CHICAGO (CBS) — The Shedd Aquarium is down to just two sea lions, after making a difficult decision to euthanize a 13-year-old male.

The Shedd had rescued the sea lion, named Otis, in 2009 as the government wanted to kill dozens of them to save the endangered chinook salmon on the Bonneville Dam.

“He was a wonderful animal that will be really missed here at Shedd,” Ken Ramirez, the Shedd’s executive vice president of animal care and training, said.

Otis began showing signs of illness two weeks ago. A necropsy performed this week shows he had urogenital cancer, something that’s becoming a problem in the wild.

“Which really opens up a much bigger question about what is in our oceans these days,” Ramirez said.

The remaining two sea lions are doing OK both physically and emotionally. The humans, on the other hand, have heavy hearts over the loss.

LISTEN: WBBM Newsradio’s Michele Fiore reports

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