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Is that a great white shark’s fin? Or something else? Here’s how to tell the difference

The dorsal fins of some less-threatening sea creatures are often mistaken for sharks, experts say.

The dorsal fins of other sea creatures are often confused for those of great white sharks.
The dorsal fins of other sea creatures are often confused for those of great white sharks.John Chisholm/Division of Marine Fisheries

Look, over there! What’s that pointy thing slicing through the ocean’s surface? Is it a great white shark?

It very well could be. But then again, it might also be something else.

With the apex predators returning to Cape Cod waters in search of seals and other food sources this summer, people may think every dorsal fin protruding from the water belongs to a great white just below the surface. But experts say it can be easy to mistake the fins of other, far less dangerous sea creatures for the ones that should send people sprinting for shore.

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That was likely the case on July 5, when an alert went out and the beaches were closed in parts of Plymouth because of an apparent shark sighting not far from the shoreline. When officials hit the waters to investigate, however, they realized it may have been a porpoise splashing around.

The Atlantic White Shark Conservancy, a nonprofit that works with state officials to tag and track sharks, also received reports of potential sightings over the weekend, this time in Lewis Bay. They later determined the fins belonged to sandbar sharks.

Although it was a false alarm, the group reminded people to keep sharing videos.

“White sharks can show up in unexpected places,” the conservancy said on Facebook, “so please continue to send your sightings in and treat any unidentified fin with respect and caution.”

Because it’s likely to keep happening, we reached out to John Chisholm, a shark researcher who works closely with the state’s Division of Marine Fisheries to identify great whites and other sharks. Here’s what he had to say about spotting fins while out swimming, boating, or just sitting on the beach, and how to tell them apart. (You can also watch this handy informational video from 2015).

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Great white

The dorsal fin of a Great White shark.
The dorsal fin of a Great White shark.John Chisholm/Mass. Division of Marine Fisheries

Seeing a foreboding fin bisecting the sea is enough to send anyone into panic-mode. But to be sure it’s a great white you’ll want to look at the shape first. According to Chisholm, the sharks have large, triangular dorsal fins that come to a point at the peak with a straight rear edge.

“It often has tears and notches in the rear edge,” he said in an e-mail.

If you think you see one, experts urge beachgoers to report the sighting to the conservancy’s “Sharktivity” app, which, if confirmed, will send out alerts to users about the possible presence of the predators.

Risk level: There was a fatality off Wellfleet in 2018, and a second nonfatal incident that same year. There are signs posted warning swimmers to remain aware of their presence.

Other characteristics: Distinctive white underbellies and small black eyes.

Basking shark

Dorsal fin of a basking shark.
Dorsal fin of a basking shark.John Chisholm/Mass. Division of Marine Fisheries

State shark experts say the basking shark — the second-largest of its species — is usually the “main culprit” of mistaken identity. But there are differences that protrude from the water: Where great whites have a sharp tip at the top of their dorsal fin, Basking sharks, which feast on plankton, have a more smooth and slightly-rounded one with a convex rear edge.

“[The] dorsal often appears to lean and flex as the shark swims on the surface,” said Chisholm. “It can also have tears and notches along the rear edge.”

Even though we’re talking about fins, people should also take size into consideration: These sharks are big. Experts say if you’re convinced your shark is in excess of 20 feet, it’s probably a Basking shark, since the average size of a great white in the region is 12 feet.

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Risk Level: Michael Dukakis picking up litter. Their presence looms large — but just observe them from afar and let them do their thing.

Feeding frenzy: These fish get down like Joey Chestnut on the Fourth of July. When feeding, they open their mouths wide and scoop up food.

Dolphin

The dorsal fin of a dolphin.
The dorsal fin of a dolphin.John Chisholm/Mass. Division of Marine Fisheries

The fins atop the backs of dolphins typically have a wave-like look to them, with a visible swoop at the back not unlike a fin on a surfboard.

“[They have] a rigid dorsal that curves backward with a concave rear edge,” said Chisholm.

But it’s their swimming behavior that can be the giveaway. For these marine mammals, the dorsal typically submerges and reemerges as it cruises along in the ocean, showing some of its upper body at the same time.

Risk Level: Like teenagers doing backflips on TikTok — mostly harmless and popular with viewers.

Fame and fortune: Dolphins also rival sharks in pop culture, including such mammalian icons as Flipper; Snowflake from “Ace Ventura: Pet Detective,” and Hall of Fame quarterback Dan Marino.

The fin of a Mola mola, or sunfish.
The fin of a Mola mola, or sunfish.John Chisholm/Mass. Division of Marine Fisheries

As Michael Bergin would say: “I ain’t seen nothing like that in my entire life.” But actually, you probably have. Mola molas — commonly referred to as sunfish — have great, big fins that often are thrust above the ocean’s surface, as they float along in the water.

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“[It’s] a tall flexible dorsal often appearing to have a convex rear edge,” Chisholm said. “It often appears to flap back and forth due to the Mola’s swimming style which involves it using its dorsal like a paddle.”

Sunfish are considered the heaviest bony fish in the world, according to the New England Basking Shark and Ocean Sunfish Project, and commonly visit the region in the summer and fall.

Risk Level: Your dad half-asleep watching TV, after eating Doritos in a recliner. Researchers have compared them to a “wobbling, struggling pancake.”

If it had its own soundtrack: The “Tuba Walk,” or the “Baby Elephant Walk” played on four tubas at the same time.

Porbeagles

A porbeagle shark washed up on Marconi Beach last year.
A porbeagle shark washed up on Marconi Beach last year.CAPE COD NATIONAL SEASHORE PARK RANGERS

Last summer, a group of friend’s fishing off Nantucket thought they made the ultimate catch: a great white shark. But, upon further inspection, wildlife experts determined it was a porbeagle, a species sometimes mistaken for its counterpart. One way to tell the difference is the bright-white spot on the backside of a porbeagle’s dorsal fin, like someone dipped it into white paint.

“[They] have a tall dorsal with a slightly rounded peak,” said Chisholm, “and distinct white patch at the base of the rear edge.”

Up close, the sharks’ snouts are pointed but compact, their coloration is more “subdued,” and their eyes are larger than that of a great white’s.

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Risk level: Pretty much zero, Chisholm said. They have sharp teeth, but their small mouths are used to strictly eat fish and squid. Incidents with humans have been very rare.

Do they ... do they eat beagles?: Don’t worry, they don’t eat dogs. According to the Florida Museum, the name porbeagle is Cornish, and probably comes from a combination of “porpoise” and “beagle” — a reference to its hunting prowess.


Steve Annear can be reached at steve.annear@globe.com. Follow him on Twitter @steveannear.