Why Sharks Attack

Will analyzing the hunting instincts of this endangered predator reduce deadly attacks? Airing May 7, 2014 at 9 pm on PBS Aired May 7, 2014 on PBS

Program Description

In recent years, an unusual spate of deadly shark attacks has gripped Australia, resulting in five deaths in ten months. At the same time, great white sharks have begun appearing in growing numbers off the beaches of Cape Cod, Massachusetts, not far from the waters where Steven Spielberg filmed the ultimate shark fright film, Jaws. What's behind the mysterious arrival of this apex predator in an area where they've rarely been seen for hundreds of years? Are deadly encounters with tourists inevitable? To separate fact from fear, NOVA teams up with leading shark experts in Australia and the United States to discover the science behind the great white's hunting instincts. Do sharks ever target humans, or is each attack a tragic case of mistaken identity? And can a deeper understanding of shark senses lead scientists to design effective deterrents and help prevent future attacks?


Why Sharks Attack

PBS Airdate: May 7, 2014

NARRATOR: It's a perfect predator, a creature that inspires fear and awe, because, on rare occasions, in the wrong place at the wrong time…

DAVE PICKERING (Shark Attack Survivor): I remember this mouth was probably about two feet from me.

NARRATOR: …tragic mistakes are made.

Today, many shark populations are falling, but off the coast of Cape Cod, sightings of great white sharks are on the rise.

GREG SKOMAL (Massachusetts Division of Marine Fisheries): If you said to me five years ago, "You're going to tag 34 white sharks in the next four years," I'm going to say, "You're out of your mind."

NARRATOR: At the same time, across the seas in Australia, a sudden spike in fatal shark attacks has shocked scientists and the community. Now, shark researchers are unlocking the secrets of these amazing ancient creatures, to understand the unusual ways they sense prey and decipher why sharks have been such effective hunters for hundreds of millions of years, in an effort to prevent future attaacks.

SHAUN COLLIN (University of Western Australia): We really have to understand sharks, to develop effective deterrents.

NARRATOR: Can scientists win the race to save people and sharks?

GREG SKOMAL: …right under us. Nice shark.

NARRATOR: Why Sharks Attack, right now, on NOVA.

Cape Cod, Massachusetts, is a popular summer vacation spot, and now, a new attraction is drawing tourists to the quaint towns, pristine beaches, and sparkling waters: great white sharks.

Even though the 1974 blockbuster Jaws was filmed in the waters off Cape Cod, these predators were only rarely seen in the area. Now, suddenly, they're here, right off the beaches.

Leslie Reynolds is the Chief Ranger for the Cape Cod National Seashore, charged with keeping the public safe. The arrival of great white sharks at the height of the summer beach season is making her job more challenging.

There's already been one attack.

LESLIE REYNOLDS (Cape Cod National Seashore): Last year, there was a bite that occurred, at Ballston Beach, on a swimmer.

NARRATOR: In 2012, a swimmer was attacked just off shore. Luckily he survived with limbs intact.


NARRATOR: With more sharks showing up, the chance of another attack is on the rise.

GREG SKOMAL: It's really not a matter of "if," it's "when."

NARRATOR: Greg Skomal is the top shark biologist for the state of Massachusetts. His job is to protect sharks and people.

According to records, the last fatal great white shark attack here was in 1936. But halfway around the world, the state of Western Australia has not been so lucky.

An unusual spate of shark attacks over a 10-month period left five people dead: September, 2011: Kyle Burden, body boarding; October, 2011: Bryn Martin, swimming at Perth's famous Cottesloe Beach, and George Wainwright, scuba diver; March, 2012: Peter Kurmann, diving with his brother; and in July: Ben Linden, surfer.

But it's not just great whites that are a threat. Other species known to attack humans are bull sharks, hammerheads, bronze whalers and tiger sharks.

DAVE PICKERING: You don't feel the teeth. You don't feel anything, a bit of pressure, like surgeon steel.

NARRATOR: David Pickering was a snorkeling tour guide on the reefs of Australia's west coast.

DAVE PICKERING: There was three kids with me at the time. I said, "Guys, I've got a bad feeling. Stick together."

And I spun around, and I remember this mouth was probably about two feet from me. The tail sort of kicked out once and it was on me. I put a hand out, like that, and actually cut 13 tendons and all three nerves.

After you've been bitten, the first thing on your mind is, "Is it coming back for me or someone else?"

Thank god I got bitten and not one of the kids, 'cause I couldn't live with that.

I can't go out in the water now without thinking about it. I can't shake that feeling, yeah? Nervous, anxious, this is what it reduces me to. When you love something so much, and you can't do it…I used to take things for granted, like having a limb that works perfectly. But, now, I appreciate every day.

