The deaths of dozens of grey whales washed ashore in recent months along the US West Coast are to be investigated by a team of US government biologist.

Declaring the spike in deaths an “unusual mortality event”, the National Oceanic and Atmospheric Administration (NOAA) said designation the deaths required greater scrutiny and allocation of more resources to determine the cause.

At least 70 dead grey whales have turned up on the US West Coast in 2019, many of them emaciated. The number is the highest recorded since 2000.

So far, 37 dead grey whales have turned up in California waters, three in Oregon, 25 in Washington state and five in Alaska, according to officials of NOAA’s National Marine Fisheries Service.

The total of dead whales documented probably represents a small fraction of those that have perished, scientists say.

Tragic photos show beached whales Show all 15 1 /15 Tragic photos show beached whales Tragic photos show beached whales A dead sperm whale lies on Hunstanton beach in Norfolk on 5 February 2016 AFP/Getty Tragic photos show beached whales Volunteers pour buckets of water over the 80 remaining live pilot whales found stranded on remote Ocean Beach on New Zealand's southern-most Stewart island, 8 January 2003 AFP/Getty Tragic photos show beached whales Contractors clear away the body of one of the dead 48ft sperm whales that were washed-up on a beach near Gibraltar Point in Skegness, Lincolnshire in 2016 PA Tragic photos show beached whales People pass by a beached whale at the Pointe de la Torche, near Brest in France on 29 November 2011 AFP/Getty Tragic photos show beached whales A woman touches the tail of a large whale carcass on Wattamola Beach at the Royal National Park in Sydney on 25 September 2018 AFP/Getty Tragic photos show beached whales Beached humpback whale in California, 2015 AFP/Getty Tragic photos show beached whales Dead long fin pilot whales at Hamelin Bay on Australia's west coast on 23 March 2009 AFP/Getty Tragic photos show beached whales A 36ft sperm whale lies dead on the beach at Sutton Bridge, in The Wash, off the Lincolnshire coast, where it became stranded in 2004 PA Tragic photos show beached whales A female fin whale opens its mouth as it lies stranded and alive on the beach at Carlyon Bay, Cornwall on 13 August 2012 Getty Tragic photos show beached whales The lower jaw of a dead sperm whale that stranded itself on a beach in Hunstanton, Norfolk on 5 February 2016 Getty Tragic photos show beached whales One of the five sperm whales that were found washed ashore on beaches near Skegness, Lincolnshire over the weekend on 25 January 2016 Getty Tragic photos show beached whales Employees at work to skin the remains of a beached 60ft whale on 25 January 2013 AFP/Getty Tragic photos show beached whales Two long-finned pilot whales are stranded on a beach in the northern French city of Calais on 2 November 2015 AFP/Getty Tragic photos show beached whales A sperm whale lies dead after becoming stranded on a beach in Hunstanton, Norfolk on 5 February 2016 Getty Tragic photos show beached whales Crowds gather as a sperm whale lies dead after becoming stranded on a beach in Hunstanton, Norfolk on 5 February 2016 Getty

The most recent dead whale in Alaska was spotted last week near Chignik Bay.

Many of the stranded whales have been found with little body fat, leading experts to suspect that the dramatically warming waters of the northern seas may have depleted the food stock the whales rely on, causing the die-off.

The grey whales summer in the Bering Sea and Chukchi Sea off Alaska, consuming most of a year’s worth of nourishment to pack on the blubber they need to carry them south to wintering grounds off the coast of Mexico and back north to feeding grounds off Alaska.

Sea ice has been at or near record lows in the Bering and Chukchi, and water temperatures have been persistently much higher than normal.

Scientists say this is an apparent consequence of human-caused climate change.

The conditions the whales encountered last summer could be hurting the animals now, as they make their annual migration north, said scientists assembled by NOAA for a teleconference on Friday.

“The Arctic is changing very, very quickly, and the whales are going to have to adjust to that,” Sue Moore, a University of Washington oceanographer, told reporters.

Another theory is that the number of whales has reached the limits of the environment’s natural capacity to sustain further population growth, scientists said on the call.

The current estimated population of eastern North Pacific grey whales is about 27,000, the highest recorded by the agency since it began grey whale surveys in 1967, said biologist David Weller.

The deaths could be caused by a combination of factors, as in other die-offs, the scientists said.

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However, it is known that some whales have ventured into new places in their search for food, facing higher risks of collisions with ships.

“We are seeing lots of live grey whales in unusual areas, some of them clearly emaciated, trying to feed,” John Calambokidis, a biologist and grey whale expert with the Cascadia Research Collective.

More dead whales are expected to wash ashore during the northward migration, the scientists said.

The last major West Coast grey whale die-off, in 1999 and 2000, was believed to have been related to an ocean-warming El Nino event. It also triggered an unusual mortality event declaration.