US officials say they will launch an investigation after an alarming number of gray whales have washed up on the west coast of North America, from Alaska down to Mexico.

Since the first of this year, around 70 whales have been found dead in what the National Oceanic and Atmospheric Administration (NOAA) has declared an unusual mortality event (UME), defined as an unexpected, significant die-off event demanding an immediate response. The actual number is expected to be higher as many whales sink to the ocean floor.

“We simply don’t have that many examples of gray whale strandings like this in the past,” Elliott Hazen, a NOAA research ecologist told IFLScience, adding that the last time such a mortality event occurred was two decades ago following a “very strong” El Niño event.

“We’re in a similar situation here, where 2015 was a very strong El Niño. The logical assumption is that the most likely cause is connected to that.”

Though El Nino events are naturally variable, Hazen noted that climate change is likely playing a role. “The other factor that is hard to ignore is that we are seeing record low sea ice in the Arctic right now where the gray whales feed in addition to extremely early melting times of the sea ice."

Gray whales have one of the most unique and longest migrations of any species, opting to spend their winter months in the lagoons of Baja and heading north in the summer to their Alaskan Arctic feeding grounds. In just a few short months, the baleen feeders must consume and reserve enough fat stores to last them until the following season.

“In the case of gray whales, most of the strandings have been malnourished whales, which suggests a lack of food, presumably in the Arctic, is most likely to blame,” said Hazen, adding that shifts in ice melt are changing how food is distributed in the Arctic ecosystems as well as shifting the quantity and quality of food all marine mammals have access to.

Dr Shawn Johnson, director of veterinary science at California’s Marine Mammal Center, told IFLScience that many of the malnourished or dying whales have come into the San Francisco Bay, sometimes spending weeks there. It’s extremely unusual behavior and may speak to their lack of food access.

“We think most likely the whales were potentially coming in and looking for some food in our estuary – there were some observations of them looking like they were trying to forage the muddy bottoms. The other is that they were just so malnourished and tired that they wanted to come into a protected area,” he said, adding that this increases their risk of being hit by ships, entangled by gear, or being disrupted by oceanic noise pollution.

The marine ecosystem in the Pacific Ocean changed dramatically following the 1999 El Niño, an event that has had a ripple effect for the last 20 years, allowing researchers to monitor and document how such occurrences impact marine animals across the food web. It’s possible that we are only now beginning to see the effects of the 2015 El Niño that was followed by the unexplained warm water “blob” event. This caused disruptions in coastal waters up and down the continent, bringing tropical species north and stranding thousands of marine animals. The blob also resulted in ecological shifts like increased red tides, Hazen added.

“We used to think of El Niño as being a canonical thing. We now know that it comes in all shapes and sizes. The El Niño following the blob had a very different ocean and ecosystem signal,” said Hazen. “One thing we might be seeing is the new normal – these top predators as sentinels are telling us that the environment is changing in pretty significant ways.”

Large marine mammals like gray whales serve as a canary in the coal mine, allowing scientists to see what might be happening in the world’s oceans at a much smaller scale. When an individual or population is having problems, scientists suspect the environment they live in is under stress. Johnson added that similar trends have been observed in other species over the last few years, such as the unusual number of deaths in California's Guadalupe fur seals recently.

“Each species has their own challenges and it’s all based on what their niche is and what they eat,” said Johnson, adding that these mass mortality events are often a combination of factors.

Gray whales were nearly decimated by whaling, but have made an “amazing” recovery to nearly 26,000 individuals today following their designation as an endangered species in 1994. Even so, experts are quick to note that a stable population will not be able to withstand rapid ecological changes currently seen in the Arctic.