Original subtitles

They're coming.

700 SHARKS INTO THE DARK

In the middle of the vast Pacific, 700 sharks congregate

around an island invisible from space:

Fakarava Atoll in the Tuamotu Archipelago.

The atoll is a crown of coral

with breaches connecting its lagoon to the ocean.

The deepest are called passes,

and the southern pass at Fakarava

contains the highest density of grey sharks in the world.

Seven hundred of them gather in the pass,

a valley of corals declared a UNESCO biosphere reserve.

Why are there so many?

What causes them to group together?

To solve this enigma,

20 divers and scientists join forces,

led by Laurent Ba/Iesta.

They set up a scientific HQ on the island.

Charlie Huveneers, from Australia, is an expert on great white sharks.

He and Laurent discuss the aims of the mission.

Amazing photo.

I want to know what distinguishes an organized pack

from total anarchy.

If this were a pack of wolves,

there would be some kind of sharing of the prey,

with the leader of the pack given priority.

Here, I don't know if they use tactics to hunt together,

but once a prey is caught,

it's a free-for—all.

-They fight over it. -Yes.

What causes these sharks to hunt together?

Is it cooperation or competition?

Could this wild horde actually be an organized pack?

These crates contain the centerpiece of the headquarters:

a detailed model of the pass.

That's the lagoon end.

Put it on the table.

It's the wrong way, turn it round.

-You say that about every piece. -This is the next one.

The model maker followed real contour lines.

We’ll use this to show where the sharks hunt

and where they rest.

I hope the model will bring the team together,

so everyone can share ideas.

It goes very deep.

This section should be raised a bit.

We make a gentle slope.

I’ve come to this pass

four years in a row, and this year,

we have 50 days and 50 nights ahead of us.

Surveying the valley of the sharks again and again,

like an adventure that thrives on routine,

revealing its secrets

and fulfilling a childhood ambition.

I didn't play cowboys and Indians, but Commandant Cousteau's adventures.

Now, in my forties, the game has become my profession,

but the dream is the same:

trying to solve mysteries of the unden/vater world.

The number of sharks seems to vary during the year.

What draws them to the pass?

I'm just borrowing this, Sané. I'll bring it back.

I'm obsessed with these sharks,

but they weren’t the reason we came here four years ago.

It was the rumor of thousands of groupers

a moon which tells them to spawn.

The rumor was true. We estimated

a world record of 18,000 groupers.

We knew such a mass of groupers would attract sharks.

But we didn’t expect so many. We were spellbound.

We made counts from one end of the pass to the other

and obtained the amazing figure of 700.

705, to be exact.

All those sharks in a valley barely 200 meters wide,

the highest density anywhere in the world.

Two.

One, two...

Five plus three, eight.

Seven hundred sharks, a silent army deployed along the pass,

impossible to behold in one glance.

Driven by our curiosity, we went to observe them at night.

We noticed that sharks

which were at mid-depth by day would suddenly swim to the bottom

and start raiding around the bottom of the pass.

We didn't dare approach them.

We filmed from 10 meters above,

with our knees pulled up underneath our chins,

afraid to dangle anything near all those jaws.

It was an impenetrable ball of sharks.

We started daring to go deeper, on the edges.

Keeping a distance, but nearer the bottom.

It was still impenetrable. What goes on in the center

when we're here?

Filming that close, we got bashed by the sharks.

Damn!

We realized that we weren't targets,

we were just obstacles.

We figured there was some sort of social system.

We were seeing various predatory strategies.

We had a hunch, an intuition.

Now I want to confirm it and see if it's scientific fact.

Laurent has 50 days.

For now, there are less than 400 sharks,

enough to observe their attacks,

before the major hunt during the June full moon.

This natural satellite is the expedition ’s stopwatch,

a reference for the scientists

and a metronome for life in the pass.

The countdown has begun.

The team plan positions for 3 surveillance cameras.

Where do you want the cameras? We said we could put one

at the entrance to the deep canyon.

The cameras will tell the biologists

when shoals of fish arrive,

how the sharks react to this potential prey,

and of possible increases in shark numbers.

There's Thibault.

-A few adjustments. —Cedric is positioning it.

That's not bad.

The 700 sharks are not here by accident.

Their presence is linked to the island’s history.

Its geological past helps us to understand its biology today.

Fakarava was created 70 million years ago.

