Original subtitles

Narrator: For centuries,

native hawaiians believed sharks

were the reincarnated spirits of their ancestors,

sent to protect them.

But that was the past.

(scream)

man: There's giant, just red clouds of blood

just bursting everywhere.

Narrator: Nine mysterious shark attacks

on the big island in three years.

Man: I was very close to dying.

Narrator: Investigators are on the case

of this terrifying anomaly.

Man: This spike is highly unique.

And we really need to figure out what's going on.

Narrator: And what they reveal could send shock waves

across every coastline on earth.

Man: Whew. I just got attacked by a tiger shark!

♪ ♪

♪ ♪

braxton rocha: So, my gear for the day.

I got my speargun, what we need to get the fish with.

I've got my wetsuit, got my mask,

and I got my fins for the day.

Narrator: Braxton rocha has been spearfishing

off the island of hawaii since he was a child.

Braxton: I always had a strong connection with the ocean.

I just love to be in the water

to put food on the table for my family.

Narrator: The big island is the biggest landmass

in the hawaiian chain.

It's here that sharks reign supreme.

40 different species prowl the water,

like oceanic whitetips, hammerheads, silkies,

and the elusive megamouth, which can grow to 18 feet long

and live at depths of up to 15,000 feet.

Historically, the presence of these predators

hasn't been an issue.

That is, until now.

Braxton: Never in my life did I ever think

that I'd ever get, like, bit by a shark.

♪ ♪

narrator: Braxton and his friend shannon

are enjoying an afternoon of spearfishing.

Braxton: We were just going after, like, parrotfish

or like koles, it's like a, like a black type of fish out here.

It was a really good time.

Narrator: The pair shoot several fish

and then consider heading back.

But braxton isn't yet happy with the day's catch.

Braxton: I was like, "nah," you know, "one more fish."

maybe I was being a little greedy that day,

and so I went back out.

Narrator: It's a fateful decision.

Just minutes later, an ominous shape emerges from the depths.

♪ ♪

braxton: I've seen sharks, like, my entire life,

but this one, I mean, it like, it literally, it blew me away.

It was the size of, like, a truck.

I was just like, just like in awe.

And then it happened.

(muffled scream)

narrator: The predator bites down on braxton's leg.

Braxton: All I could see was just teeth, just rows of teeth.

And there's, like, giant, just red clouds of blood

just bursting everywhere.

Narrator: In desperation,

braxton punches the shark in the nose.

Mercifully, it gives up.

Braxton: She disappeared and I was just telling myself,

you know, "maybe she might come back."

narrator: The injured diver races for shore.

Braxton: I was like 80, 90 yards out.

I was swimming as hard as I can.

I was just, like, freaking out the whole time.

Narrator: Finally, he makes it to the rocks.

Braxton: And I'm just, like, looking down at my leg

and it's like, it's like, split open,

looks like I got, like, blown up by, like, a bomb

in, like, in the war or something.

I was just, like, I could see everything in my leg.

I could see my bone, like, everything, like, moving.

And I'm just, like, freaking out, like, "I'm gonna die."

narrator: But while the reality of the situation sinks in,

braxton realizes that his dive partner shannon

is still in the water.

Braxton: And I'm like, "shannon! Shannon!"

I started screaming his name as loud as I can.

"shannon! Get out! Shark!"

and he finally sees my leg, and he's just like,

he starts crying, he's like, screaming.

He's like, "no! No!"

and I'm like, "bro, I'm gonna die right now.

I'm (bleep) dying."

narrator: Braxton is rushed to the hospital.

Braxton: Whew. I'm going to the hospital.

Wish me luck, guys.

Whew!

Narrator: It takes nearly 100 staples

to close up the diver's shredded leg.

He knows he's lucky to be alive.

Braxton: My surgeon had told me, if it went any lower

in the inside of my leg right here,

it would've caught my main artery,

and I never would've made it back to shore.

And it's all I ever think about is, like, how blessed I am.

(muffled scream)

narrator: Attacks are normally a rare occurrence

on hawaii's big island.

But braxton's encounter is actually

part of a much bigger problem.

Since 2013, shark bites have skyrocketed.

