(NEXSTAR) — Gray whales are continuing to wash up on Pacific Coast beaches in what officials are calling an “unusual mortality event.”

As of December 2020, an estimated 386 gray whales were stranded on the coast from Mexico to Canada, according to NOAA data released recently. The numbers signify a trend over the past two years that’s seen the massive creatures die-off in troubling numbers.

By comparison, in 2019, 214 whales were found stranded.

The number of dead whales is likely much higher, as only about 3.9 to 13 percent of gray whales that die in a year are found on land, observed and reported.

The numbers are troubling to scientists who have yet to definitively determine the cause of the mortality event.

As defined under the Marine Mammal Protection Act, an unusual mortality event is a “stranding event that is unexpected, involves a significant die-off of any marine mammal population, and demands immediate response.”

Researchers aren’t entirely sure what’s causing the mass die-offs, but necropsies have shown that several of the dead whales were emaciated with heavy loads of whale lice — external parasites sometimes found on cetaceans, such as whales, dolphins or porpoises.

NOAA has formed an investigative team to hopefully find some answers.

Once near extinction, the gray whale population is currently stable. According to a 2015 to 2016 NOAA survey, the population was hovering around 27,000.

This isn’t the first time gray whales have experienced an unusual mortality event. Scientists reported mass die-offs between 1999 and 2000, with more than 650 animals stranded along the West Coast of North America during that time.

The cause of that event is related to “oceanographic factors that limited food availability” for the whales.