The Eelpout Index A Small Fish Becomes an Indicator of Global Warming

A warm European winter has renewed concerns about climate change. New research exploring the effects of rising temperatures on a bottom-feeding fish in the North Sea -- the eelpout -- shows what global warming can do.

By Jochen-Martin Gutsch


An illustration shows an eelpout among tubeworms.
Nicolle Rager Fuller, National Science Foundation

An illustration shows an eelpout among tubeworms.

"Yep, that's the eelpout," says Rainer Knust as he bends down to peer through the aquarium glass. "Kind of lazy. It doesn't move very much." The eelpout peers back at him.

Lying on the rocky floor in cool water is a narrow, eel-shaped little fish, maybe 20 centimeters long. The Zoarces viviparus, or eelpout, is a bottom-dweller that bears live young in shallow waters of the North Sea. It's edible, but not suited to commercial fishing. Scientists are currently studying the eelpout as an indicator of the North Sea's health because its population doesn't rise and fall with fishing seasons. But nobody, Knust included, thought the eelpout would ever become famous.

Knust works at the Alfred Wegener Institute for Polar and Ocean Research in Bremerhaven on Germany's North Sea coast. The institute has a special nuclear magnetic resonance scanner which he uses to examine how the eelpout's metabolism reacts to warming water. The fish prefers ocean temperatures of 10 degrees Celsius (about 50 degrees Fahrenheit). Knust and his colleague Hans-Otto Pörtner have found that once temperatures reach 21 or 22 degrees Celsius, the eelpout's circulation fails, and the fish dies.

Pörtner and Knust recently published their findings in the journal Science, and their study has made front-page news in a country now wondering what's happened to winter. Last week, Germany's Süddeutsche Zeitung turned an obscure report on the effects of global warming on a single species of fish into a leading contribution to the debate over climate change.

The lab tests showed that warm water prevents an eelpout from absorbing enough oxygen to cope with a changing environment. Heat, in other words, can make the lazy eelpout even lazier. It sounds obvious, but Knust and Pörtner broke ground by establishing how changes in temperature directly affect the fish's physiology, and their study has also established a link between rising sea temperatures and declining numbers of fish.

"It doesn't mean that the species will go extinct necessarily, but it means they will move," Tobias Wang, a zoophysiologist at Denmark's University of Aarhus, told Scientific American. "If temperatures are going to change in (the) future, then it will have a major impact on the distribution of animals."

When eelpouts die, mullets move north

Knust's paper about the eelpout wouldn't normally interest anyone besides other marine biologists, but this year everyone seems to be talking about global warming -- on TV, on the radio, in the press. The Alps are almost bare in mid-January, and TV images show skiers on artificial snow gliding past brown woods. Researchers in Britain have predicted 2007 will be the warmest year worldwide in the past century and a half. At a recent conference, scientists meeting in the German city of Bremen discussed whether the Arctic will be ice-free in summer by 2080. And in Bremerhaven, the 50-year-old Knust gives phone interviews to journalists as far away as Washington, DC.

Knust switches on his computer and points to a graphic showing the ocean temperatures in the North Sea between 1962 and 2002. The seas have warmed in 40 years by 1.13 degrees Celsius, from around nine to ten degrees. A weather station on the German North Sea island of Helgoland measures the temperature every day at the same exact spot. "Climate change is a fact," says Knust.

But what does it mean? A rise by 1.13 degrees?

Knust clicks on a different graphic, an international climate model which estimates that the North Sea will warm over the next 100 years by at least another 1.6 degrees -- and up to three degrees Celsius. That would spell death for the eelpout. And if the bottom-feeder dies, other fish will also either perish or head north toward the cooler polar region. Knust says the eelpout's fate can be extrapolated to other organisms, pointing to dire consequences for many local types of fish if temperatures continue to rise.

Knust has devoted 14 years to his research. From March through July of the past 14 years, he has surveyed eelpout stocks between the East Frisian islands of Spiekeroog and Langeoog. Knust says he'll keep going until he retires. He'll continue to collect and compare data. This summer he'll also head towards the North Pole to catch Arctic eelpouts for a new study.

Perhaps native Baltic and North Sea species will vanish from German coastlines over the next few decades, to be replaced by Mediterranean species that are already migrating north because of warming waters.

As an example, Knust points to the striped red mullet, a tasty fish normally caught off the coast of Italy, Mallorca, and Spain. Now they're increasingly found in the North Sea, too.

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