Pink Salmon Struggle as Freshwater Becomes Acidic

Pink salmon are providing researchers with sobering hints to how carbon dioxide-induced acidity could affect freshwater fish species by the end of the 21st century.

A study  yesterday in  showed that early exposure to high levels of CO₂ during the larval stage of development had significant negative effects on the fish’s size, metabolism and ability to sense threats in their environment.

The study was among the first to look at how different CO₂ levels could affect fish larvae in fresh water, according to the lead author, Michelle Ou, a former master’s student at the University of British Columbia in Vancouver.

“We didn’t actually expect to see so many effects,” she said. “We were just poking around to see what we could find.”

Pink salmon seemed like a good species to start with. Not only are the fish abundant and economically important, but they also serve as a keystone species in marine, freshwater and terrestrial ecosystems, according to the researchers. Although pink salmon spend their adulthood in the open ocean, their first weeks of life are in freshwater streams. Once they have matured from larvae to fry, the fish leave the streams where they hatched and swim to the open ocean. Later, as adults, the fish will return to the same streams to spawn.

Ou and her colleagues at UBC created an experiment to test how fish were responding not only to ambient CO₂ concentrations but also to acidity levels expected by 2100.

After obtaining salmon embryos from a hatchery, the researchers transferred them into freshwater flow-through tanks with either ambient, high or variable concentrations of CO₂. After 10 weeks, they tested the baby fish to see whether or not their development had been affected by the different conditions.

They found that not only were they smaller and lighter, but the fish’s senses were also impaired. The pink salmon larvae were more bold around new objects and did not seem to be afraid of alarm cues in the water that would normally prompt fish to flee.

Weight loss and impaired navigation

“Think of it as a smell fingerprint of their natal stream, and they use that to find their way home,” she said.

While pink salmon are less particular about where they spawn than other salmon species, the research suggests that higher levels of CO₂ could eventually prevent the fish from finding their way to their natal streams if they are unable to adapt, she said.

Once the fish had reached the age at which they would normally swim to the open ocean, the researchers transferred the fish into saltwater tanks that had either the same or increased levels of CO₂-induced acidification.

When the fish grew up in fresh water and seawater with high concentrations of CO₂, they lost weight at double the rate of fish that were only exposed to salt water with higher CO₂ levels. Their ability to take up oxygen also went down by 30 percent, according to the study.

The findings showed that the fish and freshwater ecosystems may be more vulnerable to rising levels of carbon dioxide than previously thought, though the researchers don’t really know why carbon dioxide is having this effect.

A dearth of research

Previously, the scientific community believed that the ocean was so well buffered that higher levels of atmospheric CO₂ would have little impact on marine life. Now, researchers are struggling to figure out how conditions have changed, since there is very little data to create a base-line comparison, he said.

Part of the reason it took longer to recognize the impact of CO₂ is because adult fish tend to be more capable of handling higher levels of acidity, said Colin Brauner, a zoology professor at UBC and co-author of the study.

“People have looked at CO₂ exposure in adult fish for a long time. If you expose it to 10 times the highest concentration used in our experiment, the fish don’t have a problem. Their gills can pump out acid, so their blood stays stable. So people thought fish would be just fine,” Brauner said.

By contrast, recent studies in tropical fish have shown that fish larvae experience quite large effects from increased CO₂, he said.

Very little research has focused on freshwater species, because the conditions tend to be much more variable between streams, lakes and rivers. However, more studies on how CO₂ affects fish in these habitats are needed because about 40 percent of fish species live in fresh water, Ou said.

This new research suggests that the effects of CO₂ on larval fish may be broader than previous research had shown, according to Brauner.

“It may be during that early development that all fish are affected in a similar way. We don’t know, but most of what we see in developing salmon are seen in developing tropical fish,” Brauner said. “If the mechanism is the same, it could have a broad effect.”

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