Another gray whale was found dead and washed ashore in northwest of San Francisco and scientists believed it was the 13th whale carcass discovered since March this year, a marine conservation center said Thursday.

The Marine Mammal Center (MMC) confirmed the carcass was spotted at the beach of the Point Reyes National Seashore, a popular Northern California tourist destination on the Pacific coast, about 50 km northwest of San Francisco.

"Based on reports and visual evidence from our partners @PointReyesNPS, experts at The Marine Mammal Center can confirm the whale at Limantour Beach is a gray whale. The age class, sex, and cause of death is unknown," the center said in a tweet.

The MMC based in Sausalito, Northern California, said a necropsy date for the whale is to be set to determine the cause of the mammal's death.

On Monday, a gray whale was found floating about 400 meters off Point Knox Lighthouse on Angel Island, north of San Francisco.

Scientists of the MMC, a private, non-profit US organization established in 1973 to rescue and rehabilitate sick or injured marine mammals, have found 12 dead whales in the San Francisco Bay Area and performed 11 necropsies since March, said the center.

Gray whales migrating up the West Coast from Mexico to the US northwestern state of Alaska, where they spend the summer, are starving and coming into the San Francisco Bay searching for food to fuel the rest of their north-bound journey, researchers said.

Last month, the MMC said some of the whales were killed from injuries of ship strikes during their stay in the highly trafficked Bay Area waters.

Gray whales are one of the most frequently sighted whale species in California. They pass by California in December and January during their southern migration, and again in April and May on their northern journey between Baja California, the northernmost state of Mexico, and Alaska, according to the MMC.