Center Experts Investigate Gray Whale Strandings

The Marine Mammal Center investigates two gray whale strandings in San Francisco Bay, confirms severe malnutrition as cause of death for one

Two gray whales stranded under separate circumstances in the San Francisco Bay this week. Experts from The Marine Mammal Center and its partners at California Academy of Sciences confirmed severe malnutrition as the cause of death for one whale, but were unable to determine a cause of death for the second. The need to investigate these mortalities is more important than ever as marine mammals face increasing threats in the environment. 

Each of these investigations provide an invaluable opportunity to better understand the threats that marine mammals face.

“Each of these investigations provide an invaluable opportunity to better understand the threats that marine mammals face,” said Dr. Padraig Duignan, Chief Research Pathologist at The Marine Mammal Center. “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.” 

The two necropsies took place at Angel Island State Park Tuesday morning. The team identified the first whale as a one-year-old, 23-foot female. Upon investigation, scientists discovered a significant lack of blubber and body fat, a clear sign of malnutrition, as well as a lack of content in the animal’s stomach. There was no evidence of trauma or infectious disease. 

Experts were unable to perform a full necropsy on the second whale due to its unsafe location near the surf line. Scientists observed the carcass in a moderate state of decomposition and that it had a decent amount of blubber reserves. The second whale was identified as an approximately one-year-old, 23-foot female. Scientists plan to revisit the carcass at a later date to gather additional evidence and attempt to determine a cause of death. 

While “undetermined” is the most common cause of death of cetaceans reported by the Center’s research team, blunt force trauma from ship strikes, malnutrition, trauma and entanglements are other leading causes. The Center collected tissue samples and blubber samples on both cetaceans to contribute to various research studies and submit for further testing. 

“In recent years, biologists have observed cases of young gray whales in poor body condition migrating south in the fall from their feeding grounds in Alaska to the warmer waters off the coast of Baja California, Mexico,” said Dr. Duignan. “This species faces the longest annual migration of any whale on earth, it’s likely that after not feeding this winter she didn’t have enough reserves built up to survive her journey north.”

Experts have also noticed a migratory behavior change in gray whales entering the San Francisco Bay in the late winter and early spring months. Historically, one or two gray whales would temporarily pass underneath the Golden Gate Bridge during this time period. Last week alone, 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.

Scientists at The Marine Mammal Center have responded to more than 70 gray whales in its 44-year history. These are the first two whale necropsies the Center has completed this year. The Center responded to five gray whale strandings in 2018, including three in San Francisco Bay. 

The Center’s rescue team first received reports from Golden Gate Cetacean Research Sunday morning of a dead cetacean floating between Tiburon and Angel Island near Raccoon Straight. The carcass eventually stranded on the shoreline near Belvedere Cove late Sunday. A second dead cetacean report was received Monday morning from the U.S. Army Corps of Engineers near the Bay Bridge. The U.S. Army Corps of Engineers and a marine salvage company towed the carcasses to Angel Island Monday afternoon. 

The Center is grateful to its partners at Angel Island State Park, one of the many parks within the California State Park system, for their assistance in authorizing a landing area to perform the whale necropsies. 

For more information or to set up an interview on this topic, please contact us at media@tmmc.org.

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