"Salmon famously use their nose for so many important aspects of their life, from navigation and finding food to detecting predators and reproducing. So it was important for us to know if salmon would be impacted by future carbon dioxide conditions in the marine environment," said lead author Chase Williams, a postdoctoral researcher at the UW Department of Environmental and Occupational Health Sciences.
Researcher Chase Williams taking water samples to measure the pH in the tanks used in the experiments of the study. (Photo: University of Washington)
The analysis takes a more comprehensive approach than earlier work with marine fish by looking at where in the sensory-neural system the ability to smell erodes for fish, and how that loss of smell changes their behavior.
"Our studies and research from other groups have shown that exposure to pollutants can also interfere with sense of smell for salmon," said Evan Gallagher, senior co-author and a UW professor of toxicology.
In the behavioral tests shown in this video, juvenile salmon in two separate tanks were exposed to an odour that would normally prompt a fear response. In the first clip, fish smell the odour coming from the left side of each tank, and avoid or swim away from the smell. In the second clip, fish have been exposed to higher levels of CO2, which has impaired their sense of smell. The fish don’t react to the odour once it is introduced to both tanks, suggesting their ability to smell has been altered.
"Now, salmon are potentially facing a one-two punch from exposure to pollutants and the added burden of rising CO2. These have implications for the long-term survival of our salmon," Gallagher added.
The research team wanted to test how juvenile coho salmon that normally depend on their sense of smell to alert them to predators and other dangers display a fear response with increasing carbon dioxide.
Puget Sound's waters are expected to absorb more CO2 as atmospheric carbon dioxide increases, contributing to ocean acidification.
In the NOAA Fisheries research lab in Mukilteo, the research team set up tanks of saltwater with three different pH levels: today's current average Puget Sound pH, the predicted average 50 years from now, and the predicted average 100 years in the future. They exposed juvenile coho salmon to these three different pH levels for two weeks.
After two weeks, the team ran a series of behavioral and neural tests to see whether the fish's sense of smell was affected. Fish were placed in a holding tank and exposed to the smell of salmon skin extract, which indicates a predator attack and usually prompts the fish to hide or swim away.
Fish that were in water with current CO2 levels responded normally to the offending odor, but the fish from tanks with higher CO2 levels didn't seem to mind or detect the smell.
The head of an adult coho salmon. (Andy Dittman from the Northwest Fisheries Science Center)
After the behavioral tests, neural activity in each fish's nose and brain — specifically, in the olfactory bulb where information about smells is processed — was measured to see where the sense of smell was altered. Neuron signaling in the nose was normal under all CO2 conditions, meaning the fish likely could still smell the odours.
But when they analyzed neuron behaviour in the olfactory bulb, they saw that processing was altered — suggesting the fish couldn't translate the smell into an appropriate behavioral response.
Finally, the researchers analyzed tissue from the noses and olfactory bulbs of fish to see if gene expression also changed. Gene expression pathways were found to be altered for fish that were exposed to higher levels of CO2, particularly in their olfactory bulbs.
This study was published in the journal Global Change Biology.