[tense music playing]
[male narrator] This is the biggest pack of gray sharks on Earth.
Seven hundred of them.
These awesome predators gather under the full moon
for a single purpose:
to feed.
Now, a team of scientists is diving in,
using state-of-the-art tech
to capture the epic hunt like never before,
and find out what really goes on inside this massive shark swarm.
Seven hundred gray sharks
live in the middle of the vast Pacific Ocean,
around an island invisible from space,
about 250 miles from the island of Tahiti,
the atoll of Fakarava.
The atoll is a ring of coral
pierced by breaches that link the lagoon to the ocean.
The deepest of these are called "channels."
Hidden away in Fakarava's southernmost channel
is the world's densest population of gray reef sharks.
700 of them live right here in this valley of coral,
classified as a UNESCO biosphere reserve.
Much of their behavior is a mystery.
What attracts them here in such huge numbers?
Are there leaders whom the rest follow?
[indistinct chatter]
A team of more than 20 divers and scientists
are here to find out.
Leading the team is biologist and photographer Laurent Ballesta.
[indistinct chatter]
Laurent and his team have been here before,
four years ago, to check out rumors of vast schools of thousands of groupers
coming here to reproduce during the June full moon,
which acts as a signal to them to start laying their eggs.
The rumor was true.
The Fakarava channel is the site
of one of the largest aggregations in the world,
17,000 groupers.
Inevitably, they drew the sharks.
There were so many of the predators
that the team kept counting and recounting them.
One camera traveled the whole length of the southern channel,
about half a mile.
The footage revealed an astonishing count of 700;
705 to be precise.
That's the highest density anyone's ever seen.
Laurent and his team's curiosity is stronger than their fear.
They decided to scout the location at night,
amidst hundreds of sharks.
[in French] What we observed, when we discovered these grounds,
was that during the day, the sharks that were clustered at mid-depth
would suddenly swim to the bottom,
and start raiding hither and thither at the bottom of the pass.
We didn't dare get close to them.
We stayed above, 10 meters above them.
We filmed them from afar, with our knees curled up near our shoulders.
We were terrified to let anything dangle near those jaws.
The water was seething with sharks, an impenetrable ball of them.
[rustling]
We started daring to dive a little deeper, on the edges.
Maintaining a distance, but a little closer to the bottom.
It was still impenetrable. What goes on, in the very center of the pack
when we're around it?
Filming at that depth, once in a while we would get nudged by the sharks.
[grunting]
Damn!
We realized that we weren't targets
we were just obstacles.
[grunting, yelling]
Damn!
That's when we figured there was a semblance of a social system.
We began to suspect we were seeing various predatory strategies.
We had a hunch... It aroused our intuition.
I thought, let's test this hunch. Let's see if it's scientific fact.
[narrator] Laurent hopes to prove
that the sharks are much smarter than we think.
The next few days are critical:
the June full moon will trigger the grouper gathering.
That's when the sharks will go on the rampage.
The moon is a ticking clock for both the scientists and the fish;
a countdown to a massacre.
[loud explosion]
700 sharks aren't gathering at Fakarava by chance.
They're here because of the history of this island.
Knowing the geological past reveals the biological present.
Fakarava was born around 70 million years ago.
[rumbling]
No sooner had the volcano surged up from the depths of the Pacific Ocean
than the reefs started colonizing its shoreline.
Soon the island was surrounded by a ring of coral.
In the north, the freshwater from the rivers
stopped the coral from growing there,
which left a gap in the reef almost two miles wide.
Its fires extinct, over the next two million years,
the volcano disappeared, while the reef kept on growing.
But it still bears the imprint of the ancient rivers:
the channels.
The pass in the north takes the brunt of the tides,
so the residual current that reaches the south channel here is gentle.
This easy current is a real boon.
It's weak enough for the coral to develop,
and provides shelter for fish,
yet it's strong enough to allow a constant flow of food for everyone.
[soft instrumental music playing]
Sharks thrive in this current, too.
By day they use it to rest; by night they use it to hunt.
This perfectly adapted current is the first clue
to understanding why sharks gather here.
[birds calling]
But to see the whole picture,
Laurent needs to dissect the predators' movements.
He's tasked three scientists to create a 3D model
to help him do just that.
They'll begin by placing antennas down at the bottom of the pass.
[Yannis Papastamatiou] The strength of the current isn't equal.
