"Swamp People" star Mitchell Guist was testing out a new houseboat he was building at the time of his death ... a dream project he had worked on for months ... and now one of his co-stars has vowed to finish the job as a last tribute to the fallen reality star.

The man behind the promise is Glenn Guist -- Mitchell's bearded brother and best friend -- who is often featured hunting critters with Mitchell on "Swamp People."

Glenn's manager tells TMZ ... he's pretty broken up about the sudden unexpected death of his brother ... but he's doing better then they all expected, considering how close they were.

We're told Glenn knew how passionate Mitchell was about the houseboat project ... and believes completing the project will be the best way to honor his brother's memory.

As we previously reported, Mitchell passed away Monday after apparently suffering a seizure while on the bayou in Louisiana.

A funeral service is set for this week at a local church -- and Glenn's rep say he will begin work on the boat "after everything is taken care of with Mitchell."

10:19 AM PDT -- Law enforcement sources tell us ... Mitchell was out in his boat along the Belle River ... when he "fell." Witnesses say he appeared to have been suffering from a seizure.

We're told another person on the boat returned the vessel to the dock and called 911. Emergency personnel responded to the scene and transported Mitchell to a nearby hospital ... but it was too late.

12:15 PM PDT -- A rep for the History Channel tells us, "We are extremely saddened to report that our friend and beloved member of the Swamp People family, Mitchell Guist, has passed away earlier today. Mitchell passed on the swamp, doing what he love."

4:11 PM PDT -- The 911 call, placed moments after Mitchell Guist fell, was just released to a local news station. According to the caller, Mitchell had a seizure then went into cardiac arrest.



Mitchell Guist -- one of the bearded alligator-hunting brothers featured on the reality show "Swamp People" -- has died.

According to WBRZ.com, Mitchell was discovered at 9 AM in St. Martin Parish, Louisiana.

Law enforcement sources tell us ... someone tried to perform CPR on Guist ... but attempts to resuscitate the reality star were unsuccessful.


Sources close to Mitchell tell us ... Guist had recently fallen ill ... but the nature of his illness is still unclear.

Story developing ...