MARY LOUISE KELLY, HOST:
A few weeks ago, Eric Ste-Marie was taking an early morning walk on the Canadian side of the Detroit River when he saw something that no one has seen for more than a hundred years. So he grabbed his phone, shot a video.
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ERIC STE-MARIE: A straight-up river otter in the Detroit River. Have you ever heard of something so controversial?
EMILY FENG, HOST:
He couldn't believe his eyes. At first he thought the large, brown, furry animal was probably a mink or a muskrat or maybe even a beaver.
STE-MARIE: But when it dove, I saw that its tail wasn't flattened like a beaver's would be. And so that really only left otters as what it could be, which I didn't even think was an option in the Detroit River as an animal to see. So I got really excited.
KELLY: That is because the river was notorious for its pollution. Factories on both sides of the border had dumped oil in there before there were laws against doing that. It was a toxic mess.
JOHN HARTIG: There was so much oil floating down the river, and the waterfowl would come through on their migration. They would land in the few pockets of open water during winter, and that was filled with oil. And then 11,000 ducks and geese would die in a single day because of that.
FENG: That's John Hartig. He's a scientist who has studied this ecosystem his entire career. He didn't think it was clean enough for river otter.
HARTIG: And during the '40s, '50s, '60s and '70s, because of all that oil and petroleum products, river otter could not even have survived in that because the oil would mat their fur. And they couldn't thermoregulate, and they couldn't keep warm, and they would die.
KELLY: But in recent years, John Hartig says the river has come a long way. You can see bald eagles, peregrine falcons, lake sturgeon and now river otters, a mammal that can only survive when water is clean.
HARTIG: In this journey of revival of the river, it's so important that we have something like the river otter return because this gives hope. If the Detroit River is now cleaner for river otter, it is indeed cleaner for you and me and others who visit here or live here because we share the same ecosystem. Transcript provided by NPR, Copyright NPR.