If we know approximately where they are
in the channel, and how active they are,
and we have some understanding of current strength,
then we can really see how their behavior varies.
[Charlie Huveneers] Based on the range of testing we did,
we probably need to space the receivers about 100 meters apart.
[Johann Mourier] Yeah. Exactly.
[Huveneers] Right now we're trying to find the best position
for these acoustic receivers
to make sure that we are going to get the tagged shark
where the sharks are spending most of their time,
so on the hot spots around the walls.
Uh, but also, where they're going to be, uh, hunting at night
so we can compare the activity during the day and the night.
[Mourier] We have a big school of sharks usually staying here,
all along this place.
So, all these receivers can get, uh, the detection.
[Huveneers, off screen] Cool. We'll just have to deploy them now.
[tense instrumental music playing]
[narrator] They have to get government permission
to install 25 antennas,
since the sharks here are protected by law.
[Papastamatiou] 22 is 101799.
113122.
[narrator] But installing 25 antennas is the easy part.
[Papastamatiou] 115334.
[narrator] The next step is to equip 40 sharks
with electronic chips, so they can track their every move.
[tense instrumental music playing]
The gray sharks are keeping their distance;
like the divers, they're cautious.
These predators are also prey to 13-foot-long hammerheads.
[dramatic music playing]
[grunting]
[dramatic instrumental music playing]
[Papastamatiou] Good with that, so we have the VHF works just fine.
[narrator] With the antennas installed,
it's time to figure out how to plant the chips
in the sharks' abdomens.
The chips will let the scientists know exactly when the predators go into action;
whether they sometimes leave the channel;
and if all 700 act as one, or independently.
[Huveneers] It's pretty handy to have a shark right here.
[Mourier] Yeah.
[narrator] If everything goes as planned,
they'll be able to track the movements of each shark.
And the team has another tool at its disposal:
a spy camera covered in sensors,
ready to be attached to a shark.
[Papastamatiou] Well, let's try it and see.
[narrator] This is Yannis's first dive at night
into the middle of the pack.
He and Laurent are going to test placing the spy camera on a shark
using a surprising technique.
But the sharks may not cooperate.
[narrator] Scientists in the Pacific atoll of Fakarava
have to work in the dark,
because the sharks are too cautious during the day,
and unapproachable.
At night, they're emboldened by the hunt.
[dramatic music playing]
Research has shown that turning a shark over on its back
puts it into a hypnotized, cataleptic state.
But that was done with sharks familiar with divers
and fed by them.
Now Laurent and his team are testing it here,
amid a feeding frenzy.
Amazingly, the immobilization technique works.
This is one circus move that could end up scientific protocol.
[muffled speaking]
They need a big shark to carry the camera.
Coordination between the two scientists is crucial.
Each shark reacts differently.
Some won't be hypnotized, and that's when they might bite.
Yannis thinks this one's too small.
Laurent lets it go.
While they're searching for a larger shark,
a never-before-documented hunt is going on.
It's a moray eel.
-[dramatic music playing] -[camera shutter clicking]
A lone gray shark will never attack a moray eel this big,
but together, this pack doesn't hesitate.
After two hours, Yannis and Laurent give up.
It's too difficult to attach the camera.
They need to find another way, and soon.
[thunder rumbling]
Back at HQ, Yannis screens the first images.
Something strikes him.
It looks like the sharks might form partnerships
to hunt in pairs.
One of the most, uh, fascinating things
about the, uh, the sharks in Fakarava channel
is the social systems of the sharks themselves.
The question is, why do they form these social associations?
By remaining close to each other,
one shark could see another shark taking some prey,
and benefit from that.
So if you imagine, there's a reef fish here,
one shark sees it, charges, goes after the reef fish, it escapes.
Another shark sees that shark chasing the reef fish,
then it sees the reef fish, and it actually successfully gets it.
By hanging out close to each other,
I can see if you get some food or chase some prey, and that can benefit me.
Very simple, but that could still drive us forming social associations.
[narrator] There might be something to this partnership theory,
but it uncovers more mysteries to solve.
Are the pairs partners or rivals?
Does a pair always hunt together?
Do they hunt every night?
A shark only needs from seven to nine pounds of fish a week;
so theoretically, it doesn't need to go hunting every 24 hours.
The electronic chips may help the team find answers.
But only if they can find a way to attach them
to these deadly predators.
[splashing]
[intense instrumental music playing]
Everywhere else in the world,
scientists fish for sharks with hook and line
in order to mark them.