NARRATOR: What triggers sharks to attack humans when we are not their natural prey? And can scientists find a way to prevent these tragic events?

Off the coast of Massachusetts, Greg Skomal says great whites are appearing in waters where, for hundreds of years, they've almost never been seen. For Skomal, it's a new opportunity to study animals in the western North Atlantic whose behavior is virtually unknown. And, if he can track and monitor their movements, he may be able to lessen the risk to swimmers.

GREG SKOMAL: The white shark is probably the most charismatic fish in the ocean. White sharks can weigh in excess of 4,- or 5,000 pounds, get in excess of 20 feet long. So, that's a very big fish.

We know that these sharks can live up to 50 years.

NARRATOR: Great whites are found in every ocean. Unlike most other sharks, they produce and store heat in their bodies, allowing them to tolerate colder temperatures.

In an area where they were rarely seen, Skomal was surprised to sight and tag one great white shark in 2008, off of Cape Cod. Over the next three years, he tagged 17 great whites. Then, in 2012 alone, he tagged 17 more.

GREG SKOMAL: If you said to me five years ago, "You're going to tag 34 white sharks in the next four years," I'm going to say, "You're out of your mind."

NARRATOR: Why has Cape Cod suddenly become the summer hunting grounds for one of the ocean's most dangerous predators? For Skomal, the answer is obvious on the beaches and in the water: seals.

GREG SKOMAL: It wasn't until we had the development of these seal aggregations here, on the eastern side of Cape Cod, that the white sharks started to take notice.

NARRATOR: The growing number of great whites in the area seems tied to a population explosion among seals.

GREG SKOMAL: We have several species of seals that occur here, but the two dominant ones are harbor seals and grey seals. The grey seal's a much bigger seal species; it's the one I believe the white sharks are targeting.

NARRATOR: Seals, a rich, fatty food source for great whites, once thrived along the northeastern coast of North America, but it's believed they were killed in the hundreds of thousands for their fur, meat and blubber.

GREG SKOMAL: By the end of the 17th century, Native Americans, early settlers, bounty programs, basically had wiped out the breeding colonies. And we think that the population had been actually diminished down to something along the lines of 10,000 animals or less.

NARRATOR: In 1972, the U.S. government instituted the Marine Mammal Protection Act, prohibiting seal hunting. Four decades later, seal populations are recovering and re-colonizing old breeding grounds.

GREG SKOMAL: Grey seals get to be about 7-, 800 pounds and eight, nine feet long. They're big animals. They're big targets, big thick blubber layer on them.

And now we know that there's over 300,000 seals off Canada, alone, and tens of thousands off the coast of the United States.

It's really ideal habitat for them, but in them coming here, in essence, they've opened the café for white sharks. You've got the perfect zone for white sharks to come in and consume these animals on a routine basis. And that's precisely what we've been seeing, over the last decade, as the café emits some kind of delicious odor, I'm sure, that goes out miles into the Great South Channel. And once the sharks come in and figure it out, we've actually had animals come back each year.

NARRATOR: For the seals, or anyone swimming near them, these are potentially dangerous waters.

LESLIE REYNOLDS: We're just asking you to stay close to shore, not swim near seals, keep your feet on the ground.

GREG SKOMAL: In the last several years we've been seeing more and more evidence of these seals being attacked: bites on seals, dead seals, wounds that can really only be created by one species of fish, you know, the white shark.

NARRATOR: Sometimes Skomal finds only what's left.

GREG SKOMAL: These are the intestines of a seal, and that would be really good evidence of an attack. That's all that's left, which is pretty amazing.

It is ironic that just 10 miles or so from here is the set of the movie Jaws.

We talk about Jaws as this Hollywood production that was fantasy. And if we look at all the elements of Jaws we see, really, many parts of it that are somewhat true. What Spielberg did was just take those tidbits of truth and exaggerate the hell out of them. The white shark getting 20 feet long becomes the white shark 25 feet long; the white shark nibbling the boat, becomes the white shark sinking the boat; the white shark biting people to the white shark becoming a ravenous predator desiring human flesh.

All it takes is just one person to be killed in this area, and that fiction can become reality.

When we have this kind of geographic overlap between high densities of white sharks and high densities of people, inevitably we get interactions. Is Cape Cod likely to be the next place that happens? Sure.

CHISHOLM: This is "shark cove."

GREG SKOMAL: This is shark cove. This is the one we get most of the detections on, right?


NARRATOR: Skomal detects the presence of the sharks he's tagged over the past four years, with a series of acoustic receivers suspended in the water.

