Otto the river otter was known by many as the “most cheerful” of his kind at the Bays Mountain Park and Planetarium in Tennessee.

But last week, he died after a zoo guest tossed food into his enclosure.

In a Facebook post last Thursday, the park announced that Otto had passed away after he began showing signs of illness.

It’s not known what type of food the person threw, but a worker found grapes in the habitat where Otto lived, the park added.

The staff will be performing a necropsy to determine whether the grapes poisoned him to death, adding, “Human food is often intolerable and, in this case, even harmful to our animals.”

It’s not the first time zoo animals have suffered because of careless visitors.

Last month, a flamingo at a zoo in Illinois was euthanized after he was hit by a child skipping a rock into his habitat.

And in 2016, Harambe the gorilla was fatally shot after a toddler fell into his enclosure.

“We are both sad and furious,” one commenter said on Facebook. “People can be so irresponsible. Rest in peace, Otto. I hope those who did this come forward.”

Animal lovers also hope his memory will remind others of how fragile these animals can be.

“The best thing you can do for the park and for Otto’s memory is to calmly, kindly educate one another on the dangers of feeding wildlife,” the zoo wrote.

“That goes for any wild animals — not just the ones at parks and zoos like ours.”