AILSA CHANG, HOST:
You've probably heard that it's getting hotter outside. Last year was the hottest on record globally, topping a record that was set the year before. But some of the places we go for refuge from the heat are getting hotter as well. NPR's Nate Rott reports that two studies this week show how U.S. rivers and streams are changing and how that's affecting fish.
NATE ROTT, BYLINE: Here's a fun fact about the U.S.
BRIAN GALLAGHER: The Lower 48 in the U.S. has some of, like, the highest fish diversity almost anywhere in the world outside the tropics.
ROTT: Brian Gallagher is a researcher at Whitman College.
GALLAGHER: So for a temperate region, we have remarkable fish biodiversity.
ROTT: That is, a remarkable number of fish species living in our rivers and streams. From small, colorful darters to massive, toothy pike. Gallagher was part of a team at the Environmental Protection Agency that wanted to know how that vast array of fish is changing, so they looked at nearly 400 different species across the country over a 27-year period searching for trends.
GALLAGHER: And what we found is that it just differs a lot regionally.
ROTT: In places where rivers and streams are warm...
GALLAGHER: Those are either in, like, the middle of the country or more towards the southeastern or Gulf coasts.
ROTT: They looked at the number of individual fish, abundance in science speak, and the types of different fish, which they call richness. Both had increased. And those fish were generally smaller.
GALLAGHER: And in cold streams, we see the exact opposite, where abundance and richness are going down.
ROTT: And those fewer fish are generally larger. Those cold rivers and streams are mostly in the west at higher elevations, the kinds of places people seek out for fishing. The new study published in the journal Nature found that the number of fish in cold water rivers and streams has dropped by more than 50%. Gallagher says that's in part because of non-native species, many of which have been introduced by people.
GALLAGHER: Those streams are also areas that had basically, like, the most rapid increases in air temperature during that same time.
ROTT: Hotter air, a direct result of human activities that are warming the climate, means hotter water, which is what the second study by researchers at Penn State University found as well.
LI LI: So I'm Li Li, first name and last name. In English, they're the same. You spell the same. In Chinese, they're actually different.
ROTT: Li, a professor of ecological and civil engineering, coauthored a study published in the journal PNAS that looked at 40 years of data and found water heat waves. Days when water temperatures are abnormally high are now lasting nearly twice as long as air heat waves.
LI: Meaning these kind of days are happening every year more, which would mean it will have big impact on the aquatic systems.
ROTT: Authors of both studies say fish are an important source of food, income and tradition for many Americans, so they need to be protected.
Nate Rott, NPR News.
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