PACIFICA (CBS SF) — Boaters were being warned to watch out for a large dead whale found floating off the Pacifica Coast near shipping channels.

“We found this Humpback Whale, floating, rather bloated,” boater Tom Mattusch said.

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The whale was found just a few miles off the coast, swollen to the size of a small house.

The fishing vessel Hulli Cat was returning from a Farallon bird watching trip when Mattusch spotted it.

“I’ve been coming out on the ocean since 1967, and this is the first bloated, floating whale that large I’ve ever seen,” Mattusch said.

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An increased number of dead whales have been washing onto Bay Area shores.  Another dead whale washed onto the beach in Pacifica Sunday.

“We’ve had reports over the last two weeks that there have been large numbers of Humpback Whales and Blue Whales seen feeding off the Farallon Islands,” Moe Flannery of the California Academy of Sciences said.

Mattusch alerted the Coast Guard of the whale’s presence.

“It was not a small whale, so it’s good that it was reported to us, because it could have caused problems if it had a collision with a vessel,” David Flores of the U.S. Coast Guard said.

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Researchers say they don’t know why this whale died.