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One Fish, Two Fish—Why So Blue, Fish?

Rising stream temperatures in the Chesapeake Bay region are depriving fish of habitat and oxygen.

More than a century ago, when the U.S. Geological Survey began monitoring temperatures of the rivers and streams that empty into the Chesapeake Bay, climate change wasn’t on many peoples’ minds (except maybe this guy’s). Initially, the scientists would go to the 100 or so sites to measure flow volume for the purpose of forecasting floods and keeping track of the water supply. While there, they took temp readings, too—and it’s a good thing they did: That extra bit of work is now proving useful, giving us a picture of how global warming has affected this very important ecosystem over the long term.

A recent study by USGS hydrologists shows that over the last 50 years, rising air temperatures have warmed the streams leading into the Chesapeake Bay by more than 2.5 degrees Fahrenheit.

Who cares about a couple of degrees of extra warmth? Fish do.

Photo: Chesapeake Bay Program Warming waters are making it difficult for fish like the brook trout to find food and reproduce.

“Brook trout are very sensitive to temperature,” says USGS hydrologist John Jastram, one of the authors of the study. “The cooler headwaters and mountain streams they inhabit are one example of a place we expect to see the effects of warming water.”

As water warms, it holds less oxygen, stressing the trout and making it more difficult for them to find food and spawn. Making matters worse, large mouth bass and other fish that prefer warmer water move into the same stream segments, increasing competition for resources.

Continued warming is likely to strand the brook trout, which already live in some of the coldest upstream waters in the area. The trout in the Chesapeake region could become like the pikas of the West, the alpine rodents that have moved higher up the mountains to escape warmer temperatures but are now running out of room to retreat to.

Higher water temps do more than just make fish crowded and uncomfortable. Remember dead zones, those vast expanses rendered lifeless by algae blooms? The study suggests these oxygen-depleted areas are about to get larger and more common in and around the Chesapeake Bay. As you can see in the interactive map below, the waterway already hosts many seasonal dead zones, so this is highly unwelcome news for the bay’s crabs, oysters, and rays.

Excess nutrients from farm runoff and sewage-treatment plants are among the main drivers of dead zones. Typically, sediments at the bottom of rivers and streams absorb a portion of those nutrients and help mitigate dead zones. As water gets warmer, though, sediments release some nutrients back into the streams. In addition, higher water temperatures favor many species of algae that eat the excess nutrients, proliferate, and eventually die. As the algae decompose en masse, they feed bacteria that then deprive the waterway of oxygen. Even if you don’t care about the fate of trout, this is a major issue in the Chesapeake. The bay provides more than 500 million pounds of seafood every year, accounting for more than $3 billion in economic value.

The USGS researchers showed an average warming trend in the tributaries of 0.05 degrees Fahrenheit per year between 1960 and 2010. They haven’t yet analyzed the data to determine whether the rate is accelerating. But if that is the case—and it very well may be, since air temperatures are rising at ever faster rates—the entire ecosystem is in for major changes. And there would be very little we can do to stop it.

Oh, wait, there is one thing that could help some: We could stop emitting so much greenhouse gas.


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