But here in Fakarava Atoll,
the scientists have decided to immobilize them by hand
and then lasso them to get them up next to the boat.
Nobody's ever tried this old cowboy technique
on a gray reef shark before... only on whitetip reef sharks.
It's much less invasive for the shark,
but there's a high risk of someone getting bitten.
One problem with hypnosis is the creature's weight.
If a shark's not swimming, it sinks.
So if you immobilize one, it tends to drop down,
bringing its jaws right up against your legs.
[muffled speaking]
[Ballesta] Damn!
No!
[narrator] Laurent's taken it on as a challenge.
He's determined to get it done.
[Huveneers] Okay, Yannis, you need to do the slack,
-you can do the slack. -[Papastamatiou] Yup.
[Huveneers] Johann.
Watch out, Yannis, I'm going to pull it from the other side.
-Watch out, watch out. -[Papastamatiou] Here?
[Huveneers] Good, good.
[Mourier, in French] Slack. Give it slack, slack, slack, slack.
[Huveneers] Good one, good one, guys. We're on.
[indistinct chatter]
One, two, three.
-[thrashing] -[grunting]
[man] There you go.
[Mourier] Yep.
[Papastamatiou] Oh, there goes the...
[Mourier] Good boy.
[Huveneers] Gonna touch again. Here you go.
-[Papastamatiou] There we go. -[Huveneers] Close, close, close, close.
[Mourier, in French] When the shark is belly up,
he's much calmer, so it's easier for us to tag him.
Also, the shark is much less stressed out, when he's belly up.
[narrator] Yannis tries to implant the first electronic chip.
[intense music playing]
[Mourier] The surface of the skin is very hard, eh?
[Papastamatiou] There's a lot of really thick muscle layer
until you get into the body cavity.
So, although these incisions look quite dramatic,
there's a few things to keep in mind.
First of all is that sharks naturally have pretty violent lives,
especially when they mate.
So the male will bite the female,
and the females in particular can get pretty torn up.
So we get females with huge, huge scars, much, much larger than anything
we're inflicting with this, with this blade.
So they're naturally adapted to having a lot of damage.
And because of that, they have very impressive immune systems
and very, very fast healing rates.
So we've caught sharks a couple days after, after tagging,
and the wound is already healed up.
-[Huveneers] You can let go. -[Mourier] Yeah.
[Huveneers] Uh, one, two, three. Flip it and untie him.
Flip it. It's gonna go by itself. Perfect.
At least we know she's healthy.
[narrator] The team hopes to tag at least 40 sharks
and then follow them for a whole year.
They'll know everything the sharks get up to:
if they leave the channel or stay there;
if they hunt every night;
if they all hunt together;
and if they're influenced by the lunar cycles.
[dramatic music playing]
But tagging 40 sharks is easier said than done.
[Ballesta] Oh, my God!
[narrator] Here in the Fakarava channel,
scientists are tagging sharks using a lasso technique,
while trying to avoid being attacked by other sharks.
[Huveneers, off screen] One, two, three.
[Papastamatiou] Oh, there it goes.
[Mourier, in French] This shark may have been bitten while mating...
Or while they were attacking prey.
Sometimes they snap at each other.
[Huveneers] It's a male, guys. First male.
[narrator] This shark's bigger,
so Yannis will try to attach the spy camera.
[Papastamatiou] Come back to me. [kisses]
[Huveneers] Find it out. Yeah.
[Mourier] Okay, I know.
[splashing]
-[Papastamatiou] Good? -[Huveneers] Yeah.
[Papastamatiou] Okay.
[Huveneers] It's the best way to do it.
[Papastamatiou] Okay. Push forward, Johann, let go.
[narrator] They've tagged nine sharks on the first night,
but they're still a long way from their goal of 40.
[soft instrumental music playing]
The team's been counting the sharks every three days:
416, then 480, 520;
the number keeps rising as the full moon approaches.
By day, the sharks form three groups that the divers call "walls,"
an expression coined by explorers in the 1960s
who were the first to see hundreds of sharks lined up like this.
The walls form at very precise locations.
For four years, Laurent's team has been observing
that the sharks rest on veins of current in the channel.
Whenever they leave the vein of current, others come in to replace them:
just like migrating birds in v-formations.
They only stop to get their teeth cleaned by the cleaner fish.
The scientists discover that the camera
has fallen off the shark.
