RANCHO PALOS VERDES, Calif. -- A boat off the coast of Mexico apparently spotted the blue whale Monday that rescuers first saw several days ago entangled in hundreds of feet of fishing line near Los Angeles.

A blue whale trailing line and a red buoy was seen around 10:30 a.m. about 18 miles southwest of the Coronado Islands, according to Jim Milbury, a spokesman for the U.S. National Oceanic and Atmospheric Administration. Those Islands are off Tijuana, Baja California, and more than 100 miles south of where the whale was last seen Friday.

U.S. rescuers cannot work those waters but have been in contact with their Mexican counterparts, Milbury said.

It was a typical whale watching trip when Harbor Breeze Cruises Capt. Danny Salas saw something extraordinary reports CBS News' Mireya Villarreal.

Get Breaking News Delivered to Your Inbox

"The whale just wasn't acting right," he said. "It looked like it was a little tired. Swimming extremely slow."

They were the first to spot the 80-foot-long blue whale, entangled in a crab pot fishing line several miles off the California coast.

The Coast Guard and others rushed to help.

Peter Wallerstein is with Marine Animal Rescue and tried for hours to sever the line.

"We would have loved to cut it all off and free the whale but sometimes things are impossible and it endangers the rescuers as we're doing it," said Wallerstein. "It could ram us, it could hit us with its tail, it could do some major damage. It's a really, really dangerous situation."

As a last resort, the crew attached a larger buoy to the tangled line to make it easier to spot. But since that happened Friday night the whale had disappeared. Experts worried that the whale will have trouble eating, could get tired faster and die.

"There's not much we can do unless the whale turns north and comes back up," according to Milbury. He said his understanding was that Mexican rescuers from the group RABEN would need the whale to swim farther south to be able to help it. RABEN could not immediately be reached for comment Monday.

Rescuers in Southern California have freed other kinds of whales entangled in fishing gear before, but have not tried to free a blue whale so they do not know how it would react. They have warned boaters not to try and free the whale.

Biologist Diane Alps says saving this blue whale, an endangered species, is especially important.

"There's only about 10,000 left in the world," Alps said. "At one time there were over 300,000 and whale hunting decreased their numbers."

For now, the U.S. Coast Guard, whale watching boats and pilots are keeping a close eye on the ocean.

"Hopefully we'll get more reports and finish the job," said Wallerstein.