Dissecting wild Pacific salmon tissues for molecular analysis and viral genomic sequencing.

PRV1a in fish farms, an interview with Dr Gideon Mordecai

Editor’s note: In Dr Mordecai’s most recent study, the PRV-1a virus was reported beside fish farms in the Okisollo Channel and Raza Island. Wild salmon infected with that virus would have swam by neighbouring Cortes, Read and Quadra Islands on their way home to spawn. 

Dr Gideon Mordecai is a Research Associate with the Institute for the Oceans and Fisheries at UBC. He is also the author of more than 20 scientific publications, the most recent of which reported the PRV1a virus was in 70% of the samples they studied from 56 fish farms.

Cortes Currents asked Dr. Mordecai,”Fish farms are claiming that BC’s PRV1 isn’t deadly for wild salmon. Is that true?” 

Dr Gideon Mordecai – Photo courtesy his webpage at UBC

Dr Gideon Mordecai: “So that claim, I believe, is based on laboratory challenges,  where PRV is injected into a salmon and you see what the result is.”

“What we do know is that those lab challenges don’t reflect what it really means to be a wild salmon.  Even with the Atlantic salmon challenges, there’s very, very, very, very little, or rare, mortality in any of the challenge studies in a lab.” 

“As an analogy, I could tell you that  the one single COVID 19 challenge study that I know about, where they injected about 30 young healthy individuals with COVID 19, luckily none of them died. That doesn’t mean this virus doesn’t cause mortality.” 

“Coming back to PRV,  there is evidence linking  PRV-1a, which is the lineage of PRV in British Columbia, with disease in Atlantic salmon but also, I think much more importantly, to disease in Pacific Salmon. Whether those fish with that disease will go on to die if you infect enough of them in their natural environment, that’s a different question.”

“So to summarize, I just say mortality is a very, very high bar to use as a measure of risk.” 

Sample taken from beside a fish farm for ‘Detection and phylogenetic assessment of PRV-1 via sampling of biological materials released from salmon farms in British Columbia‘ -Photo courtesy Alexandra Morton’s Facebook Page

Cortes Currents: What about other diseases spreading from the net pens to wild fish?

Dr Gideon Mordecai: “We know there’s other diseases in the farms. I can’t tell you off the top of my head the full list of them, whether any of them have been shown to lead to mortalities in challenges. The one which is maybe most  noteworthy would be a bacterial pathogen called Tenacibaculum maritimum. That one has been associated with die offs on Pacific Farms before.”  

“I don’t believe there’s been a laboratory challenge here in BC. That’s something which hopefully we’ll see in the future, but there’s definitely links between agents like Tenacibaculum. I’m thinking of a paper, which is looking at an outbreak on a  Chinook farm in California.” 

“So really I think the question that the public are asking is, what do we know about disease agents like PRV and Tenacibaculum? What is the risk to wild Pacific salmon?” 

“There is a body of evidence suggesting that there is a risk. A lot of it  links to disease through molecular work, localizing the agent to specific disease lesions and tissues.”

“There’s another body of work, looking at transmission. We know that there’s transmission of agents from farms to wild fish, including PRV AND Tenacibaculum.” 

“The final piece is does that have an impact on population?” 

“A study came out, I believe late last year,  led by  a colleague of mine, which tried to rank all the different infectious agents which are in Pacific Salmon and correlated them with either a population, or an effect  on cohort survival. So  if a population has more of that disease, is it less likely to do well?  Also a measure of body mass index: How fat is the fish relative to its length?” 

“The two agents which came out having a negative impact on both survival and  body mass were PRV and Tenacibaculum. They were the top two out of all 35 agents.  So that was another piece of evidence. And yes,  it’s only correlative evidence, because that’s all you can have if you’re studying a disease like that  in wild fish. But it does suggest that those agents which are transmitted by farms might be having an impact on wild salmon.” 

Cortes Currents: Are we gambling with the fate of wild salmon in BC the way that we’re continuing to allow fish farms to exist, especially on migration routes? 

Dr Gideon Mordecai: “We know that there’s lots of different factors that influence the health of wild fish populations, and of course those are all important things like habitat, environmental conditions, predators. These are all big issues if you’re a salmon. I think the aquaculture example is an interesting one because it’s something which is under direct human control.”

