Salmon depend on their sense of smell to detect and avoid predators, find prey and return to their natal streams to spawn and die at their life’s end, but that critical sense may be endangered by increased carbon emissions in the ocean.
Research results released in late December by the University of Washington, in collaboration with NOAA Fisheries’ Northwest Fisheries Science Center, note that higher levels of carbon dioxide in the water can affect how coho salmon process and respond to smells.
The study, published online in mid-December in the journal Global Change Biology, is the first to show that ocean acidification affects coho salmon’s sense of smell. Researchers also looked at where in the sensory-neural system the ability to smell erodes for fish and how that loss of smell changes their behavior.
According to Professor Evan Gallagher of the UW Department of Environmental and Occupational Health Sciences salmon are potentially facing a one-two punch from exposure to pollutants and the added burden of rising carbon dioxide.
“These have implications for the long-term survival of our salmon,” he said.
Washington Sea Grant was a principal funder of the study.
The researchers wanted to learn how juvenile coho salmon who normally depend on that sense of smell to avoid predators and other dangers display a fear response with increasing carbon dioxide, because the waters of Puget Sound are expected to absorb more CO2 as atmospheric carbon dioxide increases, adding to ocean acidification.
To find out, they set up tanks at the NOAA Fisheries Research lab in Mukiteo, WA, filled with saltwater at three different pH levels, representing the current pH of Puget Sound, the predicted level 50 years from now and the predicted level 100 years from now. Juvenile coho salmon were placed in the tanks for two weeks, after which researchers ran a series of behavioral and neural tests to see how their sense of smell was impacted. The salmon were exposed to the smell of salmon ski extract, an indicator of a predator attack that usually prompts the cohos to hide or swim away.
Those cohos from the tank with water at current CO2 levels responded normally to offending odors, but fish in the tanks with higher CO2 levels didn’t seem to detect the smell. Neuro tests of each fish’s nose and brain was also measured to see where the sense of smell was altered. When neuron behavior in the olfactory bulb in the brain was analyzed, researchers saw that processing was altered, which suggests the fish couldn’t translate the smell into an appropriate behavioral response. Researchers also looked at fish tissue to see if gene expression had changed and found it altered for fish exposed to higher levels of CO2.
Researchers plan to study next whether increased CO2 levels affect other fish species in similar ways, or alter other senses in addition to smell, and hope to alert the public of potential consequences of elevated carbon emissions.