In fact, he's the seventh victim

in just over a year and a half.

And he won't be the last.

In the next three months,

two more people are savagely bitten.

Nine in total

over a three-year period.

Triple the average

for the past decade.

For marine biologists like terry lilley,

it's a disturbing trend.

Terry lilley: In the past here in hawaii,

we had very few shark attacks.

I'm seeing this spike in shark altercations with humans go up

over the last few years, to the point I've never seen that

in my entire lifetime of being in the ocean.

This is highly unique.

Narrator: The cause must be found,

or the safety of every beachgoer could be at risk.

Terry: We may have a much bigger problem.

There's going to be more shark bites,

so that's incredibly concerning to me.

Narrator: So what's behind this alarming surge of attacks?

The first thing investigators must determine

is the type of shark involved.

And there are a number of possible suspects.

Hawaii is home to the three species

most frequently implicated in attacks on humans.

The massive great white, the ferocious bull shark,

and the distinctive tiger shark.

Could one of these infamous predators

be responsible for the rise of attacks?

One way to find out is to examine the victims' injuries.

Dr. Dan huber from the university of tampa shark lab

investigates.

Dan huber: Right now we're looking at attack photos

from braxton rocha.

From these photos, we can see a severe laceration

to the victim's left thigh.

And then less severe lacerations

to the middle and lower portions of his left calf.

This would indicate that the victim was attacked by a shark

with a very, very broad curvature of the jaw,

and the only shark that matches that in coastal hawaiian waters

is the tiger shark.

Narrator: Tiger sharks get their name

from the vertical stripes on their bodies.

Their powerful jaws mean they can rip through

the shell of their favorite prey: Sea turtles.

These imposing predators are one of the most common species

of shark in hawaiian waters.

And when the data is analyzed,

it's clear they play a key role in the spike.

Dan: Looking at reports from victims,

witnesses of the accounts, and some forensic evidence,

it would suggest that most if not all of the attacks

that occurred between 2013 and 2015,

were caused by tiger sharks.

Narrator: Despite their ferocious reputations,

tiger sharks generally leave people alone.

So why have they suddenly started attacking

unsuspecting beachgoers?

According to local experts, in 2013,

reports began to surface that could indicate why.

Shark sightings near shore appear to be on the rise.

Terry: In the last couple years,

I'm seeing big tiger sharks

right in the surf zone where we're surfing.

And this puts them into more contact with us

out in the water, surfing, swimming and diving.

And there's going to be more accidental shark bites

because of that.

Narrator: Tiger sharks congregating in the shallows;

it's a terrifying revelation.

So why do these distinctive predators

suddenly seem like they're showing up near shore?

Local fisherman rick reger thinks

this frightening occurrence might be a manmade problem.

Rick reger: This is where they like to be,

right in the, the entrance

and just right in this little basin right here.

Narrator: This is honokohau harbor.

In the late 1960s, the us army corps of engineers

blasted its 20-foot basin out of solid volcanic rock,

turning it into a vital hub for fishing.

But the tiny port shelters more than just boats.

Near the harbor entrance is a rich coral reef ecosystem

that's teeming with fish, manta rays,

and green sea turtles.

But that's not all.

Rick: We see tiger sharks very, very frequently in this area.

Narrator: This video was taken in 2019.

It shows a massive tiger shark inches from land.

Man: Oh, coming straight up for you.

Narrator: What's more, honokohau harbor is not alone.

Similar boat basins have sprung up all over the big island.

And according to experts, they're full of sharks.

Man: Nine-foot tiger shark!

Narrator: So could the bountiful habitats

created by these harbors be drawing tiger sharks

closer to shore and into areas where humans swim?

It's a scenario that's played out before.

In recife, brazil,

an incredible string

of 56 incidents over 20 years

has been tied to the presence

of a nearby harbor.

So could the same thing be happening in hawaii?

According to dan huber, it all comes down to geography.

Dan: Right now, I'm looking at a map of the big island of hawaii.

And we're gonna put onto the map

the locations of all of the harbors around the big island.

You can see that there's five of them.

Now on top of that, we're gonna lay on the locations

of the nine attacks that occurred between 2013 and 2015.

