Dead salmon pulled from the water during the algal bloom in 2019.
Photo: Northern Lights Salmon.
More than 7 million farmed salmon died in northern Norway during a large algal bloom in the spring of 2019. Now, thanks to the ToxANoWa project, researchers at the Norwegian Veterinary Institute have identified the toxin responsible.
Researcher at the Norwegian Veterinary Institute's Chemistry and Toxinology Research Group and ToxANoWa project leader Ingunn Anita Samdal presented the results during the 'Havbruk 2024' conference, which was held October 22-24 in Tromsø, Norway.
This event is the most important for the latest news from Norwegian aquaculture research, as well as a significant meeting place for the fishing industry, researchers, administration, and politicians.
Led by the Norwegian Veterinary Institute and funded by the Research Council of Norway, the ToxANoWa project is a collaboration with the University of Oslo, the Technical University of Denmark, and the National Research Council in Canada, and was launched to determine what made the algae toxic and caused the death of farmed salmon in Norway in 2019.
During that fish mortality event, there was a large bloom of the alga Chrysochromulina leadbeateri (C. leadbeateri) in the affected areas. This same alga had already caused fish deaths in 1991, 1998, and 2008, so it was investigated as part of this project.
Until now, it was not known why the alga C. leadbeateri was toxic to fish, but, through work in the ToxANoWa project, a new family of chemical compounds has been identified as the toxin. The researchers have proposed naming the substance 'leadbeaterin'. Leadbeaterins are related to karlotoxins, with karlotoxin-2 being another previously known fish-killing algal toxin.
As part of the project, it was grown a large amount of C. leadbeateri algae isolated from the bloom. The harvested material has been purified and studied, and research showed that this alga is toxic to fish gill cells, which may be responsible for salmon deaths.
The Norwegian Veterinary Institute explained that a common characteristic of many fish-killing microalgae species appears to be their ability to produce polyketides. Polyketides are naturally occurring compounds that can have various functions, some of which can cause cell rupture. It is known that the polyketide karlotoxin-2 can cause gill damage and death in fish.
In addition, during the 2019 bloom, several seawater samples were also collected and have been chemically analyzed. The new toxins were found in seawater samples from Troms in 2019, confirming that they may have played a role in the fish kills. However, the researchers still have work ahead of them.
"We have managed to identify the algal toxin and demonstrate that it likely caused gill damage and fish deaths in 2019, but we have not yet mapped out the mechanism behind the gill damage. We also lack knowledge about how many algae are required before the fish die," Ingunn Anita Samdal said.
Harmful algal blooms (HABs) represent a global problem for the aquaculture industry that, according to the International Conference on Harmful Algae (ICHA), is increasing in frequency, scope, and impact. The increase in these phenomena is attributed to several factors, such as the increase of nutrients in the water column due to human activities, ballast water, or the effects of climate change, among others.
Coinciding with the announcement of the discovery of this toxin in C. leadbeateri algae, this week and also in Norway, we have learned that a new toxin has been found in farmed salmon. In that case, the Norwegian Institute of Marine Research risk report for fish farming warned about small traces of the impregnating agent tralopyril in salmon fillets. This component is used to replace copper in aquaculture cages.
