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Seeking a better understanding of complex gill disease in salmon

Researchers from Canada’s University of Prince Edward Island have received a CAD $4.7 million grant to develop an early warning system for complex gill disease on salmon farms, based on cutting-edge genetic sequencing techniques.

According to Mark Fast, PhD, professor of fish health and the project leader, the disease is called “complex” for good reason.

“There’s been a lot of recent work done on it, but there still isn’t a really good understanding of the extent of the contributions of different environmental factors or pathogenic and non-pathogenic organisms that are impacting and creating this disease,” he said.

Unravelling impact of contributing factors

Whether occurring at aquaculture facilities in Scotland, Norway or Western Canada, clinical signs of disease appear under different conditions but mostly in the fall, and different organisms have been associated with outbreaks. Some of these organisms also seem to appear after an initial trauma or insult, he explained.

A crucial part of the research by Fast’s team is risk analysis, assessing the relative impact of factors such as high temperature and net-washing frequency on the development of disease.

“It’s not that if you have one net wash, there’s effluent in the cages and that 2 weeks later, there’s a gill problem. It’s quite variable,” he stressed.

“There is a strong feeling currently that the problems may be specific to the organisms that are on the net at the time of the washing. Jellyfish that are coming into the cages are another potential impact. So, there’s a lot of individual, anecdotal information that is linked, but we don’t know the true association that any one of these has.”

Genetics provides clues on disease progression

Genomic techniques could shed much-needed light on these issues. Fast pointed to research from the University of Aberdeen,1 published last year, which showed considerable variability in gill disease on different farms. While how disease presented itself could look quite different, he explained, deeper analysis using RNA sequencing showed better agreement between cases. Such knowledge may be a catalyst of even greater understanding.

“We need to start with being able to identify the progression in the field, so that we can begin to work back to the early signs of the disease. What we’re trying to do in the lab is recreate these traumatic events to the gill, so that we can begin to identify the markers of an early, acute phase of gill trauma and gill disease, and then how that progresses into a more chronic impact,” he continued.

Beyond that, mapping out these phases of gill health should enable those working in the field to identify acute or early-stage markers before it becomes a major disease event, he suggested. From there, approaches to tackling the problem could be developed.

“If we can validate these early markers of gill disease and trauma, then we can take the next step within our program, which is to look at different resolution or intervention strategies. Those might include changing the functional feeds that the fish are getting, or changing on-site practices like reducing the frequency of net wash,” he said.

Data gathering and sharing crucial

At this early point in the research, the focus is on getting access to large amounts of field data, which will be probed for the first time using this kind of epidemiological analysis, Fast continued. As well as learning more about how gill disease develops, the work will also provide new insight into the cellular and molecular markers of healthy gills, he noted.

The research is funded by the Government of Canada through Genome Canada and Genome Atlantic, Mitacs and Genome BC. As well as Fast’s team, the project includes researchers from the University of Newfoundland, University of British Columbia, Cargill, and Fisheries and Oceans Canada. Two large salmon producers, Cermaq Canada and Grieg Seafoods, are also supporting the project — and he is hopeful that the findings of the work will have a wide practical impact.

“We’ve worked with these companies in the past, and there’s a track record of utilizing biomarkers to inform decisions with regard to some of their own internal processes, so I think there will be good uptake for this as well,” he said.

“There also appears to be a willingness to share some of this information beyond just those companies that are a part of this grant. We’ve talked with the BC Salmon Farmers Association to see how we can best disseminate the important results to everyone.”


1 Król E, Noguera P, Shaw S, Costelloe E, Gajardo K, Valdenegro V, Bickerdike R, Douglas A,. and Martin SA. Integration of Transcriptome, Gross Morphology and Histopathology in the Gill of Sea Farmed Atlantic Salmon (Salmo salar): Lessons From Multi-Site Sampling. Front Genet. 2020;11:610.