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SANTA CRUZ – Spring is an especially perilous time for female gray whales and their calves as they migrate from Mexico to Alaska. In just one day a week ago, 50 killer whales were spotted gathered around a gray whale calf carcass feeding and interacting socially just a mile off shore in the Monterey Bay at Point Piños near the mouth of the deepest submarine canyon on the West Coast, waiting to attack the newly born offspring.

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“It’s unusual for killer whales to gather in such large numbers,” said Nancy Black, a captain and marine biologist with Monterey Bay Whale Watch who has studied this species for nearly 30 years. “There is an abundance and variety of food sources available for the killer whales. In addition to gray whales there are seals, sea lions, and dolphins. Lots of activity this year.”

Killer whales with their distinctive black and white color, also known as orcas, are a prime predator of gray whale calves. Gray whales can live up to 70 years, reach a length of 49 feet, and eventually weigh as much as 36 tons providing that the female gray whale can protect its calf from a formidable foe that has no natural predators.

The geography of the Monterey Bay poses a unique challenge for a female gray whale and its calf, according to Ari Friedlaender, associate researcher at UC Santa Cruz.

“Most of the time gray whales swim tucked up to the shore in shallow water close to the kelp beds that provide shelter from killer whales who prefer deep, open ocean waters,” said Friedlaender. “However, due to the contour of the bay at some point whales have to pass over the canyon and out to the expansive ocean to continue their migration leaving them vulnerable to attack by killer whales.”

During the last six weeks there have been seven successful killer whale attacks on gray whale calves.

Kate Cummings, a captain and owner of Blue Ocean Watch, said last Wednesday that three killer whales — a male killer whale and two females killer whales — killed a gray whale calf. She tracked 11 killer whales that were 5 miles away swim at 8 miles an hour in a straight line toward the dead animal to share in its remains.

According to Black, the battle between the killer whales and female gray whales is vicious and a contest of wills and strategic maneuvering that can last more than two hours. Killer whales swim underneath the calf and attempt to ram it and separate it from its mother, who in desperation tries to push the distressed calf out of the water to enable it to breathe and to place it on its belly.

“There is lots of swishing and splashing of the gray whale mother’s flukes as she attempts to hit the killer whales,” Black said. “The mother is very protective but if she is not near the shoreline there’s not much else she can do.”

Defending its calf takes a toll on the female gray whale. “She is not only severely harassed and beat up, too,” Cummings said, “but she also ends up with tooth rake marks on her flukes and flippers.”

It takes about three to four killer whales to separate a calf from its mother and it usually doesn’t involve the young who instead wait on the sidelines. “It can be dangerous to be hit by the flukes of a mother gray whale,” Black said. “The young watch and learn from their mothers how to hunt a gray whale.”

Both the gray whales’ and the killer whales’ respective roles are learned as well as genetically determined, according to Friedlaender.

“There is a transmission of cultural knowledge,” he said. “The seasonal migration as well as the predator prey relationship is a well-established, well-known phenomena.”

And so is the group feeding that occurs after the killing of a gray whale calf. According to Black, typically several different families of killer whales gravitate toward the area and binge feed followed by socializing and playful behavior including breaching, spy hopping, rubbing each other, chasing one another, and rolling around and jumping out of the water.

“It’s like a big party with killer whales interacting together who don’t see one another that often,” Black said. “It’s all a part of nature including the predation.”

Giancarlo Thomae, a captain for Sanctuary Cruises who has a bachelor’s degree in marine biology and is a deckhand on freighters, has increasingly seen humpback whales intervene to influence the outcome of this predator prey ritual.

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“I have observed humpback whales attempt to protect gray whale calves from killer whale attacks as well as try to prevent killer whales from eating the carcass,” Thomae said. “Last week two humpback whales stayed about five feet from a gray whale carcass and were trumpeting almost as loud as an African elephant, slapping their flukes on the water creating quite a spectacle to scare off the killer whales.”

Alisa Schulman-Janiger, a marine biologist and whale researcher, has seen humpback whales place their bodies between killer whales and gray whale calves. She has also observed them following killer whales for several hours instead of feeding.

“It as if the humpback whales have taken on the role of defender of gray whale calves,” Schulman-Janiger said. “They could be doing this for altruistic reasons or perhaps expressing anger toward killer whales that harmed them in the past.”

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Tips for dealing with the Bay Area’s rat problem Still, killer whales in the Monterey Bay are like family for Black. Some have names and they all have numbers. Distinctive dorsal fins and saddle patch markings help identify whales. “I know their habits and I recognize them,” she said of the roughly 120 killer whales that frequent the area. For example, “Emma” is a mother and grandmother with a family of seven members that include a son that stays close to her and a daughter who has several offspring.

“We are only half way through killer whale season,” Black said, “and there is already more than 20 days in which killer whales have been sighted in Monterey Bay.”