He — and several wildlife experts — believe the animal could be a river otter, which hasn't been officially spotted in the Chicago River in decades, or possibly much longer.
The find would be the equivalent of seeing a mountain lion roaming through Chicago, which of course happened in 2008 before it was killed by police.
Wesley, the co-founder of nonprofit Urban Rivers, saw the mammal from the shoreline during a summer speaking engagement. Another member of the group that day, Metropolitan Planning Council director Josh Ellis, said the whole group saw the "good-sized" animal, which Ellis could have been an otter, beaver or muskrat. Wesley witnessed it again while paddling a kayak near Urban Rivers' floating vegetation island by the Whole Foods on Kingsbury Street.
"It was fleeting, within a few seconds it dove under and was gone," said Wesley, who's so excited about the possible find, he set up a camera trap in the area in hopes of capturing a photo.
River otters, which are playful, fun-to-watch apex predators, were wiped out of Cook County at the turn of the century due to water pollution, lack of open space and the value of their pelts.
But a river otter was officially discovered in an isolated pond in a Cook County forest preserve last year. Forest Preserves of Cook County senior wildlife biologist Chris Anchor says "it's absolutely possible that there is an otter in the Chicago River."
"At this point, I would not be surprised if there were valid reports of otters in any watershed in Cook County," Anchor said. "Male otters will disperse up to 90 miles overland to find non-connected waterways.”
Anchor told the Tribune in 2015 that otters caught in Louisiana were released in the central Midwest area in the 1990s, but none were let go within 150 miles of Cook County.
Dan Boehm, a zoological manager at Lincoln Park Zoo, said river otters inhabiting the Chicago River, even Downtown, "absolutely seems possible." He pointed to an example eight years ago when a wild beaver showed up in a pond just south of the zoo — an extremely rare occurrence.
That beaver likely would have had to swim up Lake Michigan from south of the city, where they're more common, and cross Lake Shore Drive to get to the pond, Boehm said.
"That was quite remarkable and that put things in perspective that a lot of things were possible," Boehm said.
Boehm said if river otters become a regular occurrence in the Chicago River, it "would be a real treat."
"It's a lot of fun to watch otters," he said.
River otters can grow up to four feet long and weigh up to 30 pounds.
Boehm said otters eat slow-moving fish like carp — the Chicago River is full of those — frogs, crayfish, insects, turtles and other prey. The make their own burrows along riverbanks and use ones already created by beavers, muskrats and other animals.
There have been no official sightings of river otters in the Chicago River, but people may have seen them. Friends of the Chicago River executive director Margaret Frisbie said otters have been sighted Downtown on the "odd occasion." Ryan Chew, founder of Chicago River Canoe and Kayak, said he has "varying degrees of confidence" in the possible sightings.
"There are loads of beaver and muskrat, and loads of observers who don't know the difference," Chew said.
But, Chew added: "It seems very possible. The population is expanding in Illinois, the Chicago is a connected waterway, and there are prey species there for them."
Seth Magle, the director of Lincoln Park Zoo's Urban Wildlife Institute, said no otters are known to live in Chicago city proper currently.
But "we shouldn't be surprised to see otters and other species begin to appear where we might not have expected," Magle said. "This can be a tremendous opportunity — imagine a wilder Chicago, one that has place for species like otters, mink, ground squirrels and others. We aren’t there yet, but if this sighting is verified, it might mean that we’re one little step closer."
Boehm said if the large mammal hanging out near Goose Island isn't an otter, he won't be discouraged.
"If an otter isn't here today, it's probably inevitable that they will be here eventually," he said.
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