Four dead gray whales have washed ashore San Francisco Bay Area beaches in the last nine days, with experts saying Friday one was struck by a ship. They were trying to determine how the other three died.

“It’s alarming to respond to four dead gray whales in just over a week because it really puts into perspective the current challenges faced by this species,” says Dr. Pádraig Duignan, Director of Pathology at The Marine Mammal Center.

The carcass of a 41-foot adult female gray whale landed at San Francisco’s Crissy Field on March 31. A second adult female was found last Saturday in Moss Beach in San Mateo County. A third was found Wednesday floating near the Berkeley Marina and the following day one washed up in Marin County’s Muir Beach.

The whales migrate 10,000 miles to winter off Mexico’s waters, where they mate and birth calves near the coast of Baja California. They head back north and stay off the coast of California in spring and summer to feed on anchovies, sardines and krill before continuing on their northerly migration to cool, food-rich Arctic waters.

In 2019, at least 13 dead whales washed ashore in the Bay Area and scientists said they feared it was because the animals were starving and couldn’t complete their annual migration from Mexico to Alaska. Biologists have observed gray whales in poor body condition during their annual migration since 2019, when an “unusual mortality event” was declared by the National Oceanic and Atmospheric Administration.

Malnutrition, entanglement in fishing gear, and trauma from ship strikes have been the most common causes of death found by the center’s research team in recent years.

A necropsy of the whale found at Muir Beach revealed significant bruising and hemorrhaging to muscle around the whale’s jaw and neck vertebrae consistent with blunt force trauma due to ship strike. But experts noted the whale was in good body condition based on the blubber layer and internal fat levels, the center said.

Experts haven’t determined how the other three whales died or if starvation was behind their deaths.

Nearly one in four gray whales migrating along the U.S. West Coast has died since the last recorded population assessment in 2015 and 2016, according to NOAA.

“This many dead whales in a week is shocking, especially because these animals are the tip of the iceberg,” said Kristen Monsell, legal director of the Center for Biological Diversity’s Oceans program.

Experts estimate the washed-up whales represent just 10 percent of the total number of the dead, with the rest sinking into the sea unnoticed by humans.

Monsell said California lawmakers need to require fishing gear that doesn’t use rope, and federal regulators should set mandatory speed limits for ships.

“Ship strikes and fishing gear entanglements kill many whales that we never see,” she said.

Her organization is suing the federal government to get speed limits in shipping lanes off California, Monsell added.