An 83-year-old Nantucket cranberry grower died Saturday after being found chest deep in mud and suffering from hypothermia less than 24 hours after he was reported missing to police on the island, authorities said.

Thomas Larrabee Sr. was found alive, with his two springer spaniels, by a retired firefighter who rode his dirt bike through a marshy area near Hummock Pond.

But he later died at Nantucket Cottage Hospital, where he was taken after being found around noon, according to Nantucket Police.

Larrabee’s son, Thomas Jr., said he believes his father, a passionate collector of Native American artifacts, was out looking for arrowheads Friday afternoon when he apparently got stuck in the marshy land.

“He loved everything outdoors,’’ Larrabee said by telephone Saturday night. “He passed away doing what loved the most, arrowheading. We should all be that lucky.’’

His family reported him missing at 8:30 p.m. Friday. He was last seen walking with his two springer spaniels in the area of Barrett Farm Road around 3 p.m. Friday, police said.

A search by Nantucket police, fire, the State Police Air Wing, and the Coast Guard started Friday and continued into Saturday, authorities said.

But Larrabee was found by resident Shawn Monaco, who jumped on his dirt bike to launch his own unofficial search.

Footprints and animal paws on the ground led him to Larrabee, Monaco said.

“I was kind of taken aback,’’ Monaco said after he spotted Larrabee, a longtime friend

“It didn’t really register at first. But I was excited, especially when he moved. I was very thankful for that.’’

The spaniels stayed with Larrabee all night — one on the shore, and one in the mud, Monaco said.

Monaco said he called State Police, who arrived with the fire department to pull Larrabee from the mud.

“The whole island community really comes together when these things happen,’’ he said.

Larrabee Sr. was a cranberry grower for 65 years at the Nantucket Conservation Foundation, where he began working in 1951, before becoming bog manager in 1959.

“We are all heartbroken at Tom’s passing,’’ said Danielle O’Dell, an ecologist with the Foundation.

About four years ago, he turned over the manager position to his son.

“He was my best friend, my dad, my mentor,’’ Larrabee Jr. said. “He was very well loved by everybody in life he touched. He had no enemies.’’

Alexandra Koktsidis can be reached at alexandra.koktsidis @globe.com.