Fish vet Kristin Ottesen warned that whatever mechanical delousing method is used, it kills some fish. Photo: Linn Therese Skår Hosteland.

Vet warns of head injury risk to fish during delousing

A fish vet has highlighted major head injuries she has seen to fish treated with warm water delousing machinery.

Published Last updated

Kristin Ottesen, of Norwegian firm HaVet, addressed lice treatment with warm water and the Thermolicer or Optilicer, during the  Fisheries and Aquaculture Industry Research Fund (FHF) conference “Prevention and Control of Lice” in Trondheim.

"No matter what non-drug method you use for delousing, it kills. Same if it's Skamik, FLS, Thermolicer and Optilice. Both the method and the logistics," Ottesen began.

She noted that temperatures of 30-34 degrees that are lukewarm for humans can be searing for farmed salmon.

'Hear the fish panic'

"But we do not know. When you stand with these machines you can still hear that something happens. One can hear that the fish panic. So the thoughts around this are not from out of the blue," she said.

She says they often see apparently healthy fish that just die after treatment, without finding the fish.

"But is it [the fish] fine? Are we looking for the right things when we look at the fish afterwards?" she asked, adding that surviving fish are not necessarily OK.

"Especially 12 to 24 hours after treatment. You will begin to see 'spare tyres' around the eyes and fluid collections in the palate after warm water treatment. This seeps in gradually. Fish farmers often say they see unconscious and lethargic fish."

Bleeding around the brain

Ottesen said that after opening such fish, she saw large bleeding around the brain and the palate of some of the fish.

"Classically, we are coming to a farm that says they have had some mortality, but are satisfied."

What farmers classify as satisfactory mortality, she adds, ranges from maybe 0.2% to 3%.

"A few weeks later we come to the same farm and see that the fish are falling and there are some sick fish. They look wrong, the eyes of the fish look strange."

Also, on fish that look fine two weeks after treatment, she finds major bleeding in the head region.

Stress-induced damage

"The fish health service has tried to document the trends in this, and the analysis companies they work with conclude that there have been major acute injuries in the head region, which can't be explained by anything other than the lice treatment.

"I see a lot of stress-induced damage to the fish after these treatments in addition to handling. I think we should think about how to build these machines. Is warm water treatment safe? It's something we must ask ourselves."

She says that as a fish health worker she has also fought a hard battle to know what the farmers treat the fish in, besides that it is hot water.

Water quality

"When the water is warmed up, it becomes nitrogen gas saturated and the fish are in there for 30 seconds. No one can tell what these seconds do to the fish. No one can say that it's not harmful, or how harmful it is.

"I think it's important to think about what you put the fish in. You put all these assets through here without looking at water quality. It's a bit shameful I think."

However, lately she has seen that people have started measuring water quality, oxygenating and getting rid of nitrogen gas.

"It's better but it has taken a lot of time to get here."

Gentler treatment

In the future she believes more knowledge must be acquired about how the fish experience mechanical treatments and how the treatments can be made as gentle as possible.

"It doesn't hurt anyone to acquire more knowledge. Both us, equipment suppliers and everyone needs more knowledge to know what we are doing, "Ottesen said.

She added that there has been a huge development to ensure fish welfare in recent years, and expects that will continue.