Now, two of these attacks happened in proximity

to the harbors,

but the other seven attacks are spread all around the island,

some of them very far from the harbors.

So it's not likely the presence of the manmade harbors

was a contributing factor to this spike in attacks.

Narrator: These manmade ports are not to blame.

But perhaps there's another reason

sharks could be congregating near the shore.

An incredible video taken in 2019

could point to what that might be.

Something enormous is lurking off the coast of hawaii.

In fact, it's the largest predatory fish...

Ever seen.

Narrator: The jaws of a tiger shark can slice through

the shell of a sea turtle like a chainsaw.

But between 2013 and 2015, on hawaii's big island,

they turn those ferocious teeth on beachgoers.

Nine vicious attacks have left

experts grappling for answers.

But an increase in sightings of the predators near shore

could be a vital clue.

Terry: In the last couple years, I'm seeing big tiger sharks

right in the surf zone where we're surfing.

Narrator: So why might these master hunters

be congregating in the shallows?

And could this increase in shark traffic

explain the attacks?

In 2019, a monstrous piece of evidence emerges from the deep.

Off the coast of maui, a group of snorkelers is investigating

the carcass of a sperm whale,

when they encounter something extraordinary.

(muffled speech)

a 20-foot-long female great white shark,

known as deep blue.

♪ ♪

this storied giant

is the largest of her kind ever recorded.

She was first spotted in 2013,

and her appearance was a global sensation.

Could her intimidating presence in these waters

be related to the observations of the tiger sharks,

which are smaller in stature, closer to shore?

In deeper waters, great whites and tigers

have sometimes been known to seek out similar food sources,

particularly whale carcasses.

So what happens when sharks of different sizes

are forced to compete for food?

At long island aquarium in new york,

curator joe yaiullo

and shark keeper roberto alfieri investigate.

Inside this 120,000-gallon tank are four sand tiger sharks

of various proportions.

Roberto alfieri: She is the bigger of the females.

Then we have bumps over there, is another female.

The two male are smaller in size.

Narrator: The team hopes to observe

how these different-sized sharks interact

when vying for a single food source.

To do so, roberto places a fish on the end of a feeding pole.

Joe yaiullo: Yeah. Well, there's one.

So was that, that was bertha on the first one?

Roberto: Yeah.

Narrator: Bertha is the biggest shark in the tank.

Joe: Bertha got pretty excited about the food

and also grabbed part of the, the feeding pole itself.

Narrator: The pattern is immediately clear.

Roberto: Bertha.

Joe: That was bertha.

Narrator: The largest shark is crowding out the competition.

Joe: So bertha is the biggest shark in our tank,

and she is pretty much the one in charge.

So she eats first, and the other sharks stay on the periphery

until it's their turn.

Narrator: Joe's experiment shows

that in the world of sharks, bigger really is better.

Joe: Same thing in the wild,

if there's a whale carcass or a source of food

that's attracting a number of sharks,

the bigger great whites are gonna eat first.

Narrator: So is something similar playing out

in the waters off hawaii?

Could deep blue be hoarding prey,

and in turn, forcing the tiger sharks

to search for food closer to shore?

It seems possible.

But as investigators dig deeper,

they find a problem with the theory.

Between 2013 and 2015,

observations show deep blue wasn't in hawaii at all.

In fact, the instantly recognizable behemoth

was spotted numerous times thousands of miles away

off the coast of mexico.

If tiger sharks are showing up in the shallows

in greater numbers, then it's not because of deep blue.

The reason for the attacks remains elusive.

But while scouring the ocean for an explanation,

experts come across something else that also ensnares

large amounts of a tiger shark's prey.

But it's not another sea creature.

In fact, it walks on two legs.

Naia rae fox: I think the reason why we are having

more sharks here actually is because the amount of fishing

that's been going on.

Narrator: This is one of the island's

most popular activities: Sport fishing.

Every year, recreational anglers haul in

more than 2 million pounds of fish.

That means that on any given day off the big island,

there are hundreds of fish being caught and reeled in.

And for scientists like dr. Dan huber,

these hooked sea creatures may be attracting sharks.

