Shark fins, unique as fingerprints
To most of us, all white sharks look similar: strong, elegant and powerful. But not to Aquarium Senior Research Scientist Dr. Salvador Jorgensen.
“In order to tell them apart, we like to think of something descriptive to call them: Middle-notch, or Split-fin, or Rooster,” Sal says. “There’s one that looks like a profile of Jay Leno. We have a shark called Hitchcock. We have one called Elvis.”
He pulls up a photo of a dorsal fin—the characteristic, triangular fin on a white shark’s back that features prominently in movies like Jaws—and compares the negative space at the tip to a profile of Jay Leno. The two are an uncanny match.
Like fingerprints and retinas are unique to each person, a dorsal fin is unique to each white shark. Each fin has scars, pockets and notches.
Sal and a number of colleagues from Stanford and Montana State University are taking advantage of these fin fingerprints to identify the same sharks as they return to Central California year after year.
White sharks in the Eastern Pacific Ocean begin their lives in the warmer waters of Southern California or Baja. When they’re born, their fins are mostly featureless. But by the time they approach adulthood and travel to the Central California Coast, their fins are more ragged, like slightly tattered flags.
Sal and his colleagues think the unique pattern of each fin might be initially caused by the bites of parasites, called copepods, which attach to the fin and cause sores when the sharks are young and their skins are soft. These bites create pits and valleys that are later expanded by wounds. The trailing edge of the fin is thin, like the foil of an airplane wing. Over time, every bite or scratch that tears becomes an indentation as it heals over. As a result, each shark wears the unique story of its battles.
“Over their life, these indentations accumulate and give [the sharks] character,” Sal says.
They also give researchers a chance to gather valuable data.
A 30-year record
Scot Anderson, a seasonal researcher for the Aquarium, was the first to discover white sharks have fin fingerprints. In the late 1980’s, while photographing seabirds on the Farallon Islands as a Point Reyes Bird Observatory volunteer, he began turning his cameras to white sharks as well.
“I saw some shark attacks on seals, and I realized that I’d never seen a photograph of these predation events,” he says.
In his photos, Scot noticed he could distinguish certain sharks by the obvious features of their dorsal fins, like a missing tip that makes a fin look trapezoidal rather than triangular. But over time, he began to recognize more subtle fin patterns, like pock marks and indentations, that can help distinguish one shark from another.
Almost 30 years later, Sal, Scot and their colleagues continue to develop this shark ID method. Each fall, during the height of the white shark season off Central California, they visit three elephant seal rookeries—at the Farallones, Año Nuevo Island and Point Reyes—that draw large groups of white sharks. They cast a seal decoy behind their boat as a lure. As each white shark surfaces, looking for a meal, the researchers try to snap a photo of its dorsal fin.
“In the 1980s, the photography and video were pretty primitive,” Scot recalls. “It used to be a roll of 36 shots on a camera covered in a plastic bag because of the harsh environment. It’s been really rewarding to see how far we’ve come.”
As apex predators, sharks are a critical part of the ocean food web, keeping prey populations in check. But many shark species are in decline due to high market demand for their fins, meat and other products. As we learn more about their specific behaviors, it can inform policies to conserve many shark species—such as protecting areas that are important for feeding or as nurseries.
Elvis is King
During the last observation season, from October to November 2015, the white shark team was able to photograph an unprecedented 117 unique shark fins.
Paul Kanive, a graduate student at Montana State University and an Aquarium Research Fellow, then compared these fins to more than 1,200 images in a database that contains all the individuals spotted in the last three decades. For example, Elvis, a 16-foot male with a fin tip reminiscent of the famous singer’s hairstyle, was first seen in 1989.
Recognizing individual sharks over time is crucial to white shark conservation research. Using shark fin identification and electronic tagging methods, the team has learned that sharks return to the same feeding grounds year after year. Subsequent tracking studies have revealed that between seasons, white sharks travel as far as Hawaii. The photographic “mark and recapture” study has also allowed them to estimate the number of adult white sharks living in central California, and track that number over time.
“It’s really exciting to see these old friends year after year,” Sal says. “We get to know them, and hope to see them again the next season.”
Last fall, when Scot saw that familiar pompadour-shaped fin tip through his camera lens, he felt a thrill like a celebrity sighting. Elvis’ return, 27 seasons since he was first spotted, set a new record—beating out a shark named Tom Johnson, who had previously been seen over 26 seasons.
“One more year,” he thought. “Go Elvis!”
Learn more about Conservation & Science at Monterey Bay Aquarium.