The Police Chief of Bristol Bay Borough Police Department in Alaska tells TMZ ... the ex-Discovery Channel star died Thursday afternoon in Naknek -- this after experiencing a medical emergency of some sort.

BBBPD wouldn't elaborate on the nature of the medical emergency -- noting it was too early in the investigation to draw conclusions. We are, however, told Nick's family has been notified of the tragic news.

Further details haven't been provided at this time -- cops are conducting interviews now.

Nick appeared in 98 episodes on "Deadliest Catch" from 2005 to 2021 ... serving as a deckhand responsible for maintaining the Northwestern and its mechanical equipment. He ended up bowing out of the show after a health scare involving his appendix -- a dramatic event that chronicled in one of his final episodes.

The health scare at the time actually led to some legal action -- Nick ended up suing Captain Sig Hansen ... this after he alleged there wasn't a proper plan in place to get him outside medical help during the pandemic.

He said the delay in getting help led to his appendix rupturing -- which he claimed had a tumor attached that he wasn't aware of -- while aboard the F/V Northwestern ... and he was in court over this as recently as last year.

Hansen fired back at the time, saying the whole ordeal wasn't on him ... but on the production company and the medical subcontractor. Last we heard, in 2023, the Hansen family was gunning for legal and medical payments linked to Mavar's injuries.

Meanwhile, post-"Deadliest Catch," Nick has reportedly been spotted in and around Alaska's Bristol Bay of late, captaining his own salmon boat and minding his business. He was 59.