Dan: A struggling fish as its body is twitching a bit,

that motion is giving off low frequency sounds.

And shark hearing happens to be tuned in

to low frequency sounds.

This is definitely something that could attract

a number of sharks to the same area.

Narrator: So could sport fishing be drawing tiger sharks

into the shallows,

thereby increasing the risk

of an accidental run-in with people?

(scream)

according to fisherman rick reger,

it comes down to where the best sport fishing takes place.

To demonstrate, he heads out to an area

well known for the popular pastime.

Rick: Okay, so this is our bottom here.

We have some of the steepest drop-offs

in the whole world right here.

This is where you're gonna see your tuna, your mahi-mahi,

your ono, your marlin.

Narrator: This location could hold a vital clue

to solving the mystery.

Called the pelagic zone,

this open-water abyss extends thousands of miles

between the continental shelves.

It's home to an array of large sea creatures:

Whales, manta rays, big game fish,

and some of the most dangerous sharks on earth.

Narrator: The island of hawaii is plagued by shark attacks.

(scream)

nine horrific encounters...

Braxton: Whew, I'm going to the hospital.

Narrator: ...Between 2013

and 2015 have left locals

and tourists in fear.

The bites appear to be the work of tiger sharks.

The cause is a mystery.

Some believe that these dangerous predators

are being lured into shore by sport fishing.

Fishing guide rick reger is mapping the island's

undersea geography to demonstrate exactly where

this activity takes place.

Rick: Right now we're at about 30,000 feet.

See, we just lose the bottom here.

Narrator: This steep drop-off is world-renowned

for its sport fishing, but it also attracts sharks.

Rick: We'll see oceanic whitetips, blue sharks,

galapagos sharks, mako sharks.

And at times, I have seen these tigers.

Narrator: So, could the presence of sharks

in the same waters popular with sport fishing

explain the attacks?

Rick is skeptical.

Rick: Right now, we're at about four miles offshore

not anywhere near where people are swimming.

A completely different area.

Narrator: It seems that sport fishing takes place

too far from shore to be implicated in the surge.

But perhaps by examining the deepest parts of the ocean,

investigators are getting closer to the truth.

And when they review the second attack in the spike,

there is one key detail lurking in the depths.

James kerrigan: So this is mahaiula beach.

You got to probably drive down a road like this

for about half an hour to get to it.

Narrator: James kerrigan came to mahaiula beach

with his wife mary in 2013.

James: So these lava fields were formed hundreds of years ago

from an eruption from mauna loa.

It's crazy how you just go from this barren, remote,

you know, empty lava field,

and then all of a sudden you walk through these trees,

and you're on this idyllic polynesian beach.

It's forever tainted for me.

I can never see it the same way again.

♪ ♪

it was sunny, probably a good 85 degrees, low humidity.

The kind of day you dream about, you know.

Narrator: Around 1:00 pm, james and mary dive into the ocean.

♪ ♪

♪ ♪

james: I just remember swimming and having a great time.

My wife was a swim instructor when she was younger,

so she was teaching me how to do the side stroke.

Narrator: But the lesson is about to come to an abrupt end.

♪ ♪

♪ ♪

a shark sinks its teeth into james' leg.

James: I felt pressure on my leg, and I felt a pull,

and all of the sudden my head was underwater.

Everything was kind of a blur.

Narrator: Suddenly the shark lets go.

James: I was bleeding a lot,

and we just started swimming as fast as we could.

Narrator: Bystanders drag james onto the sand.

James: The wound was pretty bad.

Another guy came and wrapped my leg all up in a towel.

There was a lot of blood.

Narrator: One of the good samaritans calls 911.

James is airlifted to a nearby hospital

where he's treated for multiple lacerations to his leg.

♪ ♪

james: This was the deepest one

that went right down to the bone,

and then I've got another one right up here.

So that kind of gives you an idea

how big the shark's mouth is.

♪ ♪

looking back, I was very close to dying.

Narrator: James' encounter is harrowing,

but it does contain a detail

that turns the investigation on its head.

James: So these lava fields were formed hundreds of years ago

from an eruption from mauna loa.

