Marine researchers are trying to determine what killed a 5-month-old monk seal pup found over the weekend in Waimanalo. The male pup, known as RW18, had been living with other seals on Manana Island, or Rabbit Island, and observed by volunteers, said a statement by the National Oceanic and Atmospheric Administration.

The seal appeared to be in "good body condition" when it died, NOAA said today.

The pup was born in May and was monitored by NOAA volunteers during its six-week nursing period. NOAA relocated the seal to Manana Island after it weaned so it would be near adult seals and have a chance to mature away from human disturbance.

NOAA Fisheries conducted a necropsy Sunday and no clear cause of death was found, the agency said.

"Over 40 volunteers watched over the pup from sunup to sundown while it was being reared on the beach by its mother," said David Schofield, NOAA Marine Mammal Response coordinator. "Understandably, a lot of dedicated volunteers are very saddened by the loss. With so few monk seals remaining in the Hawaiian Islands, we can't afford to lose even one."

Anyone who may have information regarding the seal's death may call the NOAA Offic eof Law Enforcement at 800-853-1964 or the state Division of Conservation and Resource Enforcement at 587-0077.