So far this year, 70 gray whales have washed up on beaches along the west coast from California to Alaska, enough that last Friday the National Oceanic and Atmospheric Administration declared an “unusual mortality event” and launched an investigation into why the whales are dying.

Reuters reports that thus far 37 dead whales have been found in California, three in Oregon, 25 in Washington, five in Alaska and an additional five along the coast of British Columbia.

The Associated Press reports that many living whales are appearing in unusual places they usually don’t visit during migration, like Puget Sound and San Francisco Bay, likely searching for food, which puts them in even more danger. Four whales found in California were struck by ships near San Francisco.

Each spring, the whales migrate about 5,000 miles from their birthing grounds in Mexico to their feeding grounds in Alaska. The whales only feed while in Arctic waters, feasting on small crustaceans called amphipods. They have to fuel up enough to survive their entire 10,000-mile round trip migration route, reports Kate Williams at The Oregonian.

If they don’t pack on the blubber while in Alaska, they won’t have enough energy to complete their journey down south and back again. That seems to be the case with the majority of whales examined so far. Most of the deceased animals are emaciated with very little body fat. It’s likely that the 70 whales are are just a fraction of the whales that have died on the migratory journey so far this spring, since most whales actually sink to the ocean floor when they die.

So far, researchers have two main theories as to why the animals are starving. One is that the North Pacific gray whale population has essentially reached the carrying capacity of its environment, or the level that food resources can sustain. It’s possible there were too many whales and too few amphipods for the entire population to get its share during last summer’s feeding frenzy.

“Keep in mind that carrying capacity is not a hard ceiling, but that it’s a shifting threshold,” NOAA biologist David Weller tells Reuters. “In some years or period of years the environment is capable of supporting more whales than in other years.”

Nat Herz at Alaska Public Media reports that warming trends in the Arctic could also be at play. Reduced sea ice may be impacting the amphipod population, reducing the food resources available for the whales.

“We have to really be on top of: Is there any relationship to climate change? And does this link to any other factors that might be affecting other species as well?” research biologist John Calambokidis at Washington-based Cascadia Research Collective tells Herz. “Could gray whales be an early warning sign of other things that we need to be watchful for?”

Last summer, waters in the Bering Sea in particular were nine degrees warmer than normal, which could have impacted the whales’ feeding. NOAA, which surveys the whales through the feeding season, is going through its records from last summer to see if there’s anything unusual they missed. This year, they will also pay close attention to the feeding season to determine if there are more whales competing for fewer resources. It’s also possible that the food is not as nutritious as it usually is.

Julia Jacobo at ABC News reports that researchers suspect the amphipods the whales prefer, which live in the sediment on the bottom of the sea, are fertilized by algae associated with the sea ice. With the ice melting away, the amphipods may be in short supply. Whales may be relying on other food sources like krill, which may not contain the amount of fatty lipids they need to build up their blubbery energy reserves. The AP reports that surveys show changes are happening in the amphipod beds, and that the creatures have moved northward in recent years.

This isn’t the first unusual mortality event among North Pacific gray whales. In 1999 and 2000, a similar die-off occurred with over 100 whales washing ashore, which was triggered by an El Nino event, or cyclical warming of the Pacific. During that die-off, however, ABC’s Jacobo reports the whales weren’t emaciated. Researchers were never able to pinpoint to cause of mortality during that event.

In 1994, the North Pacific gray whale was taken off the endangered species list because of the species’ phenomenal rebound following a moratorium on commercial whaling. Hopefully, this die-off, which is expected to continue, will only be a one-year event. But Alisa Schulman-Janiger, director of the Gray Whale Census and Behavior Project, tells The Oregonian’s Williams that there are early signs there could be trouble in the whole population. Calves born this year are equivalent to only one third of last year’s count. The population has arrived weeks late to the breeding ground, they are skinnier than usual and, of course, there’s a high mortality rate, all of which raise concerns.

NOAA will continue to investigate the mortality event as long as it lasts and is posting updates on a site dedicated to the event.