Narrator: The big island is home to four active volcanoes,

including one of the most explosive on the planet.

Could this subterranean suspect

be behind hawaii's sudden spike in shark attacks?

Narrator: Hawaii's magnificent volcanoes

are a draw for millions of tourists.

But few realize that they could play a part

in a series of vicious shark bites.

(scream)

between 2013 and 2015,

the big island is hit

with nine attacks.

Three times the average for the previous decade.

In each case, the perpetrator is believed to be

a tiger shark.

Dr. Andrew rossiter from the university of hawaii

explains this unexpected link between sharks

and the island's most unique feature.

Andrew rossiter: Some believe that tiger shark attacks

may be related to increased volcanic activity.

Narrator: On the big island,

there are four active volcanoes,

including what some consider to be

the most volatile in the world, kilauea.

This 4,000-foot-tall colossus

has been continually erupting since 1983.

And all that molten rock has to go somewhere.

Andrew: The magma comes up to the surface, lava is formed,

and the lava rushes down and goes in the sea.

Narrator: The lava can reach temperatures

of nearly 2,000 degrees fahrenheit.

And when this scorching hot torrent hits the ocean,

it creates a plume of heated water

that can extend more than a mile offshore.

To some experts, this oceanic hot zone

could help explain the spike in attacks.

Andrew: Volcanic fields may lead to the presence of sharks.

Narrator: In 2015, off the solomon islands

in the pacific ocean,

a team of researchers studying underwater volcanic activity

captured video of something extraordinary.

In the heat of the spewing lava,

the scientists thought all they would find would be bacteria,

but they also found...

Sharks.

Dubbed "sharkcano,"

the discovery makes headlines around the world

and suggests that sharks cannot only withstand

these extreme environments, but may even be drawn to them.

Dan: We know that warm water brings nutrients,

and what that means is that lots of marine life

will gather in those areas.

Because of this, tiger sharks might look for prey

in pockets of warm water such as those caused by lava flows

around the hawaiian islands.

Narrator: But for tiger sharks, warm water has another effect.

Dan: Tiger sharks are an ectothermic species,

which means that their body temperature is determined

by the temperature of the surrounding environment.

All aspects of their physiology are gonna change

with the water temperature.

When the water is warmer,

it's gonna increase their metabolic rate,

which will also increase their behavior.

This can cause the sharks to feed more often.

Narrator: So, not only could heat-seeking tiger sharks

be clustering near these volcanic outflows,

but they might also be more active,

making them more likely to bite beachgoers.

And in 2013, the year the spike began,

kilauea was especially active.

Dr. Dan huber explores the theory.

Dan: What we've got here is a map of the big island of hawaii.

And I'm gonna bring up on it the location of a lava flow

that occurred in 2013.

But now as we bring up the locations of all of the attacks,

we see that the attacks are spread around the entire island,

which would suggest that the lava flow in 2013

could not have been responsible for the spike in attacks.

Narrator: Volcanic activity

isn't drawing more sharks into shore.

But what this theory does demonstrate

is that environmental factors may cause tiger sharks

to hunt more often, making them more likely to bite humans.

This could point to a simpler

but no less frightening explanation

for the reports of tiger sharks in the shallows.

Dan: Tiger sharks often hunt in shallow coastal environments.

So if they're hunting more often,

in theory, this could be the reason why

there's reports of more sharks in the water

in the time surrounding the spike in shark attacks.

Narrator: So investigators begin to focus

on one critical question:

Is there something else that could send sharks

on a hunting spree?

The fifth attack in the spike offers a clue.

♪ ♪

brian wargo: I've been in hawaii for 24 years.

You don't get rich living out here,

but you have a very rich lifestyle.

Narrator: Brian wargo is a professional fisherman

and avid surfer.

One of his favorite surf spots on the island

is a remote stretch of coastline called kauhola point.

Brian: It's a nice surfing area.

But once you're out there and you jump off the beach,

you just never know what's gonna happen.

♪ ♪

it was a typical hawaii day.

It was, you know, 75 to 85 degrees out.

Sunny.

Nice wind conditions.

The water was clear.

There's quite a few people there.

