A 50-foot emaciated whale lumbered off the southwest coast of Kauai on Monday with its skin nearly all white and blotchy — feasted on by amphipods — and its back apparently broken, flight instructor Gerry Charlebois said yesterday.

"It was absolutely disturbing to see this poor animal," said Charlebois, who has experienced many humpback whale sightings. "The poor thing was bent in half."

The Hawaiian Islands Humpback Whale National Marine Sanctuary and the National Oceanic and Atmospheric Administration Marine Fisheries Office are on standby awaiting information on the whereabouts of the distressed whale, last seen at about 1 p.m. Monday swimming west toward Waimea.

"Coast Guard, fishermen, they are our eyes out there," said Ed Lyman, marine mammal response manager of the National Marine Sanctuary on Maui.

At about 9:30 a.m. Monday, Charlebois said he and two other instructors were descending toward Port Allen Airport following a flight training session when they spotted the whale about 100 feet off the airport traveling near the water’s surface. "He was labored and slow," he said, adding the fluke (tail) was not moving, only its flippers.

Charlebois took photos and immediately contacted Lyman.

The whale was not diving into the ocean or blowing water from its spout, Lyman said. Amphipods were feasting on its skin, another indicator the whale was in bad condition, said Lyman based on a visual assessment of the photo sent by Charlebois.

The cause of the whale’s condition is unknown, he said, noting a necropsy would be necessary to determine the exact cause of its abnormal appearance. The distorted area between the mammal’s dorsal fin and the fluke or tail stock could be caused by an early deformity that worsened as the whale aged or blunt force trauma caused by impact possibly by a vessel, said Lyman.

The mammal’s mobility is impaired as the fluke serves as the whale’s propeller and the tail stock, its engine. "It’s the powerhouse," he said.

Anyone with information on the location of the distressed whale is urged to call the National Oceanic and Atmospheric Administration fisheries hot line at 888-256-9840.