GREG SKOMAL: So, Number 4.


NARRATOR: The tags transmit a signal, indicating that Skomal's sharks are returning to their summer hunting grounds.

CHISHOLM: We've had a shark here today, within the past half hour.

NARRATOR: This kicks his tagging effort into high gear. He uses a specially outfitted boat to sneak up on and tag more sharks.

If Skomal finds sharks close to the beaches, he can warn beach officials.

GREG SKOMAL: If you've got one 20 meters away, just sitting in the water, you may not know it's there.

NARRATOR: Despite the great white's massive size, a spotter plane is the best way to find untagged sharks in the murky green water off the Cape.

GREG SKOMAL: Here we go, right up in the shallows, heading for the shore.

See the shadow ahead of you? There, he's coming back. Right under us, right under us.

That's our baby. Nice shark.

NARRATOR: Skomal estimates this great white is nearly 12 feet long, and it's swimming just a few hundred feet from the beach, in water almost shallow enough for a swimmer to stand.

Skomal and the boat's captain have modified the technique of harpoon fishing for tagging.

GREG SKOMAL: We're able to actually sneak up on the shark and place the tag at the base of the dorsal fin. I'm sure the shark feels it to some degree, but I always liken it to, perhaps, sneaking up on somebody and piercing their ear.

Any time one of these sharks swims within a couple of hundred meters of one of these receivers, we get a time stamp on that receiver. We'll look at it at the end of the year and get a sense of who's around, when they arrived, when they leave and where they spend their time.

This'll allow us to figure out whether or not that shark already has a tag in it.

NARRATOR: Skomal uses a hydrophone that will pick up the telltale acoustic signature, if this shark has already been tagged.

GREG SKOMAL: All right, hydrophones in.

Yeah, 32307; tagged it last year.

NARRATOR: He wants to know how age and sex might determine the shark's movements and how they're linked to where and what they hunt.

GREG SKOMAL: We've been able to sex only about half of the sharks we've tagged, because we have great difficulty sexing them unless we can look underneath them.

We need to tag more fish and have a better sense of what the boys and girls are doing.

I'd rather be chasing a shark than not.

Beautiful. You see it? The shadow? He's turning. He's turning, right under us.

NARRATOR: Even this close, it's impossible to determine the sex. A remote camera offers a solution.

CHAPRALES: If I can get that pole cam in,...

GREG SKOMAL: I say whatever we can get, whatever we can get on this fish.

How you doing, Niko? Take a break. Take a break.

Let's get this other shot, right here.

CHAPRALES: Right there. There you go. Those are definitely claspers, 100 percent. See them, right there.

NARRATOR: Claspers are large, long external organs that male sharks use to mate.

GREG SKOMAL: Yep. So, it's a male. This is great.

NARRATOR: In just one day, Skomal spots five great whites.

GREG SKOMAL: I see a little boat paint on that fish!

NARRATOR: Some he's tagged in previous years...

GREG SKOMAL: I see one of the tags. It's red.

NARRATOR: …and newcomers, too. Animals that have been elusive can now be studied,...

GREG SKOMAL: It's a nice fish. Perfectly clean.

NARRATOR: …as they successfully tag more and more sharks.

GREG SKOMAL: It looks really good. Let's tag him.

CHAPRALES: We got it.

GREG SKOMAL: We got it.

NARRATOR: At first, Skomal's tracking data reveals a pattern linked to the movement of the fish sharks eat with seasonal changes in water temperature.

GREG SKOMAL: By far, the bulk of the animals move between Cape Cod and an area between Georgia and northern Florida, leaving here anywhere from September to December, arriving in Florida some time in December.

NARRATOR: Most sharks follow the continental shelf, moving between the surface and the relatively shallow bottom, down to a few hundred feet.

GREG SKOMAL: So it was a very simple migratory pattern.

NARRATOR: But some large, mature female great whites do something very different.

GREG SKOMAL: You know, not stay on the shelf, not go to Florida; go out into the central part of the Atlantic and start doing deep diving behavior, every day, to depths as great as 800 meters, over 2,500 feet; very, very, very different from what we were seeing the other sharks do.

It begs the question: "Why? What are you doing? What are you doing down there?"

NARRATOR: This different pattern for female great whites is seen in the Pacific Ocean, too.

GREG SKOMAL: What's the big difference? Well, perhaps they're pregnant.

NARRATOR: Is it possible that they go there to give birth?

GREG SKOMAL: I'm not likely to find out anytime too soon. All we can really do is hypothesize that there's some sort of opportunity, usually driven by reproduction or feeding.