After the volcano emerged from the Pacific Ocean,

coral reefs colonized the coast.

The island was quickly ringed by a belt of coral.

In the north, freshwater from rivers prevented coral growth,

causing a three-kilometer wide gap in the coral barrier.

The volcano, by then extinct, disappeared after 2 million years,

while the reef kept growing.

Traces of those estuaries still remain. They are the passes.

The northern pass receives almost all of the tide.

As a result, the current in the southern pass is less violent.

This gentle current is a godsend.

It’s weak enough for coral to grow and for fish to shelter in,

but strong enough to create a nourishing flow

that benefits everyone.

The current also helps the sharks.

By day, they use it to rest, by night, it helps them to hunt.

Part of the enigma of our 700 sharks

is this miraculously suitable current.

The current allows sharks to breathe effortlessly.

They just need to face into it,

which requires less energy than swimming.

If the current is too weak, the sharks leave the pass.

To build a 3D map of their movements,

Laurent hopes to fit some sharks with tags

and install receivers in the pass.

He enlists Charlie Huveneers and shark experts Yannis Papastamatiou

and Johann Mourrier to help him.

The current isn't equal. If we know roughly where they are

in the channel and how active they are,

and we have an idea of current strength,

then we can really see how their behavior varies

based on the energetic state of the water flowing through the channel.

Based on the range testing we did, we should space the receivers

-100 meters apart. -Yeah, exactly.

So this is about 100 meters.

You don't want them too close.

Let‘s say here.

We want to position the acoustic receivers

where the sharks spend most of their time,

on the hotspots around the walls,

but also where they hunt at night,

so we can compare the activity from day and night.

We have a big school of sharks usually staying all along here.

So all these receivers can get that detection.

There‘s always a smaller school of sharks by the edge of the drop-off.

We assume there is a connection between the two schools.

We just have to deploy them now.

This is done with the permission of the Polynesian government.

The sharks here are protected by law.

22 is one, zero, one,

seven, nine, nine.

One, one, three, one, two, two.

These 25 receivers will pick up signals from tags on the sharks.

Three, three, four.

We hope to tag 40 sharks.

Fewer wouldn't be representative,

more could cause interference and skew the results.

The grey sharks keep their distance.

Like us, they are wary.

These predators are also prey

for large hammerhead sharks.

The VHF works just fine.

The first receivers are installed.

Yannis, Johann and Charlie can now tag the sharks.

They want to know when the predators go into action,

if they leave the pass, if some stay together,

and if there are leaders and followers.

Many of the answers could come from these tags

placed in the sharks' abdomens.

If it works, they will see each shark's movements,

like the demo on this screen.

We can see three axes.

For example, if the shark is moving up,

we can see the red line moving.

If it's going that way,

the green one, and that way, the blue.

So that's the data we are going to have.

If it is beating its tail, you can see the frequency.

If it gets more active, if it's doing it faster,

you can see that change in there.

The team has another more accurate tool:

a spy camera that can be fixed to the shark.

This will go on the shark's dorsal fin.

It measures depth, water temperature,

and there's a video camera that will run for 5 hours.

That will show us what it sees.

Laurent is going to try and put the shark in tonic immobility,

underwater.

The shark is going to be like this.

Do you think you'd be able to put the clamp on a shark like that?

l have no idea. We'll try it and see.

If not, we'll do it the old way.

But the goal is that we come in, I can open up the clamp,

over the dorsal fin like that, and it's spring-loaded,

to clamp together.

We release the shark, it goes off.

Once we turn it back the normal way, it will swim off.

There will be a release connecting the tag to the clamp

which, after two days, will dissolve.

So then, the whole tag should come off of the clamp

and float up to the surface.

Within a week, there's nothing left on the shark.

Let's try it and see.

It's Yannis's first dive, at night,

in the middle of the pack.

Together, we’ll try to attach the spy camera

by immobilizing a shark underwater.

One year ago, this would’ve seemed impossible to me.

The sharks are too wary by day,

but at night they are emboldened by the hunt.

Other researchers

have shown how a shark turned on its back can be placed

in a tonic state.

But those sharks were used to it and fed by divers.

Is it possible in all this excitement?

Amazingly, it works.

This circus trick could become a scientific procedure.

We want a big shark,

so I ask Yannis to be patient.

Each shark reacts differently.