Yannis Papastamatiou looks for clues to the pack's behavior.
[Papastamatiou] So one of the things we can see
when we take a look at this video
is that our shark carrying the camera
is often associating with this individual here
with the damaged dorsal fin.
So that may mean that there may be some form of social bond
between those two sharks. And next we want to know
if that also applies at night. So do these animals, for example,
form social bonds while they are hunting?
[soft instrumental music playing]
[narrator] It's almost the full moon.
So it's time to deploy Laurent's most complex brainchild.
This arch combines both photography and video
in order to capture the hunt at one-hundredth of a second,
and then, with its 32 synchronized cameras,
move around inside the 3D image.
The team has no idea what
the sharks will make of this high-tech stranger.
The first shots are taken in daytime, amidst all the fish.
But only the oncoming darkness will reveal
what happens when 700 sharks become 700 hunters.
[diver] Go, go, go, go!
[narrator] In daylight, results from the shark scientists'
3D arch look promising.
Now they must wait for light to turn to night.
The channel is not so much a well-stocked pantry.
It's more like a conveyor belt of food,
rolling ever faster as the June full moon approaches.
There's enough prey now to feed a lot of hungry sharks.
One after another, dozens of species of fish from the lagoon
are arriving to the channel, as if to a prearranged rendezvous.
They're all getting into place at the ocean gate
to give their larvae a chance of survival
far from all of the predators of the reef.
They are ready to launch their spawn into the current,
and out to sea.
[dramatic music playing]
[dramatic music continues]
For the moment,
there are only a few grouper fish in the channel;
but soon there will be as many as 17,000.
And the sharks know it.
This honeymoon is about to turn into a bloodbath.
[narrator] Just days until the 700-shark feeding frenzy,
scientists prepare to document the spectacular event.
[whooshing]
While Antonin and Thibault work on the arch,
it's Yannis's last night on the mission.
He's at last placing the 40th and final electronic tag.
As the darkness of night falls,
the fish are half asleep and less aware of danger.
That's when a shark makes its move.
The high-speed video reveals
the different phases of an attack.
The sharks are obviously attracted by sound.
Vision helps, too, but here in the shadows,
the shark has a special advantage:
It can detect the electromagnetic field of the fish
with the aid of sensitive cells
all around its mouth, called ampullae of Lorenzini.
The electrical stimulation
only works up to a foot from the shark's mouth.
[dramatic music playing]
The more agitated the fish,
the more intense its electromagnetic field.
If it could play dead, the shark would think
it was an inanimate object, and ignore it.
[intense music playing]
On its own, a shark is clumsy.
It's at its best in a pack, with everyone moving together.
The images reveal that 25% of the pack's attempts are successful.
Compare that to a pack of wolves, renowned for its efficiency,
that only catches 14% of its prey.
The photos show them encircling their prey to cut off any escape.
Teamwork is clearly an advantage.
Once the prey's cornered, though, it's a different story.
Every shark demands their share.
At Fakarava, there's no lack of prey, so it's not a problem.
Sooner or later, every one of those snapping jaws will be fed.
But jaws are useless if the prey stays hidden.
The team makes a remarkable observation:
the gray sharks follow the whitetip sharks,
also known as coral sharks,
who, with their ability to slip into holes
and come out again backwards,
are the only ones who can flush out prey hiding in the reef.
[intense music playing]
It's a new and enlightening facet of their behavior:
This shark pack can exploit the skills of another species.
The chips on the tagged sharks send information
each time they pass close to one of the many antennas.
After a week, Johann and Antonin
collect the data to assess it.
[speaking French]
The antennas will be returned
each night to continue the tracking.
[Ballesta, in French] So, there's a different color
for each of the sharks we tagged.
[in French] You can really see two groups. One group stays around here...
On the ocean side of the dropoff.
And the other group is much more active...
They mainly use that main pass by day.
These waters, here... And actually, after nightfall,
they swim much more in this area.
Another thing we'll be able to see is if there are leaders.
We see there are groups of sharks that move together, more or less.
We'll see if it's always the same shark who starts the move.
[narrator] With only three days until the full moon,
the team makes their final preparations.
The arch will make its debut among hundreds of sharks,
right in the middle of a massive shark attack.
[narrator] As night falls two days before the full moon,
the sharks seem undisturbed by the arch
and just keep on hunting, right under the divers' feet.