NARRATOR: With every new shark tagged, Skomal learns more. When one of his tagged sharks returned to Cape Cod in February, the middle of winter, it shattered another common assumption.

GREG SKOMAL: We never anticipated they could tolerate that level of cold water. So we're getting these new insights into the biology and, in many cases, the anatomy of this animal, and how it works physiologically.

I want these animals to survive. In order to be able to put in place rules, regulations that allow for the survival of this species, we really need to know about the animal and how it lives from day to day.

NARRATOR: But there are still many unknowns.

GREG SKOMAL: It's pioneering stuff! It's never been done before. We never had predictable access to white sharks in the western North Atlantic. And because of all those seals sitting out there, now we do.

NARRATOR: His acoustic tags may save lives, allowing officials to close beaches when a tagged shark is detected.

GREG SKOMAL: We can tell where the sharks are and when they approach popular swimming beaches. And, in some cases, we've actually reached out to those areas to say, "Hey, by the way, you've got a shark in your backyard."

NARRATOR: It's clear that seals are attracting great whites to Cape Cod, where the beaches are the backbone of the region's massive summer tourist season. A fatal attack could mean beach closures and pit those charged with public safety against those whose livelihoods depend on the crowds.

So far, here, sharks have become part of the landscape. They even hold shark fishing tournaments, which are controversial, because fishing for sharks like these Makos, for sport, food, and the shark-fin-soup trade, is devastating shark populations. But they do offer scientists like Skomal an opportunity to examine and take samples from specimens that were swimming in the wild just hours before.

GREG SKOMAL: The more we learn about sharks, the more we realize how critically important they are to the marine ecosystem. They fill a role of apex predator. They control, in essence, the balance of the ecosystem. So we really have to be very careful when we exploit sharks.

NARRATOR: While fatal shark attacks on humans are extremely rare, every year an estimated 100 million sharks are killed. Among those who want to protect sharks is Wendy Benchley, wife of the late Peter Benchley, author of Jaws.

WENDY BENCHLEY (Conservationalist): Jaws tapped into a primal fear that that all human beings have of apex predators, whether they're tigers or lions or sharks.

Fatal shark attacks around the world are very, very rare, but, when one does happen, justifiably so, communities are very upset and the initial instinct, always, of a human is to go out and kill whatever it is that is killing you.

Sharks are being finned and killed, up to 75 million every single year, just for shark-fin soup, and it is devastating the populations.

But, of course the solution is not to go out and kill. We learned that we needed to keep our apex predators to have an ecosystem that works. And we need our sharks in the ocean to keep our ocean healthy.

NARRATOR: In Western Australia, sharks are a recognized risk for an ocean-loving public, but interactions with great whites are no less devastating.

ELYSE FRANKCOM: There's not a day that I haven't thought about it.

NARRATOR: Elyse Frankcom was a snorkeling guide for tourists wanting to swim with dolphins.

ELYSE FRANKCOM: I was diving under the water, and I came up for a breath. That's when I felt this almighty whack. The impact was just like getting slammed by a train. I remember getting pushed out of the water. I looked down, and that's when I knew what had happened.

My first reaction was punch it, push it away.

The shark swam off, and I started sinking. I was unconscious. I remember taking my first breath. As soon as they got me on the boat, that's when the pain kicked in, very, very, intense pain. The shark cut right through my leg right down to the bone.

About 70 stitches on that side and 130-something on that side. I had a shark tooth pulled out of my bone. I had to learn how to walk again.

My mind is starting to play games with me now. I'm starting to dream about sharks a lot more. The longer I stay out of the ocean, the more scared I get.

NARRATOR: The damage inflicted by a large shark can be devastating, but most often, human victims survive the attack. Why?

Steve Huskey is a functional morphologist, specializing in understanding how animals are built and behave to capture prey.

He's studying the case of Robert Gumm who, in 2011, was surfing with friends off the Oregon coast. A great white attacked, slicing this chunk out of his surfboard. Then it disappeared. Gumm escaped without injury.

Was this a test bite? Or a full-on attack that was aborted?

STEVE HUSKEY (Western Kentucky University): My curiosity's peaked, because I want to know exactly what happened here. What does it take to cause that kind of damage to a surfboard?

NARRATOR: To find out, Huskey and colleague Chris Byrne use a machine to measure the forces when a shark's tooth penetrates a surfboard.

STEVE HUSKEY: This is the great white shark tooth?

CHRIS BYRNE: That's right.

STEVE HUSKEY: This is the perfect design for a tooth; nice, triangular shape. That point makes initial piercing, slices its way through, with those razor sharp edges.

