(CNN) U.S. authorities are investigating the disappearance of an American marine biologist who went missing at sea while serving as a monitor on a fishing boat off the coast of Peru.

Keith Davis, 41, was aboard the Panamanian flagged Victoria No. 168 to collect data and ensure the crew was adhering to international fishing guidelines. He was reported missing Sept. 10 when the boat was about 500 miles offshore.

Detectives from the U.S. Coast Guard Investigative Service and FBI agents were waiting for the Victoria when it docked in Vacamonte, Panama on Sunday evening, said Michael Berkow, director of the investigative service.

Berkow said the U.S. investigators are assisting with the probe at the request of Panamanian officials.

He said there is very little preliminary information surrounding the circumstances of Davis' disappearance.

"He's just gone," Berkow said.

Friends, family and co-workers described Davis as a big-hearted, upbeat adventurer who was passionate about the ocean and conservation. They said he was also a seasoned sailor who, over nearly two decades as an observer, spent time on boats under all conditions in oceans around the world.

They have expressed skepticism that he was the victim of an accident and have been pressing for an investigation since learning of his disappearance.

"He's not the type of guy who's just going to fall off a boat," said his cousin, Melanie Fletcher, who has been serving as a family spokesperson. "He's the type of guy who watches to make sure no one else does. He's a very smart, resourceful, MacGyver-type guy."

Armed with binoculars, laptops and video cameras, observers embed on fishing vessels to record scientific data about fish being caught, document interactions with other marine life, such as whales and turtles, and ensure that fishermen follow applicable rules regarding which fish they can catch and the methods they can use to catch them. They have no enforcement powers, but they are required to document any violations they see and their reports are provided to law enforcement.

That sometimes has negative results for fishermen. Last year, a commercial fisherman from Hawaii was convicted of gross negligence after an observer secretly recorded him sleeping night after night when he was supposed to be in control of the boat.

Observers are routinely harassed and threatened by fisherman -- and sometimes even assaulted, said Alfred "Bubba" Cook with the Association for Professional Observers.

"Every observer who has been doing it for very long has a story," Cook said via email from Guam where he was traveling on observer business.

Davis had been aboard the Victoria 168 for about three weeks, his family said. The Victoria is a transshipment vessel, which collects the catches of smaller deep sea fishing boats operating in the area, then hauls them to shore.

Crew members discovered Davis was missing when they needed him to sign off on a transfer of fish onto the Victoria and he was nowhere to be found, family members said they were told by authorities.

Fletcher, the family spokesperson, said relatives have been given shifting accounts of what happened.

"First we were told he fell and hit his head. Then were told 'we don't know'," she said.

Family members have created a Facebook page to keep Davis' loved ones abreast of what little information there is to share.

The Facebook page features a video of Davis playing his beloved ukulele on board a different boat some time ago and singing about the fates of several fellow observers who had apparently died.

Jeff Fletcher, also a cousin, said emails have been pouring in from around the globe, asking about Davis and wishing the family well.

"I'm getting emails from people in parts of the world I didn't even know existed," Fletcher said. "He made friends everywhere he went."

He said Davis was more like a brother to him than a cousin. He was trying to remain hopeful, he said, but that was growing more difficult with each passing day.

"I would almost rather know he was gone," Fletcher said. "The wondering is torturous."