(CNN) – River otters have made a return to the Detroit River after becoming locally extinct since the early 1900s.

This once in a century sighting says a lot about our environment.

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Eric Ste. Marie was walking along the Detroit River on April 25 when he noticed something unusual.

“I see something brown and furry in the water, and usually when I see something furry in the water I assume it’s a mink or a muskrat,” said Ste. Marie.

But, as a biology PHD student at the University of Windsor, Ste. Marie knew better.

“As it got closer I realized it was much too large to be either of those,” said Ste. Marie.

It turns out that this furry creature popping his head up was a river otter.

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River otters were common hundreds of years ago during the fur trades, but they were over harvested which caused them to become locally extinct.

Then in the 1940s to 1970s, the Detroit River became one of the most polluted rivers in the United States due to oil spills.

“Oil would mat their fur and they couldn’t thermoregulate, they couldn’t keep warm, and they would die,” said John Hartig, visiting scholar at Great Lakes Institute for Environmental Research.

In the mid-80s river otters were reintroduced in Ohio and have spread across the region. These animals are picky about where they live, only feeling comfortable in clean water.

Hartig says that this is a statement that the Detroit River is cleaner.

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