Salmon Escape: What Does the Science Say?

By Joe Gaydos

On August 19 and 20, a net pen owned by Cooke Aquaculture Pacific collapsed, releasing an undetermined number (estimates range from 4, 000 to 185,000) of the 305,000 Atlantic salmon being raised there into the waters around Cypress Island, just northwest of Anacortes, Washington.

In a region where vast amounts of money and effort have been spent attempting to restore wild salmon runs, this mass escape of non-native fish has caused a public uproar. How could this happen? Will the Atlantic salmon spread disease to wild fish? Will they outcompete native salmon for food or freshwater spawning habitat?

To try and answer the questions, it’s valuable to look at the established science. Unfortunately, salmon spills like this are not new events in the Pacific Northwest.

People have been farming Atlantic salmon in Washington since 1982, and in British Columbia since 1985 (McKinnell and Thompson, 1997). Despite assurances from the aquaculture industry, wherever there are fish farmed in sea pens there are escapes.

In fact, on July 2, 1996 high tidal flows destroyed seven net pens at an Atlantic salmon farm near Cypress Island, releasing or killing 101,000 Atlantic salmon (McKinnell and Thompson, 1997). Sound familiar? The lessons from that and other releases should inform us about the risk that farmed Atlantic salmon pose for the Salish Sea's five species of native salmon.

The first concern is the potential for released farmed salmon to transmit disease to wild salmon. Farmed Atlantic salmon can carry viruses, bacteria and parasites like sea lice that can infect wild salmon (e.g., Jones et al., 2015). The release of thousands of salmon that were actively experiencing a disease outbreak could have huge ramifications for wild salmon.

In Washington State, all public and private growers of salmon, including Atlantic salmon hatchery operators, are required to adhere to strict disease control polices (Waknitz et al., 2003). While we have not seen data on the health or disease status of the released Atlantic salmon, it was reported that they were treated for a bacterial infection called yellow mouth in July 2016 but were believed to be disease-free at the time they escaped.

Without detailed disease testing data it is difficult to know what the potential for disease transmission could be in this most recent release. An evaluation of the risk of disease transmission from farmed Atlantic salmon to wild Pacific salmon conducted over a decade ago (Nash 2003) classified the risk as low due to existing disease testing protocols and the State's prohibition of bringing new Atlantic salmon stocks or eggs into Washington (which limits new diseases from entering).

As to whether released farmed salmon will compete with native salmon for food and breeding or spawning space, studies (Jonsson and Jonsson, 2006) have shown that while their performance and reproductive success in nature vary, farmed Atlantic salmon often are outcompeted by wild salmon of similar size.

Between 1987 and 1996, 10,609 Atlantic salmon were caught in the North Pacific representing 4.2% of the total number reported to have escaped since Atlantic salmon farming began in Washington and British Columbia (255,554 escapees reported; McKinnell and Thompson, 1997). Interestingly, this includes Atlantic salmon caught in Alaska, even though Alaska does not allow Atlantic salmon farming, proving that the fish are capable of surviving and moving great distances after escaping.

Of the Atlantic salmon caught during that time period, stomachs were examined in 813 animals. Empty stomachs occurred in almost 77% of ocean-caught Atlantic salmon and 62% of those caught in freshwater. Washington Department of Fish and Wildlife has examined the stomach contents of about a dozen of the recently escaped Atlantic salmon and all of their stomachs have been empty. Additionally researchers and volunteers from the non-profit KWIAHT dissected 31 Atlantic salmon caught in Watmough Bight last week and found empty stomachs with the exception of two fish that each had one small mussel shell and a few crumbs of fish chow pellets. This suggests that while released farmed Atlantic salmon will compete with wild salmon for food, many also don't make the transition from being fed pellets in farms to catching and eating wild food. For those that do, though, stonefly nymphs found in the stomachs of Atlantic salmon caught in the Salmon River (Vancouver Island) suggest that escaped Atlantic salmon also can be predators in freshwater as well as in ocean ecosystems (McKinnell and Thompson, 1997).

Although the probability is low, escaped adult Atlantic salmon have the potential to colonize and exist as self-sustaining introduced species. In 1998, scientists captured twelve juvenile Atlantic salmon (and observed, but did not capture another 28) in the Tsitika River on Vancouver Island (Volpe et al., 2000). Genetic analysis confirmed that these were Atlantic salmon that were the products of natural spawning by released Atlantic salmon. More recent survey work and modeling looking at Atlantic salmon use of freshwater streams in British Columbia showed that 97 % of streams in British Columbia with high native salmon diversity were occupied by Atlantic salmon and that Atlantic salmon can occupy these rivers for multiple years (Fisher et al., 2014). Colonization can occur.

The only potential positive from this large release of Atlantic salmon is that these farm-raised fish should serve as easy prey for seals, sea lions and eagles, maybe taking some predation pressure off wild salmon.

On balance, though, the science looking at past net pen releases of Atlantic salmon in this region suggests that there can be negative impacts to native salmon including disease transmission, competition for food and breeding habitat, and the potential for long-term establishment of an introduced Atlantic salmon run.

Science informs decisions, it does not set public policy: the people and their representatives do. So while the science does not suggest that this spill will likely be catastrophic to wild salmon, in looking at the public reaction to this net pen release and the outcry against Cook Aquaculture Pacific, it seems evident that the people of the Salish Sea value native salmon runs more than they do the salmon farming industry.

The message from the public appears clear: With the billions of dollars we’ve invested to protect and recover native wild Pacific salmon, any introduced risk like farmed Atlantic salmon is unacceptable.

For daily updates, please visit the Washington Department of Natural Resources website on this incident.

To report your catch of Atlantic salmon or see where these escaped farm fish are being caught, visit the Washington Department of Fish and Wildlife's Catch Map.

Literature cited:

Fisher, AC, JP Volpe, JT Fisher. 2014. Occupancy dynamics of escaped farmed Atlantic salmon in Canadian Pacific coastal salmon streams: implications for sustained invasions. Biological Invasions 16:2137-2146.  doi 10.1007/s10530-014-0653-x

Jones, SRM, DW Bruno, L Madsen, EJ Peeler. 2015. Disease management mitigates risk of pathogen transmission for maricultured salmonids. Aquaculture Environment Interactions 6:119-134. doi 10.3354/aei00121

Jonsson B, N Jonsson. 2006. Cultured Atlantic salmon in nature: a review of their ecology and interaction with wild fish. ICES Journal of Marine Science 63:1162-1181. doi 10.1016/j.icesjms.2006.03.004

McKinnell S and AJ Thomson. 1997. Recent events concerning Atlantic salmon escapees in the Pacific. ICES Journal of Marine Science 54:1121-1125.

Nash, CE. 2003. Interactions of Atlantic salmon in the Pacific Northwest VI. A synopsis of the risk and uncertainty. Fisheries Research 62:339-347.

Volpe JP, EB Taylor, DW Rimmer, BW Glickman. 2000. Evidence of natural reproduction of aquaculture-escaped Atlantic salmon in a coastal British Columbia River. Conservation Biology 14:899-903.

Waknitz FW, RN Iwamoto, MS Strom. 2003. Interactions of Atlantic salmon in the Pacific Northwest IV. Impacts on the local ecosystems. Fisheries Research 62:307-328.

Note: if you would like to read any of these peer-reviewed papers and do not have access to them, please contact SeaDoc.



Banner photo: Farmed Atlantic salmon caught by a fisherman after the Cooke Aquaculture Pacific incident. Courtesy of Washington Department of Fish and Wildlife.