(CNN)A river otter has been spotted in the Detroit River for the first time in a century.
Eric Ste Marie, a marine ecologist studying at Windsor University in Ontario, told CNN that the remarkable sighting happened after his partner suggested they go for a walk before starting work.
Ste Marie lives on the Canadian side of the Detroit River, which divides Detroit, Michigan, from Windsor, Ontario.
"I spotted something brown and furry in the water," he said. "Usually when I see something like that, it's a muskrat or a mink, which are much more common in this area."
"I realized it was too big to be either of those. As it dove, it didn't have the flattened tail like a beaver, which leaves otter as the only possibility," he explained. River otters, semiaquatic mammals that can grow up to 30 lbs. and over 3 feet long, are found across North America, although their range has been significantly damaged by hunting and habitat loss.
Ste Marie and his partner "sprinted a few hundred yards" to the walkway where the otter was headed. He grabbed his camera and then got a video of the otter's head bobbing out of the water.
"I didn't even know it was possible to see river otters here," he said. So he contacted some local experts -- who confirmed that river otters hadn't been seen in the Detroit River for 100 years.
"I had a feeling it might be a rare encounter, but I really had no idea just how rare," Ste Marie said. "It's pretty crazy to be lucky enough to see one."
John Hartig, a visiting scholar at the Great Lakes Institute for Environmental Research who focuses on pollution cleanup in the Great Lakes, confirmed the rarity of Ste Marie's sighting.
"We are thrilled that they are back," he told CNN.
River otter populations have declined since the 1600s for numerous reasons, including the fur trade and loss of habitat. Detroit had "incredible pollution," Hartig said. There was "so much oil in the river that not only river otters but beavers could not live there. Oil would coat their fur and they couldn't thermoregulate and keep warm, so they died."
But cleanup efforts in the past few decades have paved the way for the otters' return to the river. Hartig said that in 2019, river otters were observed in Lake Erie National Park, having swum across Lake Erie. It was the first time the otters had been seen there in 100 years. "We thought we're next, Detroit River is next," he said.
River otters are an "indicator species," according to Hartig, acting as a measure of cleanliness in the water. "We look for things like river otters," he said. "If they're there, it's a good sign."
The otters and their environment still face challenges, including climate change, urban runoff, and the continued "legacy of the industrial revolution" in the form of "contaminated sediments in these rivers and harbors," Hartig said. But the otters' return is a beacon of hope for conservationists and environmentalists alike.
"If the river is cleaner for the river otter, maybe it's cleaner for you and me," he added.