Narrator: Brian is surfing with his friend mckenzie clark.

Brian: The first set came in.

I paddled for one,

but I missed the wave.

Narrator: It's then that brian sees

a surfer's worst nightmare.

(scream)

brian: I heard mckenzie screaming.

I turned around and saw her up out of the water

about two to three feet on top of the shark's head.

♪ ♪

narrator: Brian wargo is surfing with his friend,

mckenzie clark,

when the unthinkable happens.

(scream)

a tiger shark strikes mckenzie from below.

Brian: It basically looked something like

out of a jaws movie.

Narrator: The shark's teeth slice through

mckenzie's surfboard and into her hand.

Brian rushes to her aid.

Brian: She ripped her hand out of his mouth.

I just saw the blood go everywhere.

(screaming)

narrator: But mckenzie isn't free yet.

The shark attacks again.

(scream)

brian: She's coming up and screaming,

trying to get her breath,

and then getting pulled back down by the shark.

Narrator: In desperation, brian takes drastic action.

Brian: I grabbed the shark's dorsal fin,

trying everything to try to stop the shark from getting to her.

I got one hand free

and I started trying to punch the shark.

Narrator: Brian hits the shark a total of five times.

Finally, it releases.

Brian: One finger was hurt pretty bad,

and there were lots of puncture wounds throughout the hand.

(siren)

narrator: Mckenzie is rushed to a nearby hospital.

She's treated for serious injuries to her left hand

and eventually makes a full recovery.

But for her savior, brian, the trauma still lingers.

Brian: Being there and seeing it has definitely changed my life.

Many sleepless nights afterwards, you know,

post-traumatic stress syndrome

was in for quite some time and still is.

Narrator: Mckenzie's attack is just one part

of a troubling trend.

On the big island of hawaii,

there have been nine savage

encounters with tiger sharks

in just three years.

Investigators suspect that something may be causing

tiger sharks to hunt more often.

The question is...

What?

An answer may be hidden in the details

of brian's harrowing ordeal.

(scream)

the shark that attacked mckenzie came at her not once,

but three times.

Dan: The vast majority of shark attacks involve a single bite,

after which the shark leaves the victim alone.

But in this case,

the shark bumped mckenzie off of her surfboard,

and then bit repeatedly onto the board and onto her hands,

and even kept biting while mckenzie's friend

was repeatedly punching the shark.

And this indicates that the shark was extremely aggressive.

Narrator: Investigators wonder what could push tiger sharks

to behave so violently.

The answer might explain why the predators

seem to be hunting more often.

Marine biologist leighton taylor is an expert

on the significance of sharks in hawaiian culture.

He thinks a clue may be found not in science,

but in an ancient legend.

Leighton taylor: The hawaiians' creation myth,

the kumulipo, is famously coupleted

between the ocean and the sea.

For every land creature, there's a sea creature.

Narrator: According to dr. Taylor,

one of these pairings could help solve the riddle

surrounding the spike in attacks between 2013 and 2015.

It links one of the island's most prominent plant species

and its most ferocious predator.

Leighton: There is a famous proverb,

"the shark bites when the wiliwili blooms."

narrator: The adage refers to the wiliwili tree,

an indigenous species whose name means

"repeatedly twisted" in hawaiian.

For centuries, the native hawaiians revered the plant

for its colorful and distinctive flowers,

which bloom every fall.

Braxton: Out here in hawaii, people like to believe

that when the wiliwili tree out here is blooming,

that's when the tiger sharks are a lot more active

and more aggressive.

Narrator: So is the ancient proverb right?

Are tiger sharks more likely to bite in the autumn?

Dr. Dan huber thinks there could be some truth

to this timeworn tale.

Dan: When you look at the data from the attacks in hawaii

between 2013 to 2015,

most of the attacks occurred from September to December.

Narrator: A critical piece of the puzzle

has fallen into place.

The majority of the bites did indeed take place

when the old adage predicts

that tiger sharks might be more aggressive.

But what's behind this terrifying change in behavior?

The answer to this deadly riddle could send shock waves

across countless beachside communities worldwide.

