Ocean Acidification

Are Pacific salmon heading for a nosedive?


If you’re not one for the smell of fish, be thankful that you’re not a Pacific salmon. Salmon have a sense of smell that’s said to be thousands of times stronger than a dog’s, and that sensitivity is essential. Salmon smell their way to food, to their mates, to safety and even to their way back home. Salmon famously return from the open sea back to their freshwater birthplaces when they are ready to spawn, and scent is the guide that they use to get there. However, ocean acidification could be putting this strong sense — and the fish themselves — under threat.

Salmons’ discerning noses could soon be losing their power.  As carbon emissions are absorbed into the ocean, they are changing the water’s chemistry through a process known as ocean acidification. According to Washington Sea Grant-funded research from toxicologist Evan Gallagher’s lab, this chemical change to our seas could be impacting salmons’ perception of how the world around them smells — and this could threaten their very survival.

Gallagher and his UW lab have spent years researching the impacts of various pollutants on fish. “The main focus in the lab

Evan Gallagher

has been how pollutants disrupt fish behavior,” Gallagher says. In the past, this has included the impacts of pollutants from sources such as industrial, agricultural and wastewater runoff. Now, as the marine waters of the Pacific Northwest continue to acidify, carbon dioxide (CO2) has been added to the list. 

Gallagher and team conducted an experiment using a series of tanks with three different CO2 treatments. The first treatment tested Puget Sound’s current average pH of 7.8; the second tested a pH of 7.5, which is the projected average 50 years from now; the third used a pH of 7.2, which is the projected average 100 years from now. To assess the effects of the different CO2 exposures, coho salmon were placed in a simple maze that presented the fish with two distinct incoming streams of water. In one, the salmon got normal water. In the other, the incoming water included salmon skin extract — which has a smell that salmon typically avoid. 

“When salmon skin is lacerated — like when a fish is attacked by a seal or an orca — it releases a chemical compound,” postdoctoral researcher Chase Williams explains. When other salmon smell the compound, as they should from the salmon skin extract the researchers put in the tank, it is their cue to swim away from the danger. “Using behavioral tracking software, we could follow the fish in real time and measure the amount of time the fish spent in one stream versus the other,” Williams says. The research team predicted that salmon would stay in the arm of the tank that didn’t have the skin extract in it.

What the results showed was far more complicated. In the control tank (with the pH of 7.8), the salmon only spent 20 percent of their time in the arm with the salmon skin extract, meaning they displayed the expected behavior of generally avoiding the stench associated with predators. As the pH dropped, however, it appeared the salmon responded less and less to the smell. In the tank with a pH of 7.5, the salmon spent 30 percent of the time in the “predator arm.” In the tank with a pH of 7.2, they spent 50 percent of their time in the same fetid branch of the maze — indicating that by this point, they were unable to respond to the signal of danger.

While ocean acidification will likely make a pH of 7.2 more common in the years to come, such a number is currently possible. “You can already pick that up in some areas of Puget Sound,” Williams says. Which means some salmon could be starting to feel the impacts now.

The good news is that ocean acidification does not appear to similarly damage the sniffers of all fish species. The researchers also conducted a simultaneous experiment on sablefish and found that the acidified water did not impact their smell-bound behavior.

After the behavioral studies, the scientists began to investigate where, exactly, the change in behavior was coming from. Did the acidified water cause problems with their noses? Or did the low pH cause neurological change that reshaped how the salmon perceived the smell? While the former would mean no longer being able to pick up smells — say, if you were stuffed up from a cold — in human terms the latter would be more along the lines of becoming indifferent to the odor of sewage. This second case is what the team found held true for the salmon.

There are still a lot of unknowns in terms of the implications, including whether the impaired sense is permanent. Regardless, “this kind of outcome can be deleterious to the life of the fish,” Williams says. For a salmon, being able to react to smell could mean the difference between enjoying its own dinner — or becoming someone else’s.