Island Science

The Boy Who Cried “Shark!” – Fin Sighting 101

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

It is never an easy thing to try to identify a marine creature from the five percent of their body that sticks out above the water.  Unfortunately, the recent New England paranoia over great white sharks coupled with a long tradition of the aftermath of the film “Jaws” has created a perfect storm of lunacy and false shark sightings.  As with all things, it is helpful to put anything you see in nature in context.  An excellent example is the “brown recluse” spider bites people in the Northeast claim to have suffered.  This is highly unlikely as there are no brown recluse spiders in New England.  Their range is limited to the southwestern and central U.S. If you are in Massachusetts and have been bitten by a spider, odds are it is not a brown recluse. If you don’t know what state you are in, I cannot help you.

I have written about sharks four or five times over the past four years and I’ve tried to get across how unlikely a shark attack is ( The majority of sharks in our waters are gentle giants who would much rather be eating plankton, a sand lance, or other small prey fish. In Massachusetts, there have been four reported unprovoked shark attacks from 1670-2012, three of which were fatal, with the last fatal attack occurring in 1936.  Except for sport fishing, there are no major commercial fisheries for sharks except trawl and gillnet fisheries for spiny dogfish (according to the Division of Marine Fisheries website).  Of the 5.7 million pounds of shark landed in the Commonwealth in 2000, 99% were spiny dogfish with a commercial value of $1.3 million.  The remaining 1% was primarily makos, threshers, and porbeagles taken incidental to offshore trawl, longline, and gillnet fisheries based in this state.

When you see a fin around Nantucket poking out of the water it could be any of the following animals: common dolphin (long beaked and short beaked), white sided dolphin, harbor porpoise, minke whale, Atlantic white sided dolphin, Risso’s dolphins, striped dolphins, long-finned pilot whales, thresher shark (has a weird looking tale that gives it away), porbeagle, sunfish, basking shark, blue shark, mako shark, sandbar shark, sand tiger, dusky shark, great white shark, pygmy sperm whale, dwarf sperm whale, and a few other sharks which are listed below. A dozen species of sharks can be found in our waters according to the Massachusetts Division of Marine Fisheries. In fact, Massachusetts represents the northernmost range for several species of sharks. Therefore, it is an important area for monitoring the health and distribution of shark populations.

The recent rebound in grey seal (Halichoerus grypus, meaning “hooked-nosed sea pig”) numbers, from almost complete localized extirpation (extermination) to levels which seem high to present day human populations has caused many people to worry about a relatively rare predator, the great white shark, or Carcharodon carchariasCarcharodon comes from the Greek words “karcharos,” which means “sharp or jagged,” and “odous,” which means “tooth,” which sounds a bit like Little Red Riding Hood named it.  Great white sharks have 300 teeth and can grow to be 20 feet long and weigh up to 2 tons. These sharks have pointed snouts, spindle-shaped bodies, and large gill openings.  The first dorsal fin is large, high, erect and angular or somewhat rounded.  The second dorsal and anal fins are small.  The caudal peduncle (area where tail fin is attached) has one or two distinct keels. Great whites exhibit countershading, which means they are dark and a bit mottled on top in order to blend in with the dark water and sand below and white on the underbelly to minimize their silhouette from below (slimming camouflage!).  Great whites are an apex predator which tend to favor warmer waters year-round but can be seen occasionally in New England waters during late spring, summer, and fall and can even hang out into early winter.  In Massachusetts we see a variety of shark species including blue, mako, angel, basking (huge sharks often mistaken for great whites), dusky, porbeagle, sandbar, sand tiger, smooth and spiny dogfish, thresher, sharpnose, tiger and hammerheads (relatively rare tropical species), and of course, the iconic great white shark.

This time of year, the white shark sighting start to increase, some of them hoaxes and jokes, some of them real.  In mid July of 2008, a 6.5 foot long young female great white shark washed up onto the south side of Nantucket near Sheep Pond Road.  The Division of Marine Fisheries verified that this was indeed a great white shark.  This was the first time in 21 years that a C. carcharias had washed ashore in Massachusetts.  Dr. Greg Skomal, a Massachusetts Division of Marine Fisheries (DMF) biologist is the Commonwealth’s resident shark expert and he spends a fair amount of time providing factual information aimed at calming ocean users’ fears.  Some of his colleagues and graduate students have used the UMass Field Station as a base to attend billfish and shark tournaments to obtain stomach content samples in order to learn more about shark and apex fish prey species.  Dr. Skomal has also done an extensive amount of research to look at shark migration in May and June along the Atlantic Seaboard.  Massachusetts is the northernmost limit for several migratory shark species from porbeagles to basking sharks. The porbeagle shark is the only species found year-round in Massachusetts waters because of its preference for colder temperate waters.  Just like many fish species that migrate from their southern wintering locations to the productive waters off New England, so too, do sharks as they chase prey species in from deeper waters.