CHRIS BYRNE: It looks horrible.

STEVE HUSKEY: It looks beautiful.

Your machine is going to help us determine how much damage and how efficiently this tooth is able to go into that, correct?

CHRIS BYRNE: That's right, right into the surfboard.

STEVE HUSKEY: So we'll have a way of determining the force that was required to puncture a tooth into the surfboard.

CHRIS BYRNE: And determine pressure.

STEVE HUSKEY: We've got pressure right there.

CHRIS BYRNE: That's right.

So I'll bring it down to just where it's about to make contact.

STEVE HUSKEY: That's perfect.

CHRIS BYRNE: Can you hear it?

STEVE HUSKEY: I can hear it snapping fiberglass. Every serration is making more and more of a hole. It sounds excruciating.

CHRIS BYRNE: It's just sawing right through.

Let's take a look at the data. Look how it rises right up to 20 pounds, before it even punctures.

STEVE HUSKEY: So, that's our first puncture is at 20 pounds.

NARRATOR: Twenty pounds of force isn't a lot, a fraction of how hard humans can bite, but focused at the tip of the shark's tooth, it translates into massive pressure.

STEVE HUSKEY: If we look at the extremely exquisite small part of the tooth, taking 20 pounds of force over that much surface area…

CHRIS BYRNE: That tiny little area, the size of a gnat.

STEVE HUSKEY: We're talking thousands of p.s.i.

CHRIS BYRNE: Absolutely.

NARRATOR: A large great white can bite 20 times harder than a human, generating massive pressure on the tip and razor-sharp edges of the teeth, easily slicing into skin, blubber and muscle, like that of a whale or a surfboard.

Huskey's experiment shows that a shark's tooth is incredibly efficient at penetrating its target.

STEVE HUSKEY: What it reveals is that it doesn't take jack to bite through a surfboard.

NARRATOR: And what happened in the attack off the Oregon coast becomes more clear.

STEVE HUSKEY: That was no exploratory sample bite. I have complete faith that that attempt on that gentleman off the Oregon coast was a full attack.

He was coming in for the kill, and then something changed his mind. In this case it's got to be the fact that he bit something that wasn't soft, chewy, fleshy, and didn't taste bloody.

NARRATOR: Huskey believes that sharks have a "feedback loop." When they bite something, taste and touch tell them whether this is a meal or a mistake.

STEVE HUSKEY: Contact on bone is probably something that very quickly turns them away from whatever they're trying to chew on. They don't get that with a seal. We're talking inches of blubber. That's an entirely different motivation, and sensation than is something like a scrawny human.

In the event where somebody survives it, they were rejected. They weren't good enough to be shark bait.

NARRATOR: The job of finding out how sharks sense us, what triggers an attack and how to stop them before they bite falls to Shaun Collin at the University of Western Australia.

SHAUN COLLIN: One of the basic questions we're really interested in is determining why a shark attacks prey versus a human. What are the sensory cues it uses to make that decision?

NARRATOR: Collin's work delves deep into shark biology, to explore what sharks see, hear, smell and sense in ways we're just beginning to understand. His goal: test all the shark deterrents currently on the market and develop new ones based on groundbreaking science.

SHAUN COLLIN: We want to get rid of the guesswork. We want to base it on evidence that we can rigorously test and have statistical basis to rely on to develop these deterrents and bring them to market.

NARRATOR: He's come here, the coast of Western Australia, to a place with a well-earned name,"Shark Bay," to test shark deterrents based on years of research into the sensory systems of sharks.

SHAUN COLLIN: Sharks are a major part of the marine ecosystem. They've been around for 400-million years. They balance the ecosystem. They're sensory machines, which are sampling their environment continuously.

NARRATOR: Sharks have senses like us: sight, smell, hearing and touch. But they have other extraordinary senses, beyond our own. They can detect weak electrical fields, the earth's magnetic field, and minute changes in water pressure caused by passing prey.

Collin's laboratory research focuses on shark vision, especially in the creature that haunts our Jaws-inspired nightmares, the great white.

Great white sharks are ambush predators, cruising deeper water to spot prey above them.

SHAUN COLLIN: They see a silhouette of a human, but that casting of that silhouette, high contrast edges look a lot like a seal, especially if it's on a surfboard or a swimmer at the surface. So, if we can work out how a great white visualizes its prey from below, we've got a very good chance of preventing an attack.

NARRATOR: Collin pioneered the technique of determining what sharks can see and how well, for the first time deconstructing how the great white targets its prey.

SHAUN COLLIN: We've got a great white eye.

KERR: Mm-hmm.

SHAUN COLLIN: Probably an individual about two-and-a-half meters long.

