sea lice might seem like simple animals, but they have evolved to be very good at one thing: finding and attaching to hosts. Lice that fail to attach to a host won't pass on their genes, so there are strong evolutionary pressures at play. The more we understand their behaviour, such as their vertical movements in response to physical or biological cues, the better we will be at predicting their spread and preventing infestations,’’ Luke Barrett, from Deakin University, explains. 

Barrett is joined in his latest research by Sussie Dalvin, Lena Geitung and Frode Oppedal – all from the Norwegian Institute of Marine Research (IMR) – who have extensively studied these parasites. While the term ‘’sea lice’’ is often used broadly, not all species behave the same way. Caligus lice, for instance, are better swimmers and adults can be found in the water column, whereas adult salmon lice are attached to their host fish. Understanding these differences is key to developing effective control strategies.

Problems with sea lice

Lice can irreversibly impact both wild salmon and farmed salmon. They attach to their host’s skin – often close to the gills and on the fins while changing positions after they moult into mobile stages. At high numbers, this reduces fish welfare, causes weight loss and makes the host more vulnerable to diseases – which can result in death. The intensification of salmon aquaculture in open-net systems has created a surge in sea lice due to the increasing numbers of hosts. According to a risk assessment conducted by IMR, 60 million farmed salmon died or were discarded in Norway in 2024 due to various causes, including sea lice and lately also jellyfish (Apolemia). And the problem is not limited to Norwegian waters – salmon lice and similar parasites are present in the Atlantic and the Pacific Oceans and frequently observed on farmed salmon globally. 

Last year, marine heatwaves in Norway accelerated sea lice reproduction, leading to increased treatment use and greater infection pressure on wild salmon. As sea lice have developed resistance to traditional chemical treatments, farmers have been turning to thermal and mechanical methods instead. These approaches are generally effective, but cause welfare issues, especially when used frequently. Preventatively, submergence, skirts, cleaner fish and laser-removal tools can be added to the cages. However, adding all these factors together – including lice-related weight loss of the fish – costs the industry millions each year. 

What we thought we knew

Some assumptions about these parasites might have been wrong, including believing mobile salmon lice that fall off the host will sink or drift away before they can manage to reattach to other fish. Most treatments involve crowding fish, which causes lice to detach in large numbers… yet little attention has been paid to what happens to them next. Salmon lice have never evolved to be good swimmers, as detachment from the host is not likely to occur in the wild. 

‘’The release of lice during crowding and other de-licing procedures create an artificial situation the salmon louse is not adapted to and this is why we had to look more into what happens to these [detached lice],’’ explains Dalvin. 

Another misconception, according to Dalvin, is that fish already infested with lice can become reinfested by their own hatching larvae. In reality, newly hatched larvae are unable to attach to fish. The larvae have to moult twice while floating in the water column to develop into the infective copepodite stage before they can attach to a new host. This highlights the importance of challenging assumptions and using experimental approaches to better understand lice behaviour. 

Salmon lice aquaculture? 

For their experiments, salmon lice were collected directly from commercial farms around Masfjorden and maintained on host fish at IMR’s Matre research station. Female salmon lice carry their eggs in long, tail-like strings. These are fertilised as they are laid and remain hooked to the female’s body until they hatch. 

‘’To collect eggs, you simply pick up a female with egg sacs, unhook or cut the ‘string’ and put them in a hatching well supplied with fresh saltwater. Here the eggs will hatch as they finish their development into nauplius larvae,’’ Dalvin explains. 

Compared to the egg-hatching process, which was relatively simple, the actual experimentation was a tedious process requiring creativity and patience. 

‘’Following mobile lice in buckets (<50 cm) is straightforward and can be carried out comfortably sitting on a chair. In larger tanks (> 3 m deep), tracking of individual lice becomes troublesome,’’ Oppedal explains. 

To achieve this the researchers needed to use binoculars – adjusting the focus-point as the lice sank through the water column.

