A widely used method of removing salmon lice is to put the salmon in warm salt water for less than one minute, causing the lice to die.

Salmon in pain when warm water is used as delousing treatment

Salmon are briefly immersed in warm water so the lice lose their grip. The treatment is the most common non-chemical delousing method used at Norwegian fish farms. But its imminent ban comes as new research reveals the pain and injury to the salmon.

The Norwegian aquaculture industry is constantly setting new records. In 2019, Norway salmon exports came to more than € 7 billion, according to NRK broadcasting.

But salmon lice remain an unresolved problem. The parasite eats the skin and sucks blood from the salmon. This is painful for the salmon and incurs great expenses for the breeders.

Fighting lice cost the industry almost € 500 million in 2018, according to research from Nofima and reported by Dagens Næringsliv.

Aquaculture business people, researchers and bureaucrats recently met at the 2020 Sea Lice conference in Trondheim to exchange new knowledge in the field.

The industry’s only means of keeping the lice problem at bay have been to prevent infestations and the use of various treatment methods for removing lice. Vaccines are still a long way off.

Treatment in warm water widespread

The use of drugs and chemical agents to remove salmon lice has declined sharply in recent years. Instead, other systems – like flushing off lice with water and thermal methods like warm water baths – have become more widespread.

Salmon lice release their hold at high water temperatures.

In thermal treatment, salmon are transferred to a treatment tank with heated salt water between 28° and 34° C for about 30 seconds so that the lice die and fall off the salmon.

However, injury and increased mortality have been reported in the fish when using this method of delousing.

And professionals working with fish health have raised the issue of whether delousing with warm water is painful for the fish. So the Norwegian Food Safety Authority ordered a report on the method.

In recent years, the thermal method has become the most widely used non-medicinal treatment for salmon lice. It accounted for 68 per cent of all mechanical treatments for farm-raised fish in 2018, according to the Food Safety Authority.

Reacted to heated water

Salmon can survive short periods at a high temperature – but not as high as 27 ° C for an extended length of time, new research shows.

Researcher and veterinarian Kristine Gismervik at the Norwegian Veterinary Institute has studied salmon delousing in warm water.

Until now, it has been difficult to assess whether this treatment exposes the fish to undue discomfort and pain. Scientists have not known what the salmon can withstand.

Researcher and veterinarian Kristine Gismervik from the Norwegian Veterinary Institute reported at the conference that fish exposed to such warm water very likely experience pain.

She explained that fish have receptors that respond to potential bodily injury.

"In humans, we call them pain receptors," she said.

Fish have receptors that respond to warm water.

Obvious pain behaviour

Two new experiments reveal that salmon exhibit pain behaviour upon being transferred to water temperatures above 28 ° C. Researchers from the Veterinary Institute and the Institute of Marine Research conducted the research at the marine research station in Matre outside Bergen.

Salmon were put into the experimental tank and observed one at a time.

"We measured the behaviour of salmon weighing 230 grams that were transferred from 8 degree water to temperatures ranging from 0-38 ° C," said Gismervik.

To measure the pain response, the fish were filmed and their movements analysed. The researchers looked for behaviours that might indicate pain.

"Signs of pain and discomfort are head shaking, faster swimming, collisions with the tank wall, and arching the body," Gismervik said.

Salmon exposed to 28 ° C water and above immediately responded with clear pain behaviour and head shaking.

Already after ten seconds, the salmon swam around the tank faster than the control fish.

The behaviour gradually became more intense, as fish collided with the tank walls and broke the surface of the water.

Clear signs of flight response

"These are clear signs of a violent flight response, or panic," Gismervik explained.

After about two minutes, depending on the temperature, the fish bent like a banana and finally turned onto its side and had to be euthanized.

The researchers measured the time it took for the fish to end up on its side. At the endpoint, the fish was in such bad shape that it had to be euthanized. This occurred within five minutes when the salmon was exposed to water of 28 ° C and warmer. The warmer the water, the faster the endpoint was reached.

Salmon immersed in colder water than this took longer to reach the endpoint. The control fish were in 8 degree water, which was the temperature the fish in the experiment were used to.

The Veterinary Institute also takes in fish to clarify the cause of death in connection with delousing.

The cause of mortality and injury often tends to be that the fish injures itself due to panic and pain behaviours, according to Gismervik.

Food Safety Authority: method needs to be phased out

Researchers usually do not release their results until they are published in a peer-reviewed journal. But in this case, the Veterinary Institute released the results in the spring of 2019, before publication.

The findings were considered so important that the researchers wanted to share the knowledge quickly. The Food Safety Authority announced in the autumn of 2019 that the method of using water that is 28 ° C or warmer would be phased out within two years because the method does not meet requirements for proper animal welfare.

Treatment above 34 ° C is already prohibited.

The caveat against the phase-out is that it will prevent researchers from gaining better understanding about welfare-friendly treatment options.

The usual treatment time in heated water for farmed fish is about half a minute.

The study was published in the Veterinary and Animal Science journal in December 2019.

Damage to gills and cerebral haemorrhage

The handling of the fish itself can also lead to injury when the fish is transferred to a water tank or a well-boat and back again.

"We’ve observed gill bleeding, cerebral haemorrage and eye blisters," Gismervik said.

Injuries like these were also seen in the behavioural experiment, when salmon were placed in a water bath of 34-38 ° C for 72 to 140 seconds.

In another experiment, salmon that were just over a kilo were first sedated and then exposed to water in a soft bag at 34 ° C for 30 seconds. There were no significant differences between the treated salmon and the control group for most of the injuries.

Fin injury was an exception. Despite being strongly sedated, the fish showed a panic response and struggled in the water bath. The control fish did not.

Often the same farmed fish must be treated repeatedly for lice. The risk of injury increases with repeated treatments.

Gismervik believes researchers should start evaluating the number of treatments that each fish is exposed to, in addition to the treatment methods themselves.

“We can’t consider the result satisfactory if we have to go in and treat the same animal several times. That means we’re not doing a good enough job of preventing salmon lice,” said Gismervik.

Good that drug use has slowed

The concern about repeated treatments has been reinforced in the new guidelines presented at the recent salmon lice conference.

The guidelines point out the dramatic decrease in the use of drugs for treatment as a positive development. However, the total number of treatments an individual fish is subjected to has increased dramatically.

“Our new knowledge about what’s good for the salmon poses a challenge for the industry – and it means that the industry has to find better solutions for how to treat farmed salmon,” Gismervik says. “It’s critical to safeguard the fish's welfare.”


J. Nilsson et al.:Sudden exposure to warm water causes instant behavioural responses indicative of nociception or pain in Atlantic salmon. Veterinary and Animal Science. December 2019. https://doi.org/10.1016/j.vas.2019.100076

K. Gismervik et al.: The injuries in Atlantic salmon in a pilot laboratory trial. Veterinary and Animal Science. December 2019. https://doi.org/10.1016/j.vas.2019.100081


Read the Norwgian version of this article on forskning.no

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