Rosas, 41, was a beloved manager at the smoked fish shack perched on the 95th Street Bridge, deep on the city's South Side.
Owner Mark Kotlick said Rosas was sick for about six weeks and stayed at home, where he just kept getting worse. His family eventually took him to Northwestern Memorial Hospital where he was put on a ventilator and eventually died.
"It's an unreplaceable [sic] loss, basically. He was the face man, I'll say, for the store," Kotlick said. "We have a lot of repeat customers. Carlos took pride in knowing everybody."
Rosas worked at Calumet Fisheries for 20 years, serving as a manager for 16 of those years.
Calumet Fisheries was notably one of the places featured on Anthony Bourdain's Chicago episode of "No Reservations" in 2008.