‘’Trained personnel with excellent tracking skills – like those used in deer hunting or birdwatching – made a challenging task possible. Typically, individual lice had to be tracked continuously for 8 to 15 minutes,’’ notes Oppedal. 

Meanwhile in open-sea trials, individual lice were released at a depth of around 15 metres and tracked as they sank toward the seafloor. A scuba diver followed each one, maintaining enough distance to avoid influencing its natural behaviour. The process was meticulous, often requiring 10 to 15 minutes of careful observation per louse.

What the research reveals

A recent paper published by the research group examined lice survival when detached from their hosts. The researchers found that the sea lice could generally survive for many days off a host, and that adult females could survive for up to 25 days in cold (4°C), salty water – double the length that pre-adults and adult males could survive. However, at higher temperatures (16°C), adult females’ survival was no different than those of other groups.  

‘’It does not really make sense. Why do females last that much longer [in colder waters]? They are bigger and they spend a lot of energy on the continuous production of many eggs,’’ Dalvin notes with surprise. 

Furthermore, in their experiment, adult females had the best re-attachment rate, at 74 percent – 23 percent higher than adult males.

The second study explored movement. Researchers found that lice sinking speeds varied between life stages: adult females sank the fastest, while juveniles typically drifted passively with the current. Adult males often swam upwards, and females either sank passively or actively swam downward into deeper water. However, detachment does not render them harmless – 23 percent of the detached lice found a new host within five minutes in lab tanks.

Lessons for farmers 

According to the researchers, their findings could be applied by the industry to minimise lice transmission risks. Dispersal models based on the biological data showed that crowding fish (risking detaching lice) during strong tides could carry them long distances – including to neighbouring cages. However, it is not always straightforward to plan farming operations around tides, as time pressure (before hitting the regulatory lice limits) and vessel availability may limit adequate planning around tides. 

As researcher Lena Geitung says: ‘’Farmers usually know a week or two in advance that they’ll need to delouse, and they book a unit as soon as one is available. These units are often in high demand, so you more or less have to take the slot you're given. I think it’s sometimes possible to plan around favourable current conditions, and there’s likely room to do more of that. But there are probably quite a few cases where the time pressure and limited availability make it hard, or even impossible, to wait for ideal conditions.’’

Fine-mesh nets, however, could offer a practical solution during crowding. Laboratory tests showed that a 0.8 mm mesh captured all lice stages, while a 1.6 mm mesh retained most pre-adults and adult males, and a 2.0 mm mesh reliably caught adult females. In follow-up trials using a commercially available fine-mesh crowding net in a small-scale cage, researchers were able to retain above 75 percent of adult females.

‘’At commercial scale, the idea is also that the fine mesh holds onto the lice longer than a standard net, so more of them are picked up by the suction pump when the fish are transferred to the delousing unit – but we have not been able to get conclusive results from commercial scale yet. However, using a fine-meshed net during crowding could be a good strategy to reduce the number of lice released into the environment and potentially lower the risk of reinfestation for both farmed and wild fish,’’ adds Geitung. 

Mysteries remain

Despite these fresh insights, the scientists admit they are still a long way from uncovering all the secrets that either salmon or sea lice hold. Barrett, for instance, is curious to know how far lice will swim to intercept a new host. 

Oppedal, meanwhile, is exploring a different possibility.

‘’Can the lice hitchhike between farms with other species of fish? This question has come up, as farmers report new infestations of mobile lice when neighbouring farms have crowded and deloused their cages. In our model results, these farms are too far from each other for re-infection and thus hitchhiking may be an answer,’’ he explains. 

Managing sea lice means thinking beyond the fish: about currents, cages and parasite behaviour. The more that is understood, the better farming practices can be designed – protecting both farmed and wild salmon, while avoiding actions that might unintentionally spread the problem further. Clearly, these parasites have yet to give up all their secrets.