When Keith Colburn got a phone call that longtime friend and fellow captain Tony Lara had died in his sleep, the Discovery star couldn't believe the news.

"It came as an absolute shock that he had passed away," Colburn tells PEOPLE of Lara's death on Saturday.

"I know Tony had some heart problems a couple of years back, but he didn't really talk about it much."

While the Deadliest Catch costars have been friends for more than 20 years, Lara also worked on Colburn's boat in the early 2000s as an engineer.

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