Do wild salmon subsidize the aquaculture industry in BC?

AdultLepeophtheirus salmonis infesting juvenile pink salmon, Oncorhynchus gorbuscha.If these little guys don't survive, the sea lice on farms could evolve resistance faster.(photo: Alexandra Morton)

In our recent paper, just out in Conservation Letters, we make a case that wild salmon in the North Pacific might be effectively subsidizing pest control costs for the salmon aquaculture industry along the BC coast. How would wild salmon make a difference for pest control on farms? Usually we hear about the negative effects that farms have on wild salmon, but not about interactions in the other direction. This story has to do with the evolutionary dynamics of sea lice, small parasites that live on both farmed and wild salmon.

The idea for this paper started from a conversation with John Driscoll, my fellow PhD student in the lab. Because I have a bit of background in evolutionary biology, he was trying to get me to work on an idea he had. Basically, John said, BC is the only major salmon aquaculture region where sea lice had not evolved resistance to the chemical parasiticides used on fish farms to control them. BC is also the only place that still has large wild salmon populations compared to other major salmon farming regions where wild salmon either never existed, or have been nearly fished out. John's idea was that these two facts were connected: sea lice living on wild salmon comprise a susceptible pool which periodically mixes with populations of lice on farms (mature wild salmon migrating past net pens on their way back to their breeding rivers). The dilution of alleles under intense selection for resistance (on the farms) with alleles that aren't (from the oceanic pool) would change the predictions for the spread and fixation of the resistance genotype in the population as a whole, reducing the level of resistance on farms. Could this be the reason that farms in BC hadn't had problems with sea lice resistance?

We decided to reach out to Marty Krkosek's group, who are experts in quantitative ecology of sea lice and salmon in BC, to scope out the idea. It turned out that John wasn't the only person who had though about this. A current post-doc in the group, Andrew Bateman, and a former MSc student in Mark Lewis's group, Jaime Ashander, had worked on the same idea, and Jaime had already developed a genetic/demographic model for the mixing farmed and wild sea lice that showed the delay or preclusion of resistance evolution under various conditions. In addition to Jaime's model, there were a couple other models already published that also looked at the mixing of alleles between wild and domesticated sea lice but had not made connections to the ecosystem services and management implications of these evolutionary dynamics.

Life history of sea lice and salmon: when wild adult salmon migrate past salmon farms in late summer or fall they bring immigrant homozygous susceptible lice (blue) to farms. In winter, the farm population of lice is isolated and subjected to selection for EB resistant sea lice (orange). Migrating wild juveniles move past farms in spring, receiving sea lice infection from farms that cause wild salmon population declines, indicated by juveniles with an X.

After one editorial rejection at another journal and several iterations for Conservation Letters, the paper that came out of this collaboration is an interesting hybrid. It's a combination of a review of previous theoretical work, original modelling by Jaime, and global observational data on wild salmon populations and the occurrence of resistance. We end with conservation-focused implications for both wild salmon and sea lice management through an ecosystem services lens (though we stopped short of actually evaluating the dollar value saved by the aquaculture industry in BC - after a valiant effort).

If this process of seasonal allele mixing between wild and domesticated populations of sea lice is in fact happening, it is a case where the salmon aquaculture industry is both the direct beneficiary of, and a direct source of impact on, the resistance-mitigation service provided by wild salmon. The impacts of aquaculture on nearby wild salmon populations are well documented. In part, these impacts are due to the infection of young wild salmon fry that are heading out to sea with farm-origin sea lice at a life stage that they would not normally be exposed to them, causing higher morbidity and mortality than the normal situation where adult salmon are exposed to parasites out in the open ocean (see photo). By causing the population decline or local extinctions of connective salmon populations, aquaculture operations are not only causing harm to an important natural resource, they could also be cutting themselves off from the oceanic pool of susceptible sea lice that allow them to remain resistance-free. In the paper, we suggest several measures that would maintain this service, including reducing the infection rate of young wild salmon from farm-origin sea lice by correctly timing paraciticide treatments. Another idea is a payments for ecosystem services scheme that supports watershed protection/restoration around vulnerable populations. This sort of program could help reverse the decline of wild salmon populations near salmon farms, thereby serving a conservation purpose and benefiting the aquaculture industry at the same time.

This paper is the first well-theorized example of evolution generating an ecosystem service, in this case, resistance mitigation. This evosystem service involving wild salmon and their sea lice pests offers a pretty juicy counter-narrative to the conflict-ridden relationship between aquaculture and wild catch industries, maybe providing additional motivation for cross-scale conservation and management efforts.