Cambrian: Opinion

Marine experts track sharks, which in turn track elephant seals

A wounded seal heads back into the ocean, where its wounds may heal.
A wounded seal heads back into the ocean, where its wounds may heal. Special to The Cambrian

Elephant seals are top predators, but they are also prey for sharks and killer whales. 

Occasionally, a wounded seal lands on the beach, or a healed scar gives silent witness to an attack.

This wounded seal rested at Piedras Blancas during the first week of October. Shark researchers have found that sharks have seasonal migrations and spend the time between August or September through March feeding in their favorite places along the California coastline. Those places — Tomales Bay, the Farallon Islands, Año Nuevo — are where they find plenty of marine mammals, like juvenile elephant seals, to eat.

To find out whether sharks are feeding at Piedras Blancas, scientists from Hopkins Marine Station put a buoy offshore at Piedras Blancas in 2013, equipped with underwater receivers to track tagged sharks. Scientists tag sharks with acoustic tags that can be detected if the shark swims within 500 meters of the underwater receiver. Over several months, only two of the tagged sharks swam past, although it was exciting enough to know that Duke, a 17-foot male, was swimming out there.

The Piedras Blancas buoy isn’t there anymore but may be replaced in the future. The SharkNet app, available free for iOS systems, lets anyone follow the tagged sharks on their travels. 

Researcher Taylor Chapple came to Cambria in August to explain how he tags sharks to Friends of the Elephant Seal docents. Sharks are visual hunters, so he and his team attract them with a wooden dummy shaped like a seal, covered in indoor-outdoor carpet. They add some marine mammal blubber to entice the sharks close enough for the 10 to 20 seconds needed to shoot a digital tag into the dorsal fin. The unique scars on each dorsal fin also identify the sharks as individuals. Chapple has evidence that about 219 adult and sub-adult sharks live along California’s Central Coast. He and his team have tagged about 130 of them.

The dummies sure get chewed up.

Keep your eyes out for the tags on the beach. They automatically pop off after a preset period of time and float away. They continue to broadcast a signal so researchers can pick them up, but in a vast ocean, that isn’t always possible. They contain far more data than they can upload to satellites, so it’s important for the shark research project to retrieve them when possible. Researchers offer a reward of $200 to $500 for turning one in.

“We’re throwing thousands of dollars over the side every time we tag a shark,” Chapple said. 

When the juvenile elephant seals return to the beaches for their fall haul-out, sharks follow. In the Farallons and at Año Nuevo, sharks gorge on the juveniles. Juveniles make a bigger meal than the small pups but aren’t as fierce and dangerous as adult seals. 

“Hunting a pup is a lot of effort for a small benefit,” said Chapple, who said 

the researchers call them “pupsicles.”

Sharks attack from below, accelerating to hit their prey with force and disable it with a single bite. They attack from behind. A seal bitten in the tail can turn and fight back. 

A serious physical injury can interfere with the shark’s ability to hunt.

“Seals are dangerous for sharks,” he said. “If a shark loses an eye, it’s a serious consequence.”

Becoming a docent

Marine experts such as Taylor Chapple partner with Friends of the Elephant Seal docents, another benefit of being a volunteer. Contact FES at 924-1628 or the website at http://www.elephant