Acidifying ocean water—a byproduct of increased carbon dioxide in the atmosphere—messes with saltwater fish in a variety of ways. Rockfish become more nervous. Clownfish can’t always detect predators, whereas the brown dottyback actually avoids the smell of injured prey.

But it turns out that saltwater fish aren’t the only ones affected. Pink salmon reared in CO 2 -rich fresh water showed many of the same signs as their marine brethren, according to a study published today in the journal Nature Climate Change. They were smaller, less fearful of predators, and less responsive to the chemicals that help guide adult salmon back to the streams where they hatched.

The results are the first to identify these effects in a freshwater species, and they “sound a warning,” ecologist Philip Munday of the ARC Centre of Excellence for Coral Reef Studies in Townsville, Australia, writes in a commentary accompanying the study. “Some of these fishes may be more susceptible to rising CO 2 levels than we have assumed.” Munday is a leading expert on the effects of acidification on ocean fish.

To look for the effects of freshwater acidification, scientists at the University of British Columbia (UBC) in Vancouver, Canada, hatched and raised the salmon in tanks with varying levels of CO 2 dissolved in the water. One tank had the equivalent of 450 parts per million (ppm)—a little higher than current atmospheric levels. Another had 1000 ppm, a third had 2000 ppm, and levels in the last shifted between 450 and 2000 ppm. After 10 weeks, the fish were moved to saltwater tanks that also had a range of CO 2 levels, where they continued to mature.

Compared with fish in the lowest concentrations, young fish in tanks with the highest CO 2 levels were significantly shorter (31.5 millimeters versus 33.7 millimeters) and weighed less (0.18 grams versus 0.21 grams). The size differences persisted after they moved to salt water, where the salmon grew more slowly or—in the case of fish continually kept in high CO 2 —actually lost weight.

The fish in the highest CO 2 freshwater tanks were also more fearless. They spent 15% more time in water that had been dosed with chemicals mimicking those given off by a fish under attack. They also lingered in the center of the tank five times longer in the presence of a small Lego figure. All fish showed signs of reduced activity in their olfactory cells when put in water with more CO 2 , regardless of the tank they were raised in.

The findings suggest that salmon may face a tougher new environment if greenhouse gas pollution drives up CO 2 levels in their home streams, says Michelle Ou, a physiologist at UBC who conducted much of the research as a graduate student. Smaller juveniles have lower survival rates, and bolder fish are more vulnerable to predators. And because of their reduced sense of smell, the affected salmon could also have a harder time returning to the right spawning streams.

Pink salmon, the most ubiquitous of the five salmon species on the west coast of North America, are so tiny when they head to the ocean—less than 4 centimeters—that they are particularly sensitive to added stress. But the study also raises the possibility that freshwater acidification will hamper the recovery of other salmon species, including chinook and sockeye. Studies have already forecast that future higher water temperatures will take a toll on some of those fish, notes Tony Farrell, a fish physiologist at UBC and an expert in how salmon respond to environmental stresses.

The new findings on CO 2 “should set off alarm bells for salmon management officials worldwide, particularly those in the Pacific Northwest,” Farrell writes in an email to Science. And they may bode ill not just for salmon but for other fish that spend at least part of their lives in fresh water—40% of all species.