The unusual dolphin stranded in shallow mudflats of the San Francisco Bay just south of San Francisco International Airport was confirmed dead Wednesday by animal rescue officials.
The Risso’s dolphin was first spotted Saturday and appeared lethargic. The team attempting to reach the dolphin Wednesday faced muddy conditions but retrieved the 10-foot male’s carcass for a necropsy to determine why it became stranded, said Dr. Shawn Johnson, director of veterinary science at the Marine Mammal Center.
“This is a very rare thing,” said Bill Keener, a biologist with Golden Gate Cetacean Research. “I’ve seen Risso’s dolphins occasionally in Monterey Bay, but never here. They’re squid eaters. There’s no food for them out here.”
Risso’s dolphins are common offshore near the Farallon Islands, where the continental shelf ends and deep waters provide rich habitat for squid.
“This thing looked a little weak to me when I saw it on Saturday,” Keener said. “They’re pretty vigorous strong animals and it was not acting that way.”
Risso’s dolphins live in open oceans around the world where they travel in pods of 10 to thousands, sometimes with other species, said Thomas Jefferson, a marine mammal biologist in San Diego.
“It’s not by any means a rare species … They’ve been increasing along the California coast,” Jefferson said.
Keener and Jefferson agreed this dolphin’s death does not seem to be part of a trend for the species.
“Being away from the rest of its school would probably be a very distressing thing for the animal in and of itself, even if it was out in the open ocean,” Jefferson said.
Getting caught in fishing nets and hearing shipping or military sonar noise can also impact marine mammals, the researcher noted.
Its death might have been natural, Jefferson said. “It could be this animal, when it was foraging, lost acoustic contact with its school and was wandering around,” or the dolphin may have been sick, he said.
Risso’s dolphins have a unique appearance, covered in scars commonly believed to be caused by the huge squid they catch and eat. They can live up to 50 years old.
In the Bay, “the animal probably wasn’t eating, or if it was eating it was probably some abnormal type of prey that led to it being relatively weakened,” Jefferson said.
Another cause leading to the dolphin’s death could be warmer water, like the “Warm Blob” of abnormal water temperatures that heated the Pacific Ocean these last few years, said Josh McInnes, research coordinator at Marine Life Studies in Monterey Bay, where he’s helping create the first catalog of Risso’s dolphins.
“It might have been following prey that were following a warmer current. Warm water temperatures … can be very unstable and cause the animal to become disoriented,” McInnes said.
He has also studied marine life in British Columbia, Canada, where a Risso’s dolphin got stranded, beached and died in February on Graham Island, off the Pacific coast nearly to Alaska. “That’s a real abnormal situation,” McInnes said.