NARRATOR: The structure of shark eyes is similar to our own. With surgical precision, Collin removes the retina from the shark's eye, a thin, transparent hemisphere of tissue, containing light-sensitive cells, called "photoreceptors," and ganglion cells that transmit information to the brain.

SHAUN COLLIN: A bit like peeling an orange.

NARRATOR: The more tightly the photoreceptors are packed within the retina, the sharper the image.

SHAUN COLLIN: Look at that! That's the retina.


NARRATOR: Then, the transparent retina is stained with a purple dye, making the cellular structures easily visible.

Counting thousands of minute photoreceptive cells is a daunting task, made possible with a computer-controlled microscope. The result is a map of the shark's retina, showing the density of photoreceptors in different areas.

SHAUN COLLIN: In this case, the highest densities of cells occur in this region of the retina.

NARRATOR: This part of the eye has the greatest ability to see detail, contrast and movement. That means that, in the great white's field of view, their sharpest vision is of objects above them.

SHAUN COLLIN: This gives us a lot of information about what part of the eye they direct toward eating prey.

NARRATOR: Great white sharks rely on vision to target their prey, but other species rely more on their ability to sense electrical fields or their sense of smell. How can we tell which senses are of greatest importance for different species?

Kara Yopak, a member of Collin's research team, gets inside the shark's head, literally, to find out.

KARA YOPAK (University Western Australia): When you look at a brain, I can make fairly good predictions about what that animal's eating, how fast it's swimming, the general environment that it's living in.

This is the brain of a great hammerhead shark.

NARRATOR: Hammerheads are open-ocean predators, living along the continental shelf in tropical and warm temperate seas, worldwide. They feed mostly on fish.

KARA YOPAK: Its regions of the brain for vision are actually not very large. This is likely not to be a very visual animal.

NARRATOR: Hammerheads rely on other senses, like electroreception, to target their prey, more so than the great white.

KARA YOPAK: Here, we've got the brains of a great white. The regions of the brain that receive visual input are quite large, in comparison to other species, as are regions of the brain that receive smell.

But what becomes very pronounced in the hammerhead is this protrusion, here, at the front of the brain. And this is a region that we've associated with what we call "social intelligence." So, you see enlargement of this area in species that form true schools, that aggregate by sex and size, and that often have complex courtship and mating rituals; whereas, in the great white, that region of the brain is not very pronounced, which leads us to believe that the great white is, in fact, a solitary hunter.

NARRATOR: There's another conspicuous difference between the brains of these two species.

KARA YOPAK: A lot of people are really surprised when they see the brain of a great white, particularly in comparison to the hammerhead, because it looks so small. But there's an amazing range of behaviors the great white's capable of, and it's all controlled by this brain.

So, there's clearly a difference in the senses these animals are relying on. They all have the same battery of sensory systems, but the relative importance of each of those systems is going to be varied between species.

NARRATOR: While these sharks have the same battery of sensory systems, there's clearly a difference in the senses they rely on, reflected by which parts of the brain are enlarged, clues to which senses to target for effective deterrents.

KARA YOPAK: The idea of creating a blanket repellent that's going to repel all sharks in the same way really isn't realistic. But when we're developing a repellent for a great white shark we likely want to target the visual sense.

NARRATOR: If shark's eyes resemble our own, can they see different colors, like us? Collin thought that sharks, like some stingrays, their close relatives would have color vision similar to ours. For fish and humans color vision is made possible by three different types of cells in the retina, called cone cells, each one responding to a different color: red, green or blue.

In Shaun Collin's lab, Nathan Hart uses a machine that measures what color light the shark's photoreceptive cells are sensitive to.

NATHAN HART: You can see here the beam as it scans through from the U.V. to the red.

NARRATOR: What he finds, in the shark species they've examined, including great whites, is that they only detect one color of light, in the green part of the spectrum. Everything they see is in shades of that color.

For us, with no other color to compare, it's like black and white.

SHAUN COLLIN: We were very surprised to find out that sharks were colorblind.

Two out of the three cone photoreceptors were missing, so, we only found a single cone receptor. Now, this means they don't have the machinery within the retina to process color.

NARRATOR: Collin believes this inability to see color and a reliance on high contrast are keys to creating an effective deterrent for many species of shark.

The great white shark attacks in Western Australia were an unusual series of events. Collin and other shark researchers want to know what factors could have triggered the attacks, and if there's a way to stop sharks before they bite.

Shaun Collin is here, in Shark Bay, Western Australia, to test shark deterrents that are already available to the public and new ones he and his team are developing.

