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Three more dead whales have been spotted in the San Francisco Bay in recent days, bringing the total to eight this year, experts said Friday.

The gray whales were found washed ashore on April 27 in Tiburon, on Monday at the Port of Oakland and on Tuesday at Angel Island State Park north of San Francisco, according to The Marine Mammal Center.

The Tiburon whale, which later drifted to the Golden Gate National Recreation Area, had earlier been spotted alive but underweight. The animal appeared to have spent 47 days exploring the bay before dying, according to the mammal center.

Tissue samples were taken from the three whales but scientists couldn’t perform necropsies to determine how the whales died because the carcasses were in locations that were unsafe, inaccessible or had shifting tides, the center said.

Four other gray whales have been found dead in the Bay Area since early April, along with one fin whale. A pygmy sperm whale was found in February at a Sonoma County beach.

“Malnutrition, entanglement and trauma from ship strikes have been the most common causes of death in whales studied by the center’s research team in recent years,” the center said.

There are about 20,000 gray whales migrating from Baja California in Mexico to summer Arctic feeding grounds near Alaska, experts said.

In recent years the migrating whales appear to be making longer stopovers in the Bay Area, said Kathi George, director of field operations for the mammal center.

“These whales are at increased risk from human activity,” she said. “Which is why we are committed to better understanding the ongoing challenges and threats these animals face so we can safely share the ocean and bay with them.”

Five dead gray whales were found in the Bay Area from March through May of last year while 13 were found in 2019, the center said.

Since 2019, the U.S. National Oceanic and Atmospheric Administration has been investigating what it terms an unusual mortality event because of higher numbers of strandings of gray whales in poor body condition across their range of migration, the Mammal Center said.