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Calanus is the main species of zooplankton in the Norwegian Sea.

Lab experiment: Small doses of delousing agent for farmed salmon killed zooplankton

Calanus, the main species of zoooplankton in the Norwegian Sea, can be killed by small concentrations of the delousing agent hydrogen peroxide.

And survivors may be left unable to escape predators. That is the conclusion of a new article published in Facets.

“Calanus is the main food of juvenile fish along the Norwegian coast where all the fish farms are located. It spawns in spring, when the use of delousing agent peaks. That’s why we need to know whether it is harmed by hydrogen peroxide”, says PhD-student Rosa Escobar-Lux.

Hydrogen peroxide

  • Chemical formula: H2O2
  • Used as a delousing agent at fish farms. Also used as a disinfectant, for whitening teeth, and to bleach hair and paper.
  • As a so-called bath treatment (as opposed to in-feed treatments) it is mixed into the water where the fish are swimming.
  • It can be added to the sea cage, or the fish can be treated in a well-boat.
  • It isn’t quite clear why it works, but it creates air bubbles in the shells of salmon lice, which then let go of the salmon.
  • The amount of H202 used in 2018 was 6,735 tonnes (91 prescriptions).*
  • During 2014–2016, the amount used was in average 33,800 tonnes each year (around 1000 prescriptions).*

Source: The IMR risk asessment for Norwegian aquaculture: Risikorapport for norsk fiskeoppdrett 2019

Lethal the day after contact

The first goal was to find the lowest lethal doses. The researchers put Calanus in tanks containing varying concentrations of hydrogen peroxide for one hour. After leaving them to recover for 24 hours in clean water, they measured the outcome.

“A tank with ten percent of the recommended delousing dose for salmon kills all adult Calanus“, says Escobar-Lux.

A concentration of 1.8 percent killed half of them. Researchers call this point LC50 – the dose that kills half of the test animals.

Juveniles were more resistant

Juvenile Calanus turned out to be somewhat more resistant. 4.5 percent of the delousing dose was needed to kill half of them by the following day.

“When used at fish farms, hydrogen peroxide is less effective against juvenile salmon lice than their adults. That’s why we also tested its effect on juvenile versus adult Calanus”, explains Escobar-Lux.

You can see all of the results in the scientific paper.

Part of the bigger picture

At sea, the delousing agent will become increasingly diluted the further you go from where it was released. How quickly that happens depends on the currents.

“What do the lab results tell us about what actually happens in nature?”

“This research does not tell us whether such concentrations of hydrogen peroxide occur outside sea cages. What it tells us is that if such concentrations do occur, it may harm Calanus in the area”, says Escobar-Lux.

What doesn’t kill you…

The researchers also wanted to know what happened to the individuals that survived exposure. Were they unaffected, or did they suffer any so called sub-lethal effects? They tested the escape response of the Calanus.

Calanus can dash quickly out of harm’s way if they sense unexpected turbulence in the water. For example, when a fish opens its mouth to eat it.

Using a small, targeted water current, the researchers created a similar turbulence. They filmed the reactions of ordinary Calanus and of the delousing agent survivors.

The calanus is jumping to avoid the suction from the pipette, which appears as the black tube. In Norwegian, calanus is also called "hoppekreps" (litterally "jumping crustacean").

Stopped avoiding the “fish jaws”

“Juvenile Calanus that survived exposure to five percent of the delousing agent dose, did not leap away from the “fish jaws”. They mainly lay immobile on the bottom of the tank”, says Escobar-Lux.

“In other words, hydrogen peroxide can increase the risk of survivors being eaten.”

Researching how it spreads in nature

In order to estimate the concentrations that animals are at risk of exposure to in the wild, other scientists at the Institute of Marine Research are developing methods to simulate the spread of the delousing agent in nature.

“According to our models, a concentration equivalent to 1.8 percent of the treatment dose may occur several kilometres downstream from a release location. It will vary from place to place, depending on local conditions”, explains mathematician Pål Næverlid Sævik.

At the release location, such as a sea cage, in theory it can take hours for the concentration to fall below 1 percent. In the surrounding area, dilution occurs far more quickly.

“But in order to understand the effect on Calanus, you must also simulate how the animals float and swim in the affected area. Then you must look at whether their distributions overlap and for how long. We haven’t done that yet, but we are working on it”, he says.

Using a well-boat increases dilution

It makes a big difference whether you apply the treatment in a sea cage or in a well-boat that you put the salmon into. In a well-boat, delousing takes place in a tank, and the ‘bath water’ is released gradually over a distance of 2-3 kilometres.

“This means the hydrogen peroxide is diluted far more quickly and is less dangerous. On the other hand, performing delousing on a well-boat means more handling and stress for the salmon”, says Sævik.

In order to test the accuracy of their models, the researchers are also planning to measure hydrogen peroxide concentrations at sea in field experiments.


R.H. Escobar-Lux «The effects of hydrogen peroxide on mortality, escape response, and oxygen consumption of Calanus spp». FACETS 4, no. 1 (2019): 626-637. URL:

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