SHAUN COLLIN: We've worked on lots of smaller sharks in our laboratory tanks at the university, but we've got to make that progression from the lab to the wild.

NARRATOR: These metal frames are designed to hold a bait canister to attract sharks, along with the shark deterrent to be tested and a pair of cameras.

SHAUN COLLIN: The idea of having two cameras is that we have two different perspectives of the behavior of the sharks.

NARRATOR: Collin's team preps the first test with a device that emits an electric field powerful enough to shock a human, if the electrode is touched.

SHAUN COLLIN: This type of deterrent has a long electrode, which is this lead here, which extends for a couple of meters, typically, behind a diver or the wearer, and, once turned on, creates an extensive and fairly strong electric field around the device.

This is the other type of commercially available electric device that we're going to be testing. It's an anklet-style device, again, worn by divers or swimmers.

NARRATOR: Bait canisters are filled with a mixture of smelly, bloody, ground fish and the electro-repellent rigs are deployed. Each rig is anchored to the bottom, and suspended in the water column by floats.

They'll be left for two to three hours, enough time, they hope, for the bait to attract a shark.

This is what happens when these rigs are deployed without deterrents: bites that would inflict serious wounds, but which will be stronger, the scent of food, or the electric deterrent?

While the frequency of some electric fields seem to repel sharks, others attract them. So which frequencies make animals into targets, and which might protect us?

Ryan Kempster has developed techniques in the lab to find out.

Bamboo sharks, small and easily kept, hunt for prey hiding under the sand, given away by the electrical impulses of their muscles.

RYAN KEMPSTER (University of Western Australia): So, we know that all organisms give off bioelectric fields, and these can, essentially, be measured.

NARRATOR: With every muscle movement, even the beating of a heart, an electric field is generated.

RYAN KEMPSTER: The actual size of the organism itself determines how far that electric field extends in the water column. A larger animal's going to give off a larger electric field.

NARRATOR: With electrodes attached to this acrylic plate, which he buries under the sand, Kempster simulates the tiny electric field of the shark's prey, a small crustacean or fish. With a camera mounted above the tank, he records when the shark bites at the electrodes, indicating it has sensed the electric field.

So, just how sensitive is the bamboo shark's electro-reception?

RYAN KEMPSTER: They can sense down to around one nano-volt. Essentially, this is a billionth of a volt, so it's an incredibly low electric field. So, they're really, really very sensitive to the bio-electric fields of their potential prey.

NARRATOR: Kempster measures the strength of the electric field and the distance at which the sharks can sense it.

RYAN KEMPSTER: Oh. There we go. That was a good one. You can really see, it's kind of this zone, here, where they have to get within to react.

It's really a very close-range sense. They would typically use their other senses to hone in on where the prey is. Then it's really that last minute feeding strike that they would use their electroreception to assist them to locate that prey.

Oh, yep. There you go.

NARRATOR: Electroreception is a short-range sense. It's only when a big shark gets close that its electro-sense locks onto a living target.

Electroreception can help sharks find prey, but it can also warn them of danger. Even these embryonic bamboo sharks can respond to an electric field that simulates the approach of a big fish.

RYAN KEMPSTER: We can see the shark breathing away, very rhythmically. Its tail is moving in the egg case to help flush oxygen around the egg. But then, as soon as we turn on that electric field,…

MACHINE VOICE: Electrode on.

RYAN KEMPSTER: ...it stops breathing. It stays really still; tail goes really still, and this is essentially a response to that electric field that's mimicking the predator in the water.

MACHINE VOICE: Electrode off.

NARRATOR: Kempster has found the electric field that disturbs bamboo sharks, but is there a specific electric frequency that will turn large sharks away?

He thinks that finding the right frequency of an electric field is crucial to deterring dangerous sharks like the bull, tiger and great white.

RYAN KEMPSTER: And this is really key to determining how to create an effective repellent, in finding out exactly what that frequency is that they're most repelled by.

NARRATOR: Back on Shark Bay, the team tests other deterrents, based on their research into shark vision. This one is a flashing light.

They'll transfer the camera's footage to the computers to watch later.

KERR: There you go, Dave. Here is the cameras from Rig 27.

This is the strobe lights.

DAVE: Brilliant. Let's see if we have had some luck this time.

NARRATOR: In the meantime, they've got another deterrent to test: scientifically designed patterns for wetsuits.

NATHAN HART: This is designed to be camouflaged under water, certainly to the visual system of the shark.

NARRATOR: As Collin and Hart discovered, sharks are colorblind. They see the world in shades of black and white. This material is designed to fade into the background, as the light changes at different depths, each color tailored to what a shark's eye sees.

