Roughly 20 percent of the humpback whale calves born in Hawaiian waters each winter don't survive to reach their summer feeding grounds off Alaska, said Dave Matilla, science and rescue coordinator for the Hawaiian Islands Humpback Whale National Marine Sanctuary.

Matilla said most of the young whale deaths are from natural causes, such as being too small, weak or sickly, or from being injured by sharks.

"There's a fair amount of natural mortality" among the 500 or so humpbacks born each season in Hawaiian waters, he said. Only about 400 of those eventually show up at the feeding grounds, he said. He called the estimates "very rough numbers."

Matilla said it was unusual that the entangled whale had debris caught in its mouth, but it's perhaps not strange that some get entangled, given the presence of marine debris in the North Pacific.

"Humpbacks are very curious about things. They'll poke around, and I've even seen them lift a log with their heads," he said.

Photographer Ken Held, who regularly goes out with boats from the Pacific Whale Foundation, caught images of the propeller-damaged calf on Monday, about a mile and a half off the coast of Puamana, south of Lahaina. It was photographed from the same boat, skippered by Chris Nesbitt, that spotted the entangled whale.

The animal appeared to have two sets of deep slices on its back, as if it had been hit twice or run over by a boat with twin propellers.

"I don't think I've seen a calf with those kinds of wounds survive," Matilla said.

Whale researcher Mark Ferrari of the Center for Whale Studies saw the second injured calf off Lahaina. Most of its tail flukes were gone. It appeared able to feebly swim on its own, but its mother would regularly lift it with her head, where it could get a free ride in her slipstream as she swam, Ferrari said.

"The mom was doing the best she could. She would behave differently when sharks were around" and keep the calf close, he said.

With the mother humpback's support, the calf could live and grow for an extended period of time, Ferrari said. But he doubted the animal could survive independently as an adult, given the extent of its injuries.

Whale researchers say the population of humpbacks in the Islands appears to be on the rise, and the growth in numbers should be a caution to boaters.

"As the population increases and there are more and more animals out there, anybody on the water during whale season needs to know that they are there and to keep a good lookout for them," Matilla said.

Calves are particularly vulnerable, both because they often have no visible spout and because they sometimes pop to the surface alone to breathe while their mothers rest 60 or so feet below. While resting mothers need to take a breath only a few times every hour, calves can hold their breath only four minutes or so, he said.