Warming Oceans May Kill Off Coldwater Fish Species By Overloading Respiratory Systems - Nature

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The warming of the oceans is having a cruel effect on some fish: they can't breathe fast enough to survive in a hotter home.

Hans Pörtner and Rainer Knust from the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, studied the viviparous eelpout (Zoarces viviparus), a fish that lives in the northern Wadden Sea. When summer water temperatures were about 20 degrees C the fish were fine, but after a hot summer of 25 degrees C, the fish population crashed to nearly zero.

The reason, the team concluded after lab studies of the fish, is that the animals' cardiovascular systems were working at the limits of their comfort zone. As the fishes' metabolism speeds up in higher temperatures, they need more oxygen, but their hearts can't pump fast enough to provide it. Every species has a temperature range, or 'thermal window', within which it can breathe comfortably. The eelpout of the Wadden Sea are now butting up against the upper limits of their window, says Pörtner. The fish don't like to move too far from their natural habitat, so are unlikely to swim north to cooler waters. The alternative is suffocation.

The largest of the species die off first, says Pörtner, because it takes even more energy to pump oxygen around a large animal than a smaller one. What makes things worse is the fact that warmer waters contain less dissolved gas, including oxygen. And warm conditions may become more common in the future: these waters have warmed by 1.13 degrees C over the past 40 years.

Combined, the future looks bleak for fish struggling to catch a breath, they report in Science1.


EDIT

http://www.nature.com/news/2007/070101/full/070101-5.html