From: Emily Sohn , Discovery News
Published December 9, 2009 03:14 PM

Increased Temperatures Turn Fish into Daredevils

As the world grows warmer, some fish may stop acting like themselves. With a small rise in temperature, a new study found, some fish become more daring and more aggressive than they would otherwise be. The finding suggests that climate change could put fish in peril in unexpected ways.

"The fact that big effects on behavior were happening over the course of just a couple degrees surprised me," said Peter Biro, a fish ecologist and evolutionary biologist at the University of New South Wales in Sydney, Australia. "I would have never expected that from the things I had read in the literature."

Biro stumbled on the discovery by accident. He was working with damselfish in an indoor lab that naturally got warmer over the course of a day. His original goal was to study personality traits in the fish.

But as the lab heated up every day, he noticed some fish becoming more active, aggressive and bold.

To figure out what was behind their personality transformations, Biro and colleagues put 30 young damselfish in individual aquaria. All fish were the same size and the same age, and the researchers gave them as much food as they wanted so that hunger wouldn't affect their behavior. Then came the personality tests.

To measure boldness, Biro used what he calls "the scary test." First, he shoved a wooden stick into each fish's tank, sending it into hiding. When he removed the stick, he timed how long it took for the fish to emerge from their shelters.

In a test of aggressiveness, each fish was placed near a jar that contained another fish. Scientists then watched to see how frequently the fish tried to attack or scare their intruders.

To gauge activity levels, the researchers simply counted how many times each fish crossed the center of its aquarium over two minutes.

One round of tests happened at a temperature of just over 26 degrees Celsius (79 degrees Fahrenheit). In a second round, the water was warmed by just under 3 degrees Celsius to 29 Celsius (84 degrees Fahrenheit). Testing lasted for two weeks.

In warmer water, fish became an average of six times more active (with some fish becoming as much as 23 times more active), four times more aggressive and four times bolder, the researchers reported in the journal Proceedings of the Royal Society B.

Fish that had cowered in their shelters for up to 10 minutes during scary tests in cool waters emerged immediately after the stick was gone from more tepid conditions.

A shift in metabolism probably explains why timid fish can become more daring in milder times, Biro suspects. Fish are cold-blooded, so their bodies grow warmer and more active with a warmer environment. To sustain that higher rate of energy burn, they need to eat more. And in order to get more food, they need to be bolder more aggressive.

The new findings suggest that even small changes in temperature -- from hour to hour or season to season -- can alter fish personalities, said Steven Cooke, an environmental biologist at Carleton University in Ottawa, Canada.

"In the wild, fish experience frequent fluctuations in temperature," he said. "This new research illustrates that personality attributes are not consistent across even a narrow range of temperatures."

Bolder fish are more likely to become lunch for their predators, Biro said. However, it is still not clear what his findings mean for the future of fish in a warming world.

Not all of the fish in his study actually changed their personalities when the surrounding temperature changed, for one thing. There is probably also a limit to how active or aggressive a fish can become.

"In terms of climate warming, I don't think that in 30 or 50 years we'll have these incredibly active and aggressive fish eating everything in sight," Biro said. "I think these fish are going to adapt."