Scientists have figured out why hundreds of dead gray whales are washing up on US shores.

A peculiar pattern of mass deaths of gray whales in the northern Pacific Ocean may have an explanation, with scientists correlating the main die-off occurrences to Arctic sea ice levels.

According to a new study, three mortality episodes have struck gray whales (Eschrichtius robustus) off the coast of North America since the 1980s, lowering the population by 15% to 25% each time.

More than 2,000 gray whales are known to have died in total. The first huge die-off occurred between 1987 and 1989, and it was the most severe, killing at least 700 whales. 

However, because reporting procedures and survey efforts only grew in the 1990s, the number of strandings related with this mortality event may be underestimated, according to the report.

The most recent die-off occurred in 2019 and resulted in the death of more than 70 gray whales in just six months.

It is still going on, and as of September 26, 2023, 688 whales had killed. It was unknown what prompted each of the episodes.

"These are extreme population swings that we did not expect to see in a large, long-lived species like gray whales," said study co-author Joshua Stewart, an assistant professor at Oregon State University's Marine Mammal Institute.

Commercial whaling until the mid-twentieth century nearly wiped off the North Pacific gray whale population, but it has since recovered and now numbers around 14,500 individuals, according to the statement. 

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