Most people don’t know that there used to be huge numbers of sand sharks around the Cape and islands in the first few decades of 1900’s. The sand shark is also known as the dogfish shark or ground shark and has an obvious second dorsal fin and large anal fins. From this Gulf of Maine shark facts site: “The facts that a catch of about 1,900 sharks by three boats on Horseshoe Shoal, in Nantucket Sound, June to September 1918, was mostly of this species, as was another catch of 350 sharks, taken near Nantucket in the early 1920’s, illustrate their numbers there. Scattered sand sharks are also caught along the outer beaches of Cape Cod by surf anglers (published records are for Monomoy, Chatham, and Provincetown) and there are enough of them along this stretch of beach in some summers (1951 was a case in point) for them to be a nuisance to anglers casting for striped bass in the surf at night” from, an excellent resource accessed July 22, 2012.

According to the Massachusetts Division of Marine Fisheries white shark tagging research page ( accessed July 22, 2012): “Despite their size white sharks are difficult to spot as they are a deep water species; their characteristic dorsal fin is usually not visible above the waterline.”  A recent paper entitled “Shark Nursery Areas in the Coastal Waters of Massachusetts” may keep you up at nights, but it is an excellent summary of the various sharks in the neighborhood and the reality is, there have always been a lot of sharks near here, and yet shark attacks are extremely rare; accessed July 22, 2012. From this article I learned that smooth dogfish are extremely common sharks in our waters accounting for 92.5 % of sharks captured by longline. You might be catching my drift by now which is if you see a shark, chances are it is something other than a great white.

In fact, it could easily be a sunfish or mola mola like the creature seen last week at Fishermen’s beach (; accessed July 23, 2012). I love sunfish or mola molas so much I wrote a whole column on them back in 2010 (; accessed July 22, 2012. The ocean sunfish, Mola Mola, or common mola, is the heaviest known bony fish in the world.  It has an average adult weight of 1,000 kg (2,200 lb). According the 1995 edition of the Guinness Book of World’s Records, the largest recorded one weighed in (actually a guesstimate) at two tons four hundredweight, which converts to 4,927 pounds.  The fish measured 3.1 meters (10 feet) in horizontal length, 4.26 meters (14 feet) in vertical length.

This portly species is native to tropical and temperate waters around the globe.  It resembles a fish head with a tail, and its main body is flattened laterally.  Sunfish can be as tall as they are long when their dorsal and ventral fins are extended.  The common name “sunfish” is used to describe the marine family, Molidae, as well as the freshwater family, Centrarchidae.  The common names “ocean sunfish” and “mola” refer only to the family Molidae and can be applied all three Molidae species.  Presently, three distinct species are recognized within the family Molidae including: the common mola, Mola mola Linnaeus 1758; the sharp-tailed mola, Masturus lanceolatus Lienard 1840; and the slender (relatively!) mola, Ranzania laevis Pennant 1776.

The German name for this fish is “Schwimmender Kopf” meaning swimming head, which gives you some idea of what a mola mola looks like. Mola molas are often seen near the surface, swimming either upright, dorsal fin flailing from side to side as it seems to waddle underwater, or flipped over swimming on its side, basking on its sides soaking up the rays of the sun and hoping for some free grooming by birds (; accessed July 22, 2012).

The term for an abnormal fear of sharks for trivia fans out there is “galeophobia” which is derived from the Greek words “galeos” meaning “dogfish, small shark” with a derivative or alternate meaning from galē for “shark with markings resembling those on a weasel” and the often used “phobos” (fear).  Galeophobia is also sometimes used as alternate term for ailurophobia, or the fear of cats, from the Greek word “galee,” meaning “polecat” and/or “weasel.” According to, “sufferers from this phobia experience anxiety even though they may be safe on a boat or in an aquarium or on a beach.”  Galeophobia of ichthyophobia (fear of fish) specifically focused on the fear of sharks. Recently (within past 10 tears) psychologists have started to use the term selachophobia for fear of sharks. I was extremely impressed to find that the website Ehow has some useful advice on how to address galeophobia; accessed July 22, 2012.

