Initiative sets its sights on complex gill disease in farmed salmon

Published: December 14th 2021

For reasons unknown, farmed east coast Atlantic salmon have so far escaped the costly rise in complex gill disease (CGD) experienced in net pens on the west coast and as far away as Norway and Scotland.

“CGD does occur in the North Atlantic, and in quite severe cases in European salmon aquaculture operations, but it has not had the same severity in Atlantic Canada,” confirms Dr. Mark Fast, Professor of Fish Health and Immunology at the Atlantic Veterinary College, University of Prince Edward Island (UPEI).

Whether the east coast can remain largely unaffected by the disease is also an open question. However, a new research initiative, begun in August, is aimed at checking CGD nationally and sparing the east coast salmon industry the scale of the west coast problem.

As the academic project leader, Dr. Fast, and his long-time research collaborator, Dr. Matthew Rise, at the Department of Ocean Sciences, Memorial University, have joined forces with British Columbia-based industry, government and academic scientists on a three-year, $3.6 million Complex Gill Disease Initiative (CGDI) under the Genomic Applications Partnership Program of Genome Canada. The initiative is managed by Genome Atlantic, in partnership with Genome British Columbia.

Together the researchers are developing genomic tools to pinpoint the risk factors for CGD and help aquaculturists spot and manage it for better outcomes. At the moment there are no vaccines or therapeutics for CGD available.

Dr. Fast readily admits there is plenty of scientific mystery here to investigate. CGD is found only in farmed fish and shows up as “a range of gross and microscopic lesions” he said.

These lesions are associated with various environmental conditions as well as some fish attributes among other factors. Yet linking cause and effect remains murky. Dr. Fast says CGD is more of a syndrome than a disease, but whatever the label, costs are staggering, even though salmon with CGD is considered safe for human consumption.

Overall, sea mortalities, slowed fish growth, and delayed harvests due to CGD are estimated to be costing the Canadian industry at least 12,500 tonnes in lost annual production, valued at $130 million. Factoring in indirect losses, the figure rises to $250 million.

As for costs to this region’s salmon producers, Dr. Fast said, “There has been some impact on Atlantic Canada but we have not estimated this since it has been a greater concern in British Columbia for a longer period of time.”

Given the revenue at stake, Cermaq Canada, with salmon farms on the east and west coasts of Vancouver Island and Grieg Seafood, with British Columbia salmon operations and a site in development in Placentia Bay, N.L. are leading the charge on the CGDI. Cermaq Canada’s Fish Health Director, Dr. Kathleen Frisch, is the industry lead on the CGDI, while Grieg Seafood’s Director, Fish Health and R&D, Tim Hewison, is the co-lead. Cargill Canada is involved too as an industry partner.

Also on board are Dr. Colin Brauner from the University of British Columbia and Dr. Simon Jones from the federal Department of Fisheries and Oceans.

Dr. Fast expects,” the genomic tools developed in the CGDI will assist in our understanding of gill health in general and could be applied to any gill issue of salmon and potentially other species.” Once risk factors for the disease are better known, he says, the disease will become easier to control, leading to fewer incidents of CGD and measures to prevent outbreaks.

He is optimistic too that “there are likely different therapies and or strategies that already exist to mitigate CGD impacts. For instance, identifying farm practices that contribute to the disease may allow these to be altered to reduce their contribution and there are gill health diets on the market, some of which we will be testing to determine their ability to help heal or resolve CGD.”

Before mitigation work can go ahead, though, Dr. Fast says, the team needs to chart the acute to chronic phases of the syndrome, in order to test various relief strategies throughout the course of the disease for efficacy.

The key aim of the initiative is to validate biomarkers to indicate when Atlantic salmon gills are healthy or compromised. Dr. Fast explains, “First we will be conducting a series of controlled lab experiments at U.B.C. and U.P.E.I. to develop histological and molecular markers associated with a number of different gill trauma events… Once we have found these robust markers from the lab we will test these same markers out in the field during CGD events on multiple farms in B.C. to validate their use during the different states of CGD in the field.”

Markers assigned to different stages of CGD will allow veterinarians to diagnose the syndrome and determine how far advanced it is. Once developed, the cellular and molecular markers can be tested for the proteins they produce when different fish feeds, developed for gill health, are trialed. Knowing the proteins being expressed, enables assessment of how well the feeds are working to treat the condition.

While European salmon producers have had an earlier start on trying to solve their CGD problem, their results are not necessarily transferrable. Dr. Fast said “while they have made some progress toward identifying contributing factors to CGD, they are still working on intervention strategies. The issue we have in Canada is that we do not know that we have the same contributing factors to CGD here, and we cannot treat it without knowing what is causing it.”

He added that it’s possible some European mitigation strategies may work in Canada. That possibility, he said, “is also why we are collaborating with Dr. Sam Martin, at the University of Aberdeen, and others to determine synergies to help the industry as a whole. It’s also possible that some of these strategies may be proprietary…so we will need to develop agreements to use and test these.”

Meanwhile, the initiative will also be building genomics research capacity for aquaculture at the Atlantic Veterinary College. Dr. Fast said the college has been able to purchase a multi-slide scanner to associate histological lesions, the hallmark of CGD, with biomarkers. The link will be made via high throughput image scanning and analysis, sharable with CGDI collaborators and others. A fish pathology resident has been hired under the initiative who will add new expertise to the university. Moreover, the sequencing pipelines and genomics collaborations developed with Memorial University during a previous GAPP-funded project are now being expanded to take on gill transcriptomes. Three post-doctoral fellows and student trainees hired for the initiative are expected to strengthen UPEI’s relationship with MUN and forge new ones with the University of British Columbia, the University of Connecticut and various universities in the United Kingdom.

Other key investigators for the CGDI are Dr. Rachel Balder, a Senior Scientist and Manager for the Nutrition Sciences group at Cargill Animal Nutrition, Elk River, MN; Dr. Simon Jones, Research Scientist, leader of the Marine Parasitology Program in Nanaimo, B.C. for the Department of Fisheries and Oceans; and Dr. Sonja Saksida, an aquatic epidemiologist at the Atlantic Veterinary College, UPEI.

Besides Cermaq, Grieg Seafood and Genome Canada, the CGDI collaborators include the Department of Fisheries and Oceans, Cargill, and MITACS.