“There’s specific management actions, like as you mention, Removing farms or keeping them, which can control exactly how much of that stressor impacts wild salmon.” 

“As you’re probably well aware, right now the federal government is going through this transition consultation over what should happen in the future. What that means to different groups and different people. Out of that we’ll expect a plan about what that transition means and, to use your language, if they’re going to take that gamble or not.”

“It’s not my decision. I’m just doing my best to make sure that as much of the evidence which is out there informs those consultations. Hopefully that risk, which I just described in your earlier question, will be taken seriously.”

Cortes Currents: Your most recent study found 70% of the fish that you sampled had PRV1 A. What does that mean going forward? 

Dr Gideon Mordecai:  “The result from that study, that’s not a new result. We know that as fish are put into the ocean, the proportion of those fish which have PRV increases. By the end of a production cycle, nearly all of the fish on the farm are infected with PRV.”  

“The study that you refer to was testing out a new method for monitoring the farms. So it was important to us for the result we found, was similar to  what was already understood about PRV infection dynamics on farms and that was the case. When we averaged out the samples we had, there was a 70% infection rate.” 

“We know that if you look at it in terms of how long those fish have been in the farm, the prevalence can become higher and in  most cases, by the end of that production cycle almost a hundred percent of those fish are infected.”

 We know those farms act as a source of transmission to wild fish and this new method we developed to monitor those farms amplifies that message.

Cortes Currents: Do you believe the PRD one is coming from the hatcheries and is there room for other explanations?  

Dr Gideon Mordecai: “Where shall I start?”

“There are different types of hatcheries. There are salmon enhancement hatcheries, which raise Pacific salmon and then release them into the ocean. There are also Atlantic salmon hatcheries, which are used to rear Atlantic Salmon, which are then put into the net pens.” 

“We know from a previous study that PRV has been detected in those Atlantic Salmon hatcheries. That study was a few years ago, using some data from over the last decade. So the data is a little bit older.”

“We’ve been told by the industry and by DFO that they’ve cleaned up the hatcheries and they have a new system which stops PRV transmission, and their hatcheries have been cleaned up.” 

“I haven’t seen any data. Maybe that is the case, but I’d really like to see the data showing that and how they came to those conclusions. Just as  when I make claims in my papers,  I show the data and show how we came to those conclusions. So there’s a bit of a transparency issue.”

“The sequencing data suggested that there is a similar lineage of PRV in farms from the same company, which does suggest there’s this shared transmission source from somewhere like a hatchery.”

Cortes Currents: What’s the next step, when it comes to evaluating the pathogens in fish farms and the effect they’re having on wild salmon.  

Dr Gideon Mordecai: “This is an interesting question and there will be two different arguments here.” 

“Some people will say, and I think I’ll be one of them, we always want to do more work. You could always do more follow up studies with more fish and bigger studies. I think that’s going to be the scientist’s answer. We are going to want more money to do more research, to know more things.” 

“There’s another way of looking at it, and I think that’s reflective of the consultation that’s happening right now where there’s people who have to make a decision today about what’s going to happen next.”

“From that perspective, well maybe you don’t need to be a hundred percent certain of the size of the impact to make a precautionary decision based on the amount of evidence which is already there, which suggests we should be acting to mitigate this risk. It really appears to be having an impact.”

“It  reminds me of action on climate change.  How certain do we have to be that CO2 is causing  the climate to warm before we start to act?  I’d suggest  if you’re fairly certain that there’s a problem, better to act early and mitigate that impact than to wait  for 99.99% certainty. By then it’s a bit too late and the planet’s already heated up.”

“Many of your salmon populations are already in these tremendous declines.” 

 “There’s a science versis management action question there, and I think it really depends who you ask.”

“I’m not out here suggesting that if you remove fish farms, everything’s going to be solved because it’s a complicated problem.  You’ll speak to the expert in various fields, and they’ll tell you about what they work on.”

“I don’t like to come across preaching for this one thing that’s going to solve the problem. Our understanding is growing. PRV was only discovered about a decade ago.”

Top image credit: Dissecting wild Pacific salmon tissues for molecular analysis and viral genomic sequencing – Amy Romer photo

Sign-up for Cortes Currents email-out:

To receive an emailed catalogue of articles on Cortes Currents, send a (blank) email to subscribe to your desired frequency: