Marine scientists in the United States say the death of a ninth grey whale on beaches around San Francisco this year is a "cause for serious concern".

Key points: Grey whales have the longest yearly migration of any whale, moving from Alaska to northern Mexico

Grey whales have the longest yearly migration of any whale, moving from Alaska to northern Mexico But they have been dying of malnutrition and boat strikes while stopping in the San Francisco area

But they have been dying of malnutrition and boat strikes while stopping in the San Francisco area The have also been 13 whales found dead along the Washington State coastline this year

The female whale washed ashore at Ocean Beach on Monday morning (local time) and people could be seen standing on top of it and taking photographs.

The whale appeared to have been hit by a boat, according to Padraig Duignan, the chief research pathologist at the Marine Mammal Centre who performed the necropsy.

"It looks very much like a blunt force trauma injury that took her life," Dr Duignan told the Washington Post.

Dr Duignan said the whale was not in good health either way.

"She was in poor body condition, [with a] depleted blubber layer and no internal body fat relative to what an adult female should have at this time of year in this area."

A necropsy revealed the whale had probably been hit by a boat. ( AP: Jeff Chiu )

That would make it the fifth whale this year to die of a ship strike, with three more having died of malnutrition.

The cause of death of another whale, which washed ashore on April 30, is not yet known.

Dr Duignan said the whale deaths were the continuation of a wider trend that has researchers worried.

"It is very unusual. A normal year for us, we have maybe between one and three [dead] grey whales in this season," he told the Washington Post.

"This is triple that already, in less than a two-month period."

People stood on top of the whale as it lay on Ocean Beach, San Francisco. ( AP: Jeff Chiu )

A further 13 grey whales have been found dead further north, along the Washington State coast, according to biologist John Calambokidis.

The Marine Mammal Centre said grey whales are a common sight along the California coast as they make their migration between Alaska and Baja California in northern Mexico.

Their 17,000-kilometre journey is the longest yearly migration of any whale species.