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Two dead gray whales — one that died of severe malnutrition and the other of undetermined causes — were found floating in the San Francisco Bay this week, launching a scientific investigation into why.
Experts from the Marine Mammal Center and its partners at the California Academy of Sciences conducted necropsies on the whales, but were unable to perform a full examination on the second whale because of unsafe conditions. Both whales are believed to have been female, about 1 year old and 23 feet long.
The necropsies — animal versions of an autopsy — were conducted on one whale on the shores of Angel Island on Tuesday. Experts determined that the whale had suffered from severe malnutrition. They found a significant lack of blubber and body fat on the whale, which is a common sign of malnutrition, and the whale’s stomach was empty.
The second whale appeared to have sufficient blubber reserves; scientists plan to revisit the carcass another time to gather more evidence and attempt to determine a cause of death.
Although common causes for deaths in cetaceans range from blunt force trauma from ship strikes to malnutrition, trauma and entanglements, there was no evidence of trauma or infectious disease in either whale.
Officials from the Marine Mammal Center collected tissue and blubber samples on both whales to contribute to various research studies, and to submit for further testing.
Gray whales make the longest migration of any cetaceans, traveling 11,000 miles from feeding grounds in Alaska to the warmer waters off Mexico’s Baja California and back again. They are one of the most frequently sighted whales in California, passing along the coast in December and January during their southern migration, and again in April and May on their northern journey.
In recent years, biologists have found younger gray whales in poor condition during the migrations, said Dr. Padraig Duignan, chief research pathologist at the center, which could have contributed to the first whale’s death.
“It’s likely that after not feeding this winter, she didn’t have enough reserves built up to survive her journey north,” Duignan said.
Experts have also noticed a migratory behavior change with gray whales entering the San Francisco Bay in the late winter and early spring months. Historically, one or two gray whales have temporarily passed beneath the Golden Gate Bridge during this time period, but last week, experts counted five entering the bay.
“The number of gray whales entering the San Francisco Bay this year has been abnormally high, and they’re staying much longer than in years past,” said Bill Keener, a whale expert at Golden Gate Cetacean Research. “There’s likely a few factors at play here, including food source availability and a relatively sheltered habitat for juvenile whales that are in weaker body condition.”
In its 44-year history, the Marine Mammal Center has responded to more than 70 gray whales in distress. These are the first two whale necropsies the center has completed this year. In 2018, the center responded to five gray whale strandings, including three in San Francisco Bay.
These dead whales were first seen Sunday and Monday, with one floating between Tiburon and Angel Island near Raccoon Strait. The carcass became stranded on the shoreline near Belvedere Cove late Sunday. A second report was received Monday morning from the U.S. Army Corps of Engineers near the Bay Bridge. The Army Corps and a marine salvage company towed the carcasses to Angel Island on Monday afternoon.
Officials at Angel Island State Park authorized a landing area so the experts could perform the necropsy.
“Each of these investigations provide an invaluable opportunity to better understand the threats that marine mammals face,” Duignan said. “The gray whale population along the U.S. West Coast is a conservation success story, but the species continues to face numerous environmental threats including entanglements, ship strikes and shifting food availability.”
