The eelpout--a shallow water, bottom-dwelling fish--is an indicator species. Zoarces viviparous, or the one that gives birth to live young, rests near the top of its local food chain in the shallow waters of the southern North Sea, an area also known as the Wadden Sea. Although it is not commercially fished, records on its population go back more than 50 years and its health is taken as a general proxy for the overall health of this sea. New research combining lab tests on its ability to thrive in warmer waters and these long-term records show, for the first time, how climate change might be driving the eellike fish out of its ancestral home.

Animal physiologist Hans Prtner and animal ecologist Rainer Knust of the Alfred Wegener Institute in Bremerhaven, Germany, compared field data with lab tests to show that the fish face an oxygen constraint as the water warms up. Because the eelpout gives birth to relatively few live young and does not migrate, environmental challenges quickly register in its population numbers. Plotting the instrumental record of temperatures against these population numbers revealed that the eelpout declined during hotter years--where there was not any significant change in other factors such as predation or fishing--the two researchers report in Science on January 5.

Meanwhile, lab tests showed that when exposed to warmer water, the fish were unable to get as much oxygen as they needed to cope with the increased temperature. "When you say I get less oxygen in, then I have less oxygen available for aerobic energy and I have less energy for the diverse tasks that a species needs to fulfill in an ecosystem, such as being ready to prey, grow, move and reproduce," Prtner explains. "It's a general weakening of the individual once it is going beyond its thermal limitation."

On top of that, warmer temperatures mean less oxygen dissolves into the sea's waters; not only are the fish able to get less oxygen out, but there is less oxygen available. This oxygen deprivation provides a mechanism for how climate change directly affects the ability of a species to thrive and now has been demonstrated in aquatic animals ranging from worms to the eelpout, Prtner notes.

Eelpout populations did recover during relatively mild summers, because the shallow-water species is adapted to deal with larger temperature fluctuations better than some of its aquatic counterparts. But this climate-based fluctuation may be interacting with other pressures in other species to produce dire results. "There is relatively strong evidence that the cod in the North Sea find it too warm to maintain high productivity," Prtner says. "When the fishing industry maintains the same high fishing pressure--that has not been a problem before--with the lower productivity, this turns into overfishing."

And with even greater global warming predicted for the future, the eelpout may simply be an indicator of change to come. "It doesn't mean that the species will go extinct necessarily, but it means they will move," notes Tobias Wang, a zoophysiologist at the University of Aarhus in Denmark. "If temperatures are going to change in [the] future, then it will have a major impact on the distribution of animals."