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Krill had low tolerance for hydrogen peroxide in lab experiments
One two-thousandth of the “salmon dose” was lethal to the krill.
“Of the species that have been studied so far, krill is the most sensitive,” says research scholar Rosa Escobar Lux, referring to her latest study.
Hydrogen peroxide is a delousing agent that is added into the water where the fish are swimming, either in a sea cage or well-boat. In both cases the chemical ends up in the sea where it is diluted and gradually broken down. Scientists are therefore studying how well it is tolerated by species other than salmon and lice. (Much less hydrogen peroxide is used now than in the past – see fact box.)
The findings of this study fed into a new, major report on an investigation into 79 cases of mass mortality of krill.
Krill are near the shore when most delousing takes place
“Krill are very important organisms both out in the open sea and along the Norwegian coast. Particularly between January and May, they stay close to the coast. That is also the time of year when the use of hydrogen peroxide has traditionally been highest,” explains Escobar Lux.
Previously, her research team have studied the impact on Calanus finmarchicus, lobster, sugar kelp and various species of shrimp. This is the first time they have tested krill.
How long does it take to become toxic?
“In this experiment we monitored the krill for two days after exposing them for one hour. That allows us to determine the dose that is acutely toxic, as well as the dose that may kill them within a few days,” says Escobar-Lux.
In order to find the minimum lethal dose, the researchers put six groups of krill in tanks of seawater containing varying concentrations of hydrogen peroxide. The highest concentration was 1,700 milligrams per litre of water – the dose added to sea cages.
After one hour any surviving krill were returned to tanks containing pure seawater, where they were kept for two days. “A concentration of ten percent of the dose given to salmon killed all of the krill within the first hour,” says Escobar Lux.
One two-thousandth was dangerous within days
They estimate that 1.9 percent of the dose used for salmon will be acutely lethal to half of the krill. This is a standard measure of toxicity which researchers call LC50 – the dose that kills half of the test animals. “The amount of hydrogen peroxide that is lethal to krill within two days of exposure is 0.86 milligrams per litre, or one two-thousandth of the dose used for salmon,” says Escobar Lux.
At sea, the delousing agent will become increasingly diluted the further you go from where it was released. In practice, how quickly that happens depends on the currents.
Not a death sentence for krill
“What do the lab results tell us about krill and hydrogen peroxide in the sea?”
“This research does not tell us that these concentrations of hydrogen peroxide are found outside sea cages. It does tell us that if krill are exposed to a given quantity for a long enough period of time, it may harm them,” explains Escobar Lux.
Nevertheless, the exposure period of one hour was not chosen randomly. Other research has modelled delousing in sea cages and estimated that concentrations of 100 mg/l may be found in surface waters up to two kilometres away from fish farms for several hours.
“We believe it is an appropriate length of exposure based on what we know about how the chemical is broken down and what the models tell us about dilution. But we still need more research on that, ideally including field experiments,” says Escobar Lux.
Other modelling done at the Institute of Marine Research (IMR) shows that there are big local differences in how quickly dilution occurs. It also occurs far more quickly with a moving well-boat than with a sea cage.
Dead krill washed ashore – but has it become more common?
There are stories of large quantities of krill washing ashore from as far back as the start of the 20th century.
In recent years the number of these events reported by the general public has risen sharply, as the IMR has recently thoroughly reviewed in a major report.
Scientists don’t yet know whether this phenomenon is occurring more frequently, or whether people are just noticing it more.
In three out of 79 cases, the researchers were able to conclude that delousing agent was a probable cause of the krill washing ashore. But in those cases the agent used was not hydrogen peroxide.
In some cases it was impossible to rule out a link, and in many of the cases it was unlikely, because there were no reports of delousing taking place in the vicinity.
Part of a growing knowledge bank
“The knowledge we obtain from lab experiments is a prerequisite for those kinds of studies. Previously, no-one knew the tolerance of krill to delousing agents,” says Escobar Lux.
“Based on this new information about the sensitivity of krill to hydrogen peroxide, we cannot rule out that delousing may contribute to krill mortality. The big question is how much overlap there is between hydrogen peroxide and krill in time and space. That is something we will continue to work on,” says Escobar Lux.
These new results will be added to the IMR’s growing knowledge bank on how delousing agents affect species in the wild. Also see our Risk assessment for Norwegian aquaculture (in Norwegian).
Rosa H. Escobar-Lux & Ole B. Samuelsen: The Acute and Delayed Mortality of the Northern Krill (Meganyctiphanes norvegica) When Exposed to Hydrogen Peroxide. Bulletin of Environmental Contamination and Toxicology, 2020.
Quick facts about 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.
- Over the period 2014–2016, the average amount used was 33,800 tonnes (approximately 1,000 treatments) per year.*
- In 2019, the amount used was 4,523 tonnes (76 treatments)**
*latest figures come from the IMR’s Risk assessment for Norwegian aquaculture 2019
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