Believe it or not, there are several useful resources for the lay person to explore that help us determine what type of fin we are looking at when we gaze across the ocean.

The website tells us how to identify a shark fin in the same nonchalant manner as how to change a spark plug or build a doghouse. Here’s their advice:

  1. “Locate the dorsal fin in the water. Since sharks are swimmers and aren’t stationary for very long, it’s imperative that you learn how to look quickly at the dorsal fins and remember their shape.”[Really?? I was going to take my time, maybe have a cocktail while I check it out from my surfboard].
  2. “If the fin is triangular and slightly slanted, it’s a shark. If the fin is triangular but severely slanted and dramatically curved inward, then it’s most likely a dolphin.” [If it is saving your life or starring in a TV show or exploited needlessly for children’s amusement, it is most likely a dolphin].
  3. “If you’ve identified a shark fin, get out of the water immediately. If you can’t tell because of a poor sighting, don’t take chances – leave the water. “[Once again, some solid advice for the 0.02 percent of people who might linger].

Ahhhhhhhh run, or more accurately, swim away!!!!!!!!!

Read more: How to Identify Shark Fins ( access July 22nd, 2012).

If you are truly serious about learning how to identify shark fins that have been illegally taken, the Pew Trust has spent a lot of time and effort to show us how to identify some of the more endangered species

And here are a few more tips I found that help one distinguish between a dolphins versus sharks.

  • Most species of dolphins have just one dorsal fin, situated toward the middle of the back. Many shark species have a second dorsal fin, located closer to their tails although one might not see this second dorsal fin unless the shark is swimming close to the surface of the water. The secondary dorsal fin is usually much smaller than the primary one, but tends to mirror the shape of the larger fin.
  • A dolphin’s fin has a curved silhouette, while most shark fins have a straight forward edge. In some shark species, the rear edge of the dorsal fin may be slightly curved.
  • Some types of shark can also change the attitude of their dorsal fin, so that it droops slightly. However, a change in fin attitude is rarely severe enough to make it look like a dolphin’s fin. Some porpoises have straighter fins which may be mistaken for shark fins, but it is less likely you are looking at a porpoise.
  • Notice the texture of the fin. Although possibly damaged by boats or other previous encounters, dolphin dorsal fins are smooth in texture and streamlined. Sharks have a notched or ragged appearance at the rear of their fins. For some species of shark this may only be noticeable when very close and most of us may not want to get that close. But often someone like a lifeguard or birder may be around with a pair of binoculars. If you have a camera, take a picture.
  • Notice how the fin is attached. While only visible when the animal is very close to the surface, a dolphin’s fin ends at the rear in a smooth transition to the animal’s back while a shark’s fin has a gap between the rear edge of the fin and the back creating a detached tip.
  • Read more at accessed July 22, 2012.

For comparison, the mola mola dorsal fin is usually very tall and curves in a convex manner. This web site has a lot of great pictures of mola molas alongside divers so one can see how huge they can get, accessed July 22, 2012. Remember they do seem to flail about a lot and they are much slower swimmers than sharks. The speed of movement is key to determining whether you are looking at a shark or dolphin or a slower species.

The official web site for the State Department of Marine Fisheries has information updates as of 2010 on the great whites that frequent our waters accessed July 22, 2012. You can find an excellent photo essay of the great whites trapped in a Cape Cod salt lagoon in 2004 at

After the shark sightings in 2010, a lot of publicity for sharks occurred up and down the coast of New England. Over the past 10 years, scientist and photographer Michael Sholl has taken pictures of 1,500 great white sharks. Now, he’s sourcing the public to help him create a data set to track and study great whites; accessed July 22, 2012. Dr. Greg Skomal recently published an awesome and very fun book about sharks called “The Shark Handbook: the Essential Guide for understanding the Sharks of the World” with photography by Nick Caloyianis. I love this book and really appreciate its useful tables and lists of sharks by common and Latin names. If you have a shark lover in the family you should get it. And last but not least you can learn about more creatures along our shores and see of video of some swimming by at accessed July 23, 2012. Enjoy your summer beach days, fall will be sneaking up on us faster than a great white.

Articles by Date from 2012