Ocean acidification is a growing problem that scientists are scrambling to understand.
It occurs when carbon dioxide absorbed into the ocean mixes with sea water to create carbonic acid. This substance reduces the pH-balance of the surrounding waters making them more corrosive.
A new study from the Scripps Institution of Oceanography found that such ocean acidification may be making some fish extremely anxious.
Martín Tresguerres studies the California rockfish for Scripps.
He and co-author Trevor Hamilton of MacEwan University in Canada wanted to see how the behavior of the fish changed when it was exposed to high levels of acidic ocean waters for a week. They had a hunch the fish would become anxious based on previous studies of other fish.
This led to an odd question.
"How do you measure anxiety in fish?" Tresguerres said.
It turns out that anxious rockfish like to hide in the dark.
So the researchers built a fish tank that was half in light and half in the dark. Normal rockfish explored both sides.
Rockfish given an anxiety-producing drug hid in the dark.
And, as Tresguerres predicted, so did the rockfish exposed to acidic waters.
"They also go to the dark part of the tank, without any drugs," Tresguerres said.
In the wild, he said, this could mean rockfish exposed to acidic water may not explore past their shady kelp forests and fail to find new food or spread throughout a region.
The change in behavior has to do with neural receptors known as GABBA receptors.
They are an important part of the sensory system and a common feature of vertebrate animals. Human anxiety levels are related to GABBA receptors as well.
Tresguerres says a number of studies have shown that highly acidic ocean water changes how GABBA receptors work in various types of fish.
But, he points out, not all of them become nervous.
For example, he says, clownfish in acidic waters seem unable to recognize predators, making them easy prey. But he adds, some predators are changed too.
"There are other studies showing that the predators don't recognize the prey," Tresguerres said.
A growing problem
There are some limitations on the current research. For instance, the fish in these studies were exposed to acidic water quickly, leading to a rapid change in their environment.
In reality, the oceans are acidifying relatively slowly, and the pH levels can rise and fall at different times. This variability may allow fish in the wild to adapt to ocean acidification over time.
Still, as humans continue to burn fossil fuels, the amount of CO2 in California's waters is expected to double within 100 years unless emissions are curbed.
Tresguerres says this could mean strange changes in many species' behavior and that could spell trouble for the marine ecosytem.
An earlier version of this story incorrectly referred to the Scripps Institution of Oceanography. KPCC regrets the error.