Viruses from salmon farms are not likely to harm wild salmon.
The idea that salmon farms can “amplify” a natural virus to the point that it will harm wild salmon is pure speculation not backed up by facts.
Since salmon farmers are having trouble with the IHN virus right now, let’s talk about this virus specifically.
IHN is an RNA virus, which replicate themselves very quickly. In fact, RNA viruses replicate so quickly that a single infectious particle can reproduce itself three times a second!
However, RNA viruses do not “proof-read” themselves during the replication process like DNA does. Skipping this step allows for more rapid replication but leads to a high level of errors — mutations — when RNA viruses reproduce.
This sounds like a recipe for disaster, doesn’t it? IHN gets into a salmon farm, where there are lots of hosts, replicates a huge amount of itself and mutations are inevitable. Isn’t something terrible inevitable?
No, it isn’t. There is no reason to assume that any of those mutations will make the virus suddenly more harmful to the infected farmed salmon, or to the wild fish swimming by.
In fact, research suggests that most of those mutations are random noise. A normal virus population is full of mutants which do nothing to alter the virus’ overall survival strategy (infect a particular host, replicate, repeat).
In the case of the IHN virus, although it will certainly replicate quickly and mutate, it is extremely unlikely that a mutation will evolve on a salmon farm which is suddenly harmful to wild salmon swimming by. Wild salmon are already highly resistant to the disease and often carry it as hosts without any ill effect. It’s incredibly unlikely that a random mutation from a farm will suddenly overturn hundreds, if not thousands, of years of evolution and start killing wild salmon when it hasn’t before. Viruses live to replicate themselves. If they kill their hosts, they can’t replicate.
But suppose such a harmful mutation did happen. It’s very unlikely that the few fish swimming by which would be exposed to such a harmful mutation would survive to spread it.
If they did, it’s practically impossible that the new more harmful virus strain would somehow out-compete and and replace all the other strains of IHN already out in the ocean. What’s more likely is that the harmful strain would kill its hosts, eliminating further chances for it to reproduce and spread itself.
Harmful mutation is “snot” likely
Consider this analogy. There is a portable building full of kindergartners in the middle of a sports field. These kids are all highly susceptible to the flu virus. There are windows on all sides of the building and they are all open.
Outside, in the sports field, other kindergartners are running around, playing. These kids all have a natural resistance to the flu virus. Occasionally they pass close to the open windows.
One kid passing close to the open windows sneezes through the window, spraying saliva and mucus at several of the kids inside. Since they have no resistance to the flu virus, they quickly get sick with the flu and spread it to other kids in the classroom.
Now the entire portable full of kids are sneezing and coughing out the windows. Occasionally, one of the outside kids passes by and passes through air carrying mucus and saliva from the sick kids inside.
Like all the other kids running around the field, this one already has a natural resistance to this virus. How likely is it that he is going to suddenly get sick? How likely is it that the virus inside the portable is suddenly going to mutate into a form which will make the kids outside sick? If it does, how likely is it that the outside kid running past the window is going to get sick but still keep running around enough to pass it on to the other kids running around the field?
We think it’s pretty unlikely the sick kids in the portable will have any effect on the health of the kids running around the field.
Environment prompts evolution
Back to salmon, it’s important to understand that the only cases where IHN virus has been observed to do any harm to wild salmon is in hatcheries, where man-made spawning channels contain a high density of eggs and young salmon. When spawning salmon return to those channels, if they are carrying the IHN virus (and they regularly do) the high density situation in the spawning channel can lead to high infection rates, and mortalities among the young salmon. The environment is different from the environment where the virus and wild salmon have co-existed for millennia, resulting in a different outcome.
But in natural spawning settings, there is a low risk of IHN virus harming young salmon because the natural densities of young salmon in freshwater are low.
On the other hand, farmed Atlantic salmon are highly susceptible because they are raised in densities higher than how fish normally school in the ocean. And, more importantly, farmed Atlantic salmon come from a different ocean and did not evolve alongside the virus, developing a natural resistance to it. This plus a higher density changes the environment for the virus, resulting in different outcomes.
Let’s look at a real-world example. The Hagerman Valley in Idaho is home to a large number of rainbow trout farms, which are relatively isolated from nearby rivers and from the ocean. There are very few, if any, new introductions of IHN virus particles into the valley. For 21 years, scientists tracked and studied the evolution of the virus and found that over time the relatively few strains present in the valley grew into many.
The presence of a general trend toward divergence over time suggests that the virus is actively evolving in the valley rather than exhibiting the relative genetic stasis observed in Alaska and the Washington state coastal region (Emmenegger et al. 2000, Emmenegger & Kurath 2002).
The generation of this diversity may have been facilitated by conditions specific to Hagerman Valley aquaculture. Year-round trout production with the constant introduction of immunologically naïve fish may allow more rounds of viral replication per year than in anadromous hatchery or wild fish, where low-level chronic or carrier infection may be more common.
In addition, partitioning of fish populations into numerous facilities, each with numerous rearing units may result in a lack of competition and purifying selection, allowing multiple variants to be simultaneously maintained. Rapid evolution of IHNV may also have been initiated by the process of virus adaptation to the unique Hagerman Valley environment which includes the rainbow trout host and constant 15°C water temperature.
There are several reasons why the virus was able to evolve such diversity. Being isolated into an artificial system is at the top of the list. And it’s important to note that in the ocean, the IHN virus is in relative genetic stasis because it is experiencing no environmental pressures to significantly change itself.
And even in the Hagerman Valley example, there is no evidence after 21 years that the virus had mutated to be more harmful.
Amplification is for music
There is simply no good reason to assume that a salmon farm will somehow “amplify” a naturally-occurring virus to the point that it will be harmful to the wild salmon which already carry it.
And there is certainly no good reason to assume that random mutations of the virus at an infected farm will do any harm to wild salmon.
Once again, we point out that the ocean is full of viruses. Fish swim through them every day. The ocean is not “pristine” in the sense that it is free of viruses and diseases. Nature is brutal, and in the ocean just like on land creatures get sick and die.
Wild salmon have evolved to be incredibly hardy to viruses and diseases. It’s grasping at straws to suggest that a string of incredibly unlikely scenarios, which get more unlikely as they stack one on the other, is going to do any harm to wild salmon.
And it’s cynical and manipulative for anti-salmon farming fanatics to suggest this, especially the ones such as Alexandra Morton who purport to be scientific experts.
We hope people will ignore the silly speculations and research the science for themselves, and they will see the risks posed by salmon farms in B.C. are very low to wild fish.