Some resist hypnosis and there’s a risk of being bitten.

Yannis says this one is too small, so I let it go.

As I'm looking for a compliant shark,

we witness something never filmed before.

No grey shark would attack a moray eel this size on its own,

but with the school comes greater audacity.

After two hours, we give up.

Attaching the camera is too difficult.

We need another method.

Still got all ten fingers?

l have gloves.

I've been fortunate to dive all over the world with sharks.

l have never in my life seen something like that.

That was beyond belief.

I'm happy to hear that, coming from a guy like you.

My God.

Look at his face

In 25 years of diving, I've probably seen one or two predation events.

-And here? —I stopped counting!

Unbelievable.

Probably the best dive I've ever done.

-Best dive? -Yeah.

You judged that it wasn't the right way to place the camera?

We could have done it, but if it wasn‘t perfect first time,

you wouldn't have been able to keep on holding it.

I realize with the small ones, I can really immobilize them.

With the big ones, I can just hold them.

And you want a big one.

I‘m sorry. When I held one, it moved and jumped on you.

That‘s right, I just stopped moving and hoped for the best.

It's funny now, but it could've been serious.

The endless night dives are exhausting.

By dawn, tiredness catches up with us.

Like the sharks, we rest by day.

The moon continues its course.

We need to be ready for when the shoals arrive.

After a short nap,

Antonin and Thibault continue installing receivers in the pass.

At HQ, Yannis examines our first images.

A hypothesis emerges.

The idea of sharks forming duos

that hunt together.

There are so many sharks here,

and so much predation going on,

that for the first time, we can try and understand

sharks' hunting tactics, and, more importantly,

how that applies to the social system of the sharks.

One of the most fascinating things about the sharks in Fakarava channel

is the social systems of the sharks.

Why do they form social associations?

By remaining close to each other,

one shark could see another shark taking some prey and benefit from that.

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 successfully gets it.

By being close to each other,

I can see if you get some food and that can benefit me.

Very simple, but that could still drive us to form social associations.

The duo theory seems plausible.

Are the hunting pairs always made up of the same individuals

and active every night?

Grey sharks only need to eat 3 to 5 kilos of fish per week.

In theory, they don't need to hunt every night.

Electronic tags will tell the team more.

Laurent has a plan for fitting them.

I grab the shark, turn it over, and rope it.

I'm not sure I can tighten the slipknot with one hand.

Do you know this knot?

It works.

But that also means

I need some slack to get it off my wrist.

He lets go of the tail and you guys pull.

-Not before that. —Then we really go for it.

The most important part of the expedition is at stake here.

Will this non-violent method work?

Or won't it?

It's a trial.

It's a trial, but it would help if it works.

You got the bag?

Good luck!

Most scientists catch sharks with hooks to tag them.

We aim to immobilize them by hand then put a lasso on them.

It’s less violent, but also untested,

and there's a risk of being bitten.

One of the difficulties is the weight of the animal.

A shark sinks when it stops swimming.

When I immobilize it, it sinks downwards

and its jaws are close to my legs.

Damn it!

It has to work tonight,

to convince Yannis and Charlie who doubt our method.

-Yannis, you can do the slack. -Yeah.

I'll pull it from the other side.

-Here. -Good.

Pick up the slack!

Another one, guys. We're on.

A little bit more.

One, two, three.

There goes the...

Good boy!

-Don‘t touch... -There we go.

Close! Close!

When the shark is belly-up, it's much easier for us to work.

And the shark is less stressed when he's belly-up.

Yannis Papastamatiou fits the shark with an electronic tag.

This will reveal all of the shark's movements

and periods of activity and rest.

The surface of the skin is very hard.

A really thick muscle layer until you get to the body cavity.

Although these incisions look quite dramatic,

you must remember,

first of all, sharks naturally have pretty violent lives,

especially when they mate.

The male will bite the female, females can get pretty torn up.

We get females with huge scars,

much larger than anything we're inflicting with this blade.

They're naturally adapted to having a lot of damage.

So they have very impressive immune systems

and very fast healing rates.

We've caught sharks a couple of days after tagging

and the wound is already healed up.

Lassoing sharks like this

is more than an underwater rodeo show,

it allows us to catch the specimens we want.

We need a range of sizes, from small to large sharks,

but also males and females

to have a good population sample.

It also means we don't have to fish for the sharks.

