Sydney - Mercy requires that an orphaned whale calf that has touched the hearts of Australians be put down if the latest rescue attempt for 2-week-old Colin fails, marine scientists said Wednesday.

They said Colin has likely been rejected by its mother and has not fed since Friday.

Earlier this week the baby humpback was spotted nuzzling a yacht anchored off a Sydney beach. It apparently had mistaken the yacht for its mother and was hoping for a feed.

Colin stayed alongside when the yacht was towed out to sea. It was hoped that either its mother or another lactating female would respond to its calls.

But when yachtsman Peter Lewis woke from sleep to the noise of Colin suckling the hull of his boat, wildlife officers knew their ploy had failed.

"It sounds like someone with a great vacuum cleaner trying to suck the bottom of the boat," Lewis told national broadcaster ABC. "It's suffering and I certainly wouldn't want to see it starve to death."

National Parks and Wildlife Service spokesman John Dengate said taking Colin into captivity was not an option and a mercy killing was being considered. (dpa)