SHAUN COLLIN: So the idea is, the shark just swims by and has trouble seeing it.

NARRATOR: A second design is very different.

NATHAN HART: We also have something which has been suggested for a number of years to be a good shark deterrent, and that's a pattern that mimics a sea snake's coloration.

NARRATOR: Sea snakes are highly venomous, and they advertise their toxicity with bold patterns that warn predators to stay away.

SHAUN COLLIN: Most sharks don't like eating things that are striped, such as a sea snake. It actually is a noxious animal to them.

NARRATOR: Collin thinks that many sharks naturally avoid stripes. Again, the team's research into what sharks can see is crucial to recreating the sea snake design in the right proportions.

SHAUN COLLIN: We had to actually work out the spacing of that pattern, the bars or the stripes, by our knowledge of the spatial resolving power or the resolution of the eye.

NARRATOR: The underwater cameras run continuously. For the scientists, finding that instant when a shark approaches means scanning hours of footage.

Finally, their patience is rewarded.

RYAN KEMPSTER: That's cool.

NARRATOR: A bronze whaler, a shark known to attack humans, has found the bait.

This test rig has the flashing light as a visual deterrent.

SHAUN COLLIN: That's nice, isn't it? He has got a little remora following him on his dorsal flank.

NARRATOR: The shark makes five passes at the bait, never getting too close, each time turning away.

These sharks are opportunistic feeders, clearly interested in the bait, but the flashing light seems to keep this one at a distance.

SHAUN COLLIN: Well, it is certainly being deterred.

NATHAN HART: We can probably analyze this in slow motion, as well, so you can time when the light is on with how the shark reacts.

NARRATOR: Next up, video footage from the test rig with the anklet-style electrical deterrent.

KERR: Oooo!


NATHAN HART: Look at that!

SHAUN COLLIN: Look at that! God!

NATHAN HART: It is a hammerhead.

NARRATOR: Hammerheads hunt with electroreception and they seem especially sensitive to electrical fields.

SHAUN COLLIN: Wow! He came straight in, right towards the bait and then almost did 180-degree turn straight away.

NATHAN HART: Yeah, looks like he was deterred all right.

SHAUN COLLIN: This is where the device was turned on, with the electric field extending out. That was amazing.

NARRATOR: Finally, the bold, sea snake wetsuit design seems to turn another hammerhead on its tail.

SHAUN COLLIN: It reacted almost instantaneously. It did the same behavior. Didn't get as close this time. It was deterred from a greater distance from the actual device.

NATHAN HART: Definitely.

NARRATOR: Their research seems to be paying off, but to test their ideas on the most dangerous sharks, they need to be in the right place at the right time.

SHAUN COLLIN: Despite the spate of attacks that have happened over the last year or so, it is still inherently hard to find large predatory sharks off our coastlines.

NARRATOR: For now, there is no one answer; no surefire way to protect humans from attack.

SHAUN COLLIN: We need more sharks interacting with our deterrents before we can really make any judgments about how effective they are.

NARRATOR: Halfway across the globe, on Cape Cod, Greg Skomal continues to tag and monitor great whites as they come close to the beaches where people swim. Chief Ranger of the Cape Cod National Seashore, Leslie Reynolds, and head lifeguard Keith MacFarland, are on alert, in a place where great whites are a new threat.

LESLIE REYNOLDS: People are intrigued by the white shark. They want to learn about them. They would actually like to see one, from the beach.

NARRATOR: Researchers are going further than ever before to understand how sharks sense their world and hunt for prey.

SHAUN COLLIN: There's no holy grail that we can see in front of us yet.

They are wild animals; they're hard to study, but we really do feel that we can deter them from our beaches.

NARRATOR: That's the goal that these scientists are racing to achieve, to better understand these powerful predators and try to save the lives of both humans and sharks.

Broadcast Credits

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Special Thanks
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Produced by Prospero Productions in association with

SeaLight Pictures for NOVA

Produced with the assistance of ScreenWest, Lotterywest and Screen Australia

© 2013 Prospero Productions, ScreenWest Inc. and Screen Australia

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Additional Material © 2014 WGBH Educational Foundation

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This program was produced by WGBH, which is solely responsible for its content.


Image credit: (Great White Shark)
© Stuart Westmorland/CORBIS


Wendy Benchley
Shaun Collin
University of Western Australia
Steve Huskey
Western Kentucky University
Ryan Kempster
University of Western Australia
David Pickering
Snorkeling Tour Guide
Leslie Reynolds
Cape Cod National Seashore
Greg Skomal
Massachusetts Division of Marine Fisheries
Kara Yopak
University Western Australia

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