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 ...