In her work as stranding coordinator at the Noyo Center for Marine Science in Mendocino County, Sarah Grimes investigates reports of dead marine mammals up and down the coast. So when she began finding the decapitated bodies of harbor seal pups dragged up near the high-tide line in MacKerricher State Park, just a few miles north of Fort Bragg, she was on the case.
“It was so gruesome,” said Grimes. “I was like marine mammal CSI, seeing all the dead pups with their heads torn off, and I’m like, ‘What the heck did that?’ ”
The headless bodies began turning up in 2016, and Grimes wasn’t able to pin down what was behind them. She suspected it might be coyotes, as she had found tracks in the area — but coyotes were not known to go after live marine mammals.
It wasn’t until the fall of last year that Grimes got proof, when Frankie Gerraty, a Ph.D. student at UC Santa Cruz, caught one of the culprits in the act at MacKerricher.
“We set up camera traps and got one really solid video of a coyote dragging a harbor seal pup and beheading it,” Gerraty said. “We are pretty confident there has been predation at four sites along the Northern California coast.”
Gerraty began placing motion-sensitive camera traps on local beaches to study the role of coyotes and other animals in scavenging the carcasses of marine animals. He and his team of undergraduates make regular trips to Point Reyes, following coyote tracks across the sand and up into the dunes and collecting scat for DNA analysis.
They’re trying to figure out if seafood makes up a large part of the coyotes’ diets or if they are simply opportunistic eaters.
“Not many people think of them as beach dwellers, but they definitely are,” said Gerraty.
So when his camouflaged cameras captured coyotes attacking harbor seal pups, Gerraty wasn’t shocked. “Coyotes are underappreciated predators in shoreline ecosystems, and marine mammals are the largest and most calorically rich nutrient parcels in the ocean, and really anywhere in the world,” Gerraty said.
Why they only eat the pups’ heads, however, is still unclear.
The coyotes’ taste for marine mammals could be genuinely new, or it could be that researchers are just beginning to notice it. It might also be the resurrection of a habit that existed when large predators from coyotes to Grizzly bears freely roamed the California coast before they were hunted down by ranchers determined to protect their livestock.
Either way, this newly observed dynamic could have consequences for the region’s marine mammal populations, which have bounced back since the passage of the federal Marine Mammal Protection Act in 1972.
In 2021, volunteer Sue Van Der Wal was counting harbor seals at Drakes Estero in Point Reyes when she witnessed a coyote cross onto a sand bar full of seals at low tide.
“I was shocked and upset to witness the one coyote attacking and pulling a pup away from its mother,” Van Der Wal said. “The pup gave out stress calls — it was terrible to hear and watch.”
Sarah Codde, a marine ecologist with the National Park Service at Point Reyes National Seashore, has found that coyotes are also attacking young elephant seals. Coyotes were known to make meals out of carcasses and placentas during pupping season and had even been seen going after abandoned newborns. But in 2020, a few coyotes got bolder, taking chunks out of the flippers of weaned elephant seal pups, which weigh about 300 pounds.
“At first, we didn’t know what was causing it,” Codde said. “We didn’t know if they were bites or a human coming in and cutting off pieces, or a skin disease.”
By the end of the season, her team spotted around 30 weaned pups with flipper bites. In 2021 and 2022, they didn’t notice the behavior. But in 2023, a handful of weaned pups turned up with bitten flippers.
Codde enlisted Gerraty’s help, and the team set up camera traps — and caught a coyote on video.
“The coyote was really struggling to pull at it, and the seal was trying to pull away. We caught it for about six minutes of the coyote pulling and biting, and then the seal would get away, and the coyote would get it again, and then they went off-screen,” said Codde.
The researchers are fairly confident that coyotes have not killed any elephant seals. They have not seen any carcasses or noticed a decline in the Point Reyes population. “For all the pups we saw with injured flippers, there were no other bites anywhere else on the body. We didn’t know why they would do that, what was the purpose, what is the nutritional value,” Codde said.
Tali Caspi, a Ph.D. student at UC Davis who studies coyotes, isn’t convinced that the coyotes are trying to kill the elephant seals. “Maybe they think it’s a carcass, and then they go there, and they take a bite, realize and run away,” she said.
In the first half of the 20th century, elephant seals were almost hunted to extinction, and state-financed bounty hunters decimated harbor seal populations to protect fishermen from competition. By the time marine mammal populations started bouncing back, most terrestrial predators had been chased off the land by ranchers. Researchers believe that seals and sea lions got more comfortable having pups on beaches, rather than offshore rocks and sandbars.
But now terrestrial predators such as coyotes are expanding their ranges toward the coast.
“Everyone was fat and happy, and no one was disturbing them. Then populations of large predators started to rise, and suddenly, now, there’s a killer in the mix,” said Grimes.
Researchers are now monitoring marine mammal populations to figure out if they are responding to coyotes as an emerging threat. In the last few years, Grimes has seen fewer harbor seals at MacKerricher Park during the spring pupping season.
“Harbor seals are pretty skittish,” she said. “Maybe the smarter ones are realizing they failed to pup for the past few years, and they are choosing a different site.”
The harbor seal population at Point Reyes might be experiencing a slight dip. “There is definitely more research that needs to be done on a statewide population look of harbor seals so that we can see if they are moving to different areas,” Codde said.
If harbor seals feel pressure to move, they might be pushed to less suitable locations. “Drakes Estero is a good place for pups to learn how to swim, dive and feed,” said Codde. “If harbor seals have to find new locations because of coyotes, we have to be mindful of that and protect those areas from human disturbance.”
There may be consequences for coyotes, too. Marine mammals sit high on the food chain, so their tissues collect high concentrations of mercury, which is emitted from coal-fired power plants and deposited in the ocean.
“There have been prior studies on California condors scavenging marine mammals — and because of that, having high contaminant levels,” said Gerraty.
Whether the coyotes’ behavior is new, a resurrection of old relationships, or something scientists have simply just noticed, it is providing researchers with a unique opportunity to study the interaction between two native species. “
I think it’s an exciting thing,” Codde said. “It’s a predator-prey relationship that’s developing. It’s nature happening.”