Not having a hook in its mouth,

the shark will recover more easily.

-Ready to let go? -Yeah.

One, two, three. Flip it out.

Flip it, and let it go. Perfect.

At least we know she's healthy.

OK, guys, get ready for the next one.

-How many sharks have you tagged? -In my life?

Probably over 2,000.

And these? Are they easy to handle?

You know, you have to be cautious with them.

But, as you see, they will switch their behavior in a second.

They'll go from being calm, and then, suddenly,

if you don‘t watch the head, your hand's going to be in the mouth.

The tagged sharks will be monitored

for one year.

It should tell us if they leave the pass,

if they hunt together every night

and if the lunar cycles influences them.

The lasso method works.

The other sharks don't attack.

The school shows no solidarity in defending its members.

One, two, three.

There it goes.

This shark may have been bitten while mating or while attacking prey.

Sometimes they snap at each other.

It‘s a male, guys. First male.

This shark is bigger, time for Yannis to try his spy camera.

Come back to me.

Turn it up.

-Good? -Yeah.

It's the best way to do it.

OK.

Push forward and let go.

When Johann told me divers could catch sharks for us to tag,

l was like: "No, that's not going to happen."

So you've proved me wrong.

He didn't know us.

You didn't know me. Us, I mean. Us.

We're smiling now, but we could easily have been bitten.

-We were close. -You had some close calls.

They turn on you.

-They go like this... -We had a few close calls too.

We don't want to get bitten.

This first night is encouraging.

We tag nine sharks.

But we 7] need more nights like this to reach our target of 40.

Careful, now.

Raise this end so the other end is lower.

Is it clear? Careful, now.

Easy. Wait until we get it undennater.

It took nine months to build this arch conceived by Laurent.

Antonin Guilbert and Thibault Rauby are the pilots.

The arch combines photography and video

to film a predation in 100ths of second,

using synchronized cameras

to circle around the image in 3D.

How will the sharks react to this imposing oddity?

Two laser beams help the divers to aim and focus on the predation.

The two lasers will really help us to aim it.

If we manage to put it down on the bottom,

what it films will be the middle.

You position it where you think it's going to happen,

you focus the cameras, then switch off the lasers.

We can't see them now, but in murky water or at night,

they'll look like Jedi light sabers in the shot.

Johann’s counts reveal 413 sharks,

then 480.

That number should rise as the full moon approaches.

By day, the sharks form three groups which divers call "walls”.

The term was coined by explorers in the 19603

who first saw hundreds of sharks in formation.

A scary sight at the time.

The "walls” form at precise points in the current flows.

Every time a shark leaves the flow, it returns to join it again,

like migrating birds flying in a ”V" formation.

They pause for dental attention from cleaner fish.

The spy camera detaches itself from the shark as planned.

Yannis Papastamatiou looks for clues to the school’s behavior.

One of the things we can see when we take a look at this video

is that our shark is often associating

with this individual with the damaged dorsal fin.

That may mean there is a social bond between those two sharks.

We need to know if that association occurs more than just by chance alone.

That could suggest there is a social network for the shark population here.

Next, we want to know if that also applies at night,

if they form social bonds while hunting.

The arch is finally ready.

With Antonin and Thibault,

we start the buoyancy tests.

Designing it was a challenge.

We needed the right cun/ature and camera spacing

and materials that were both light and strong.

-Not bad. —It floats. It's not moving.

We don’t know how easy it will be to film a predation,

but it feels like we are holding

a futuristic machine,

a magic key to seeing the undenivater world differently.

/ can't wait to see the images, to show them to the scientists

and to see their reaction.

The full moon approaches.

Everything gathers pace.

Shoals of fish arrive in the pass.

They will soon spawn and attract more and more sharks.

After crossing our minds as an idea,

the arch now crosses the pass.

The idea has become a reality.

We'll soon be ready to use it with the sharks.

Antonin and Thibault are the pilots.

Even undenivater, I hear their excitement.

The results are promising.

It will give us a more precise idea of the sharks’ position as they attack.

The pass is not like a well-stocked larder,

it is a conveyor belt of food

that speeds up as the full moon approaches,

finally providing enough to satisfy our school of sharks.

One after another, dozens of species arrive from the lagoon,

as if they have synchronized agendas.

The pass brims with life.