Narrator: Between 2013 and 2015,

the island of hawaii is in the grip

of a disturbing string of shark attacks.

(scream)

nine incidents.

In each case, the most likely

culprit is a tiger shark.

Investigators have zeroed in on an ancient proverb

that suggests that this ferocious predator

might be more aggressive in the fall months,

the very same time period as many of the attacks.

Could the old adage be right?

And if so, what could cause

this terrifying change in behavior?

♪ ♪

biologist dr. Kevin weng is on an expedition

to catch and study tiger sharks in the region.

The results of this research could finally reveal the truth.

And they've just caught their first subject,

a 14-foot female.

Kevin weng: Still swimming, swimming good.

Narrator: Kevin and his team plan to take

various measurements and samples from the massive shark.

Kevin: Let's get the floats off, but then you wanna probably

leave that amount of line in the water.

Okay, I'm gonna move back, move back.

Dan: Yeah, go ahead. Kevin: Okay.

Narrator: Before the data is collected,

they must first immobilize this potentially dangerous predator.

Kevin: Okay, I'm gonna move up, dan.

Are you guys ready with the tail rope?

If we can get the animal flipped upside down for a few minutes,

typically it will become very, very calm.

♪ ♪

you got it? Okay.

It's a phenomenon known as tonic immobility.

Narrator: Tonic immobility is a reflex

seen in many invertebrates, including some sharks.

For reasons still not fully known, when turned upside down,

a tiger shark will enter a coma-like state,

similar to hypnosis.

This allows researchers to collect data

without harming the animal or being bitten.

Kevin: Hannah and brooke are using

a medical ultrasound machine to look inside of this animal.

Narrator: Before long, the team finds what they're looking for.

Kevin: Did you see any eggs?

Hannah: Yeah.

Kevin: Cool.

Narrator: The captured shark is getting ready to reproduce.

She could soon be carrying

anywhere from 8 to 80 pups inside her womb.

Studies like this one have revealed stunning information

about tiger shark breeding habits.

Kevin: Based upon the available evidence,

it looks like mating is happening in the wintertime

in January, February, kind of a period.

And the giving birth to the pups is happening in the fall.

Narrator: So could pupping be causing tiger sharks

to attack bathers?

According to dr. Weng, an explanation might be found

in the toll the gestation process takes

on the pregnant shark.

Kevin: They are growing up all of these young

inside of their uterus.

That's a big job that takes a lot of energy.

Narrator: By the time their pups reach full term,

a tiger shark's energy levels are near exhausted.

And so after they've given birth,

these tired mothers must eat.

Kevin: You could infer from having lower energy stores,

they might be more motivated to feed.

Narrator: As a result,

these hungry mothers might hunt more often

and may be more aggressive,

meaning that violent encounters with humans

would be much more likely.

But that doesn't fully explain the spike.

In hawaii, tiger sharks have been giving birth

during the fall for centuries.

So what's changed?

The final piece of the puzzle

lies with hawaii's number one industry: Tourism.

2013 through 2015 saw a dramatic rise

in the number of visitors to the big island.

A nearly 10% increase over the previous decade.

That's thousands more swimmers, surfers and kayakers.

According to dr. Dan huber, it's a recipe for disaster.

Dan: So the combination of more people in the water

and tiger sharks hunting more often

to replenish their energy reserves after giving birth

increases the chances of shark attack.

Narrator: It seems that tiger shark pupping,

combined with record numbers of tourists,

are the two likeliest reasons behind the spike in attacks

on the big island from 2013 to 2015.

These revelations could have global implications.

While the reproductive cycle of tiger sharks

is now well understood in hawaii,

elsewhere in beach communities such as florida or the bahamas,

much less is known.

And so the aggressive behavior of this fearsome predator

cannot be predicted.

It's an ominous warning.

But for the residents and tourists of hawaii,

it's another sign that our oceans

must be treated with both caution and respect.

Terry: Sharks are kind of an indicator species.

We need to spend more time and effort studying

to see what's going on out there.

Braxton: We're quick to blame sharks for attacking us

or being the cause of attacks.

Always know that that's their home

and that you're a guest when you're entering the ocean.

Captioned by side door media services