Pacific gray whales are showing up dead in North America’s oceans and shores at a rate four-times greater than typical. As of July 11, 182 gray whales have been found dead or beached this year in Mexico, the US, and Canada according to the US National Oceanic and Atmospheric Administration (NOAA).

While that might seem like a relatively small figure for a species that numbers in the tens of thousands, the dead whales that are spotted by mariners or found on ashore only represent roughly 10% of total deaths: 90% of whales simply sink to the ocean floor when they die, according to Jeffrey Boehm, CEO of the Marine Mammal Institute.

The gray whale migrates from the Arctic to Baja California in Mexico during the winter before migrating back north in the summer. The whales were removed from the US endangered species list in 1994. Nearly 22 years after the Marine Mammal Protection Act made it a federal offense to harm or kill any marine mammal. Researchers estimate the population has recovered to roughly 27,000 whales, levels not seen since before the peak of whale hunting in North America in the 19th century.

NOAA is investigating this as an “unusual mortality event,” and so far, the cause of the deaths is inconclusive. Pádraig Duignan, chief research pathologist at the Marine Mammal Center, told Quartz there are several theories that rely on a combination of a lack of food, climate change, and ship strikes.

The overpopulation theory

Before their journey south, gray whales feed on sea bugs at the bottom of the ocean in what is known as the benthic zone. They consume around one ton of these critters each day until they’ve consumed enough to get them to Mexico. Once there, they’ll fill up again.

Examinations of the whales found in the San Francisco Bay area this year revealed that half of the 12 dead grays died due to starvation. “There’s been a rise in malnourished strandings,” said Duignan, who conducts examinations of dead whales at the Marine Mammal Center in Sausalito, California. Scientists agree their appearance along the coasts—and in particular the Bay area—is due to whales trying find more food during their migration.

If they’re stopping for food, there’s a concern for the quality of the whale’s food supply in the Arctic Circle. With the gray whales reaching their pre-hunting population size, their habitat’s resources could be stretched thin and be incapable of providing them the sustenance they need, said Bill Keener at a Marine Mammal Center event.

The lack of food theory

Another theory is that their food source is dying off due to retreating Arctic sea ice. Ocean temperatures in the Arctic Circle shrinks the area of the Bering sea ice. There was virtually no ice cover in 2018. Warmer temperatures and retreating ice sheets harm benthic habitats and the creatures within (PDF) that the gray whales eat.

If the whale’s food source is endangered by climate change, researchers would expect to find more stranded and emaciated whales during their next migrations.

The ship theory

Ships are still the greatest threat to migrating sea mammals. Gray whales don’t echolocate like orcas and narwhals do. That means barges coming in and out of harbors and large boats can surprise them. Half of the dead whales found by the Marine Mammal Center in 2019 died from ship strikes. “We sometimes see a few human interaction deaths each year,” said Duignan, “but this year we’ve seen a lot of deaths from ship striking that we haven’t seen in previous years.”

These collisions typically happen when ship traffic out of bays and harbors coincide with whale traffic. Because these whales are struggling to feed, they may be swimming closer to the coast looking for food. That puts them at a higher risk of encountering humans, said Duignan.

The extent of the whale deaths remains a mystery. Duignan reflected that our knowledge of these mammal deaths peak around shipping harbors and populated areas because of the resources available to recover and study the stranded animals. “Information that comes out of this investigation will come out of what minimal cover is available to scientists.”