At the entrance to the ocean they discharge eggs and sperm

into the current, towards the open sea,

giving the larvae a chance to survive

far from predators on the reef.

So far, there are only a few groupers, but there will soon be 18,000.

The sharks know it. This romantic rendezvous

is about to become an ambush.

By day, the fish are too alert.

The sharks save their energy.

A tacit truce is obsen/ed.

While Antonin and Thibault master the arch,

Yannis, on his last night on the expedition,

fits the last of the 40 electronic tags.

At night, the fish are half asleep, they don 't spot danger.

Hunting alone, the majority of the sharks' attacks fail.

Filmed at 1, 000 frames per second, we see the different phases.

The shark is attracted by noise and movement,

but in the dark, it has another advantage.

lt detects electromagnetic energy from the fish

thanks to sensory cells all around its snout

called Ampullae of Lorenzini.

This electric stimulation only works 10 to 20 cm from the shark's snout.

Beyond that distance,

the shark only detects movements.

If the fish plays dead, it goes unnoticed.

The shark on its own is clumsy.

The team shows that salvation lies in the pack.

Analysis of our images

shows that the pack succeeds in 25 percent of attacks.

By comparison, a pack of wolves, despite its reputation,

only reaches 14 percent.

But wolves, unlike sharks, will share their quarry.

Our photos show how they circle the prey to block any escape.

While the group is clearly a force,

it is a handicap once the prey is caught

and everyone wants a piece.

At Fakarava, prey is abundant.

Sooner or later, all mouths are fed.

To understand the pack, no stone is left unturned.

Eric Parmentier, from Liege University in Belgium,

has a passion for shark morphology.

His specialty: Jaws.

Everyone has this idea that sharks

are streamlined, especially the head.

But when the animal is feeding,

it can change shape.

It's no longer a straight line, there is a step here

preceding the opening of the mouth.

As a morphologist, I want to know

how the animal manages

to deform its body in order to feed on fish.

If sharks feed on small prey,

they do what many fish do and just open their mouths wide.

The wide mouth sucks in a lot of water

and any prey contained in the water.

That suction is essential

to get the prey into the mouth.

Take your favorite rubber duck, for example.

Lie in the bath, with your hands behind you,

and try catching it in your mouth.

It won't work. You need suction

to draw water into the mouth and the duck will follow.

For large prey like groupers,

suction alone isn't enough,

the shark has to use a different technique.

We need to understand how a shark's upper and lowerjaws work.

What happens when it opens its mouth?

The lowerjaw ligaments tighten,

the upperjaw ligaments tighten,

and that tension makes the teeth stick up straight.

As well as that, the shark has a muscle

between its upper jaw and its eye.

When this muscle contracts, the jaws are projected out.

Like unfastening false teeth and pulling them fonnard.

The jaws shift and the teeth stick out towards the front.

With the upper teeth and lower teeth all biting vertically

the prey is completely caught.

Like the groupers.

But only half of it is in the mouth.

The shark shakes its head from side to side

and these rows of teeth

act like saws and slice through the prey.

The jaws are ultra-efficient.

but they’re no use if the prey remains hidden.

Our observations led us to a shocking discovery.

The grey sharks follow white-tip reef sharks,

or "mamaru", that can swim into small gaps.

Only they can flush out a prey hidden in the reef.

A bit of action!

When we started really getting into the melee,

we noticed these guys underneath.

They‘re "mamaru“, whitetip reef sharks.

They rummage around in the coral.

The grey sharks hang around above them.

-They‘re always first. -At every melee.

There are two possible outcomes.

Either the whitetip finds a small prey inside the coral

and he just eats it there.

The grey sharks get nothing.

Or he flushes out a grouper too big for him

and the grey sharks are waiting for it.

When the "mamaru" hunt in the coral they flush out other fish by accident.

The whitetips search the coral thoroughly,

every nook and cranny.

-They flush out other prey. -They're flexible.

They're smashing the corals.

Biting everything.

This opportunism is another facet of the school’s behavior.

It's a significant discovery.

The school is able to exploit the virtues of another species.

The electronic tags in our 40 sharks

send back data each time they pass one of the receivers.

After a week,

Johann and Antonin extract the data.

The receivers are then put back.

There's a different color for each of the sharks we tagged.

By day, they're by the wall, as we saw.

We see them looping

and returning to the same spot.

You can really see two groups. One stays here by the drop-off,

and the other group is much more mobile.

