Antarctic Fish "Hibernate" in Winter

Helen Scales
for National Geographic News

March 6, 2008

Antarctic cod go "on ice" and take a nap during the long winter months, a new study shows.

The cod hunker down on the seafloor, reduce their feeding, and slow their heart rates—probably as a way to survive Antarctica's dark winters, when the fish might have a harder time spotting prey.

This is the first time fish have been seen actively becoming torpid—a state similar to hibernation in land animals—as part of an annual cycle.

"A lot of freshwater fish go [unexpectedly] dormant in winter because a drop in temperature lowers their metabolism," said study co-author Hamish Campbell, a zoologist at the University of Queensland, Australia.

"By contrast, these Antarctic fish actively reduce their 'cost of living,'" he said.

The findings appear this week in the journal PLoS One.

Dramatic Slowdown

Campbell and colleagues attached heart rate monitors to wild fish and tracked their movements for a year using acoustic tags.

"The fish became 20 times less active in winter compared to summer," said co-author Keiron Fraser, a marine biologist from the British Antarctic Survey.

"Antarctic cod are not the quickest swimmers, but in winter they become semi-comatose," Fraser said.

About every week or so the cod wake up and swim around for a few hours, the team observed.

"This is quite similar to 'denning' in bears, where the hibernation isn't so deep and the animals can be disturbed, then spend some time awake before going back to bed," Fraser said.

(Related news: "Hibernating Animals Suffer Dangerous Wakeup Calls Due to Warming" [February 2, 2007].)

But for now the researchers aren't sure why the cod employ this biological trick.

"Annual sea temperatures in the Antarctic vary from around 1 degree Celsius [33.8 degrees Fahrenheit] in summer to around -1.8 degrees Celsius [28.7 degrees Fahrenheit] in winter," Fraser said.

"This change isn't enough to explain the changes in metabolic rate we see in these fish."

And, he said, "warming up winter-acclimatized fish by two degrees [Celsius] in the laboratory doesn't increase their heart rates."

Lack of prey also doesn't appear to be a motivating factor.

"There still seems to be plenty of food available, but the fish adopt a strategy which doesn't involve going to look for it," Fraser said.

"Perhaps they are visual predators, and in the [dark] winter they basically can't see their food."

Sparse Sunlight

The team is also unsure what triggers the fish to start their winter slowdown.

"Like many other hibernating animals, they may be responding to changes in day length" study co-author Campbell said.

In the summer the Antarctic Peninsula—where this study took place—is bathed in 24-hour daylight. But during the winter there are only three hours of dusky light every day, around midday. (Explore an interactive map of Antarctica.)

Arthur DeVries is a marine biologist at the University of Illinois, Urbana-Champaign, who was not involved with the new research.

"This study is a very interesting observation, especially from an ecological point of view," DeVries said.

Further studies on the cod could show whether they also alter their body chemistry to survive so long without much food—which would offer proof that their hibernation is a survival strategy.

"It would be interesting to see whether the activities of [the fish's] enzymes involved in energy production are also depressed during this period of dormancy," he said.

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