It's becoming clear that whether
they're resting during the day or on the attack at night,
the sharks form partnerships, subgroups that work together.
High-speed footage from the arch reveals what happens
at the heart of an attack.
The shark on the right is driving the prey
straight towards the other.
Slow motion reveals hunting duos.
In addition to the pack's overall synchronization,
they seem to have a deeper level of organization
based on either momentary or more durable mutual interests.
The prey can escape once, maybe twice, but not forever.
In fact, this coordination is a mixture
of cooperation and competition.
They watch more than they help each other; but it works.
If one doesn't win out, the other does.
Nearly 600 sharks hunt here now:
The ranks of the predators are swelling every night.
[beeping]
[Ballesta, in French] We're part of the pack, now!
[Cedric Gentil, in French] That's the feeling I had.
And there you go... You're with them.
Sometimes you feel like taking one under each arm, and walking along with them.
Sharks everywhere! They didn't let us down!
It seems like there are more every year.
[Yanick Gentil, in French] Youngsters. Lots of youngsters.
Right... There are lots of youngsters who are... about yay big.
[Gentil] Didn't you call one "Papa?"
[Ballesta] I didn't dare admit it.
[soft instrumental music playing]
[narrator] For 50 nights they've been diving,
and now on the 51st, it's the day before the full moon,
the most important night for the sharks and for Laurent and his team.
The surveillance camera gives the go-ahead.
And just like every year at this same time,
the groupers are back.
They're solitary all year round,
but get together for the annual spawn.
No one misses the date: the full moon in June.
And you don't need to study the stars
to see that all the females are pregnant.
This is the day the sharks have been waiting for...
all 700 of them.
Tonight will put to the test all the latest theories
about how the pack functions.
The attack will come from within the channel
where the sharks are resting.
The countdown to a massacre has begun.
[ominous music playing]
The groupers come here to reproduce once a year,
entirely driven by instinct.
While waiting for the females to spawn, the males hide.
Don't hide, and you're dead.
Night is falling now.
The pack is waking up,
and they're hungry.
[narrator] 700 sharks swarm their prey.
[intense music playing]
A tornado of jaws, followed by a rain of scales.
It's a deadly roundabout.
[lively, intense music playing]
Electroreceptors help the sharks zero in on prey.
For the groupers, this is the price they pay
to spawn the next generation.
Though the sharks hunt in a pack, they don't like to share.
[intense music playing]
Some bear deep scars,
perhaps from battles with other sharks at the hunt.
[intense music playing]
This grouper has managed to slip through three sets of jaws.
But not the fourth.
[intense music playing]
A parrotfish, half asleep, bumps into a coral shark.
In its haste, the gray shark bites the coral shark
and the parrotfish escapes.
Weighed down by her huge belly, a female can't get away.
Her eggs are scattered... and lost.
At midnight, the full moon is at its zenith
and lights up the whole ocean bed.
But by 5:00 in the morning it's low on the horizon
and the sharks are all left in the dark, exhausted.
The hunt finally eases, the groupers know it,
and the spawning begins.
In the darkness, the sharks seem less alert.
The lights from the cameras don't affect them;
they're too patchy.
Sharks have evolved into the ultimate predators;
but their prey has evolved, too.
The groupers, with speed and perfect timing,
have learned to spawn at this opportune moment.
[lively music playing]
Both prey and predator have strengths and weaknesses.
The groupers are less vulnerable
than the biologists imagined,
and sometimes the sharks fail to hit their target.
Four months have gone by since the start of the mission,
and the electronic tags continue to reveal the secrets of the shark pack.
They seem to obey some rules.
By day, they stay in three groups,
make their rounds, and sleep a bit.
They begin to form pairs.
As night falls, they band together for the hunt,
forging partnerships that persist for the night,
like sub-units within the pack,
although it does not mean these pairings will be the same
on the next night.
This behavior has never been documented before.
Three thousand hours of underwater filming
reveals for the first time that a big school of gray sharks
is more deadly than a pack of wolves.
Even more remarkable and never before revealed,
the sharks' activity follows the lunar cycle
much more than it follows the abundance of prey.
The level of moonlight determines the intensity of the hunt.
And come morning, the sharks all return
to their respective walls,
with the exception of at least two of the tagged 40,
who leave the channel, and to this day have never returned.
There are still mysteries to be solved in the Fakarava channel,
but one thing is clear:
In these pristine waters,
gray sharks rule.