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Thousands of pink salmon can be seen swimming upstream to spawn in this file photo.LUCAS JACKSON/Reuters

When a small Lego figurine was placed in a tank of freshwater where the carbon dioxide level had been elevated to project conditions caused by future climate change, researchers noted a surprising response in pink salmon.

Instead of fleeing, as fish in a control group in non-acidic water did, the young salmon hovered near the tiny figure, apparently unafraid and without the flight instinct that is vital for avoiding predators.

Michelle Ou, a master's student working with Colin Brauner in the zoology department at the University of British Columbia said in an interview the failure of the fish to flee the funny Lego figure is one of several observed changes that could have serious implications for pink salmon.

"You could say it might have a large impact on predation rates and thus survival," said Ms. Ou, lead author of a study that looked at the responses of pink salmon to acidification caused by increased carbon dioxide levels.

The researchers found that when CO2 levels were increased to levels predicted to occur under climate change, the higher acidification affected "the growth, metabolism, olfactory responses and anti-predator behaviour in the early life stages."

The natural reaction of salmon to the placement of "novel objects" in a tank is to flee to the outer perimeter. But Ms. Ou said when the water became more acidic, the fish were smaller, less energetic and they failed to show anxiety when the Lego figure was introduced to the tanks.

Ms. Ou said the fish appeared to have lost some of the reactive skills they need to survive.

"Usually with the behaviour of pink salmon they are not very bold, especially when exposed to a novel object. So these fish don't seem to have that behaviour. High CO2 has altered their behaviour somehow," she said. "If they are more bold and less cautious, that could affect their survival."

She said the more acidic water also seemed to rob the fish of their olfactory abilities, which are important in both detecting the presence of predators and in identifying the waters of their natal streams.

All salmon spawn in fresh water and imprint on the chemical make-up of their home streams, returning there to spawn after migrating to the ocean to mature. Pink salmon, a keystone species on the Pacific Coast because they are so prolific, migrate to the ocean within weeks of hatching, returning two years later to spawn.

Ms. Ou said more research is needed but the study suggests the fish might have difficulty relocating their streams because of olfactory impairment.

"One of the ways they find their way back is when they are young they are able to recognize the different amino acid composition of that [home] river," she said. "So it's kind of like the amino acid fingerprint of that stream. When they come back as adults they use that fingerprint to find their way home and because we see there's a reduction in olfactory response, their ability to smell those amino acids might be affected."

Ms. Ou said the study shows increased acidification has numerous "sub-lethal effects" that don't directly kill fish, but which make them weaker, smaller and generally more vulnerable.

"What that translates into in the future we don't necessarily know because fish populations can adapt," she said. "Natural selection can work in a way that a more CO2 tolerant fish does better and is selected for, so the population might evolve. We don't know if that's possible, so more research has to be done."

The study was published this week in the scientific journal, Nature Climate Change, and comes on the heels of other recent research that shows how increased ocean acidification is affecting a wide range of aquatic life.

Just last week U.S. researchers reported the Arctic Ocean is becoming so acidic it could soon threaten the ability of marine creatures to build shells. One of the organisms that might be affected is the pteropod, or swimming sea snail, which makes up nearly half of the pink salmon's diet.

This story corrects an earlier version that incorrectly spelled Ms. Ou's first name