They mainly use these waters during the day,

and at night, they swim into this area a lot more.

It would be great if we noticed

specimens who leave the pass for a few weeks or months.

We will. A percentage of them at least.

We now have the peak number of sharks for the year.

For the management of the pass,

if we could show the reserve authorities

that sharks don't only live here.

Some leave and need to go somewhere else

to complete their cycle.

That could be positive and create

an interest in extending the protected area.

We'll see if there are leaders,

if there are groups of sharks that move together.

Is it always the same one leading?

We'll try to identify interactions,

like these two playing around at the mouth of the pass.

Must be buddies.

Plenty to get your teeth into!

Nine days from the full moon, all our experiments are set up.

We have just enough time to complete our scientific mission.

Are there other schools of sharks?

Do they also hunt in packs after sunset?

There are rumors, so I need to go check

with Thibault, Cedric and Yanick.

One, two, three, four, five, six,

seven, eight atolls worth visiting.

We'll start with Toau.

There are two spots.

A few days here and there,

then back to Fakarava.

The interesting pass at Toau, for groupers and sharks

and deep dives, is very exposed.

I know we have three years' experience at Fakarava,

but that doesn't mean a thing, it could even be dangerous.

The outgoing current at Toau is way too strong.

Like a catapult.

Like being fired through a blowpipe.

We'll start with the most difficult.

-Great. —No, seriously. Why not?

We'll be very careful after that.

They are only just visible on the map.

Atolls are low-lying rings of land, but in my dreams of empty oceans,

they are mountains of which we see only the peaks.

To understand the pass, we have to consider the outer reef

How can we appreciate an oasis without the desert around it?

Do sharks have any reason to go to these depths?

Fish are rare down here.

This 2,000 meter high fortress

is just a pile of fossilized corals,

the giant skeleton of a reef which has grown over 70 million years

as the original island subsided.

On the surface, corals build and erode at the same time,

crying endless tears of sand into the abyss below.

A vertiginous void.

These barren depths make the life in the pass even more precious.

Are the sharks here

from a school that lives higher up?

At 100m, there's no current,

but in the Toau pass, it's another story.

No chance of seeing sharks resting here like they do at Fakarava.

It sucks you up and then pushes you down.

Your fins are pushed up.

It was hard enough with the lighting rig.

I could see Yanick with the camera, like this...

It's hard to dive in this current.

It's not like Fakarava where we can always dive.

You can‘t work here. You just drift.

Drifting is OK, but you can't stop to do anything.

There‘s less predation here.

Fewer hiding places for coral fish than at Fakarava.

Fakarava is full of them.

Nothing here is like what we see at Fakarava by day.

But what about at night?

Myself, Thibault, Cedric and Yanick are going to see.

If I get injured, I'll use this as a tourniquet.

Or a band-aid for a shark bite.

-Imagine your arm hanging off. -Put your hood on.

Getting lost is not allowed.

We stay together.

Nobody tries to get lost.

Or fall off a boat, but it happens.

Yellow card. OK?

If you don't put your hood on.

I hope we won‘t be fired through the pass in 25 minutes.

Is the drop-off at 25 meters?

Guys, I'm struggling with the map.

What‘s the depth?

30 meters.

All the passes we visit are scraped and eroded by powerful currents.

The coral has nothing to hang onto.

Sharks have little to gain from hanging around here.

The current catapults us out of the pass in under 20 minutes.

Let's get the others.

20 meters.

Impossible.

It's disappointing.

We hoped to find something similar to the southern pass at Fakarava

at some of these other atolls we're visiting.

We haven't found it yet.

The current isn't linear.

It pulls you up and down and rips you in half.

We didn't realize its potential. Fakarava is the perfect pass.

You were scared.

There! Now I‘m scared.

No more dives like that, thanks.

It's late. Let's sleep.

With its moderate currents and burgeoning corals,

Fakarava is unrivalled.

The 700 sharks are here and only here.

Three days to go until the full moon and the spawning,

a major event for the pass.

Acousticians have set up microphones from the entrance to the lagoon

to the ocean drop-off.

The aim is to see if the acoustic intensity changes

when shark attacks increase during the spawning.

This acoustic backdrop is formed by dozens of species.

Eric Parmentier and Loi'c Kever hope to record them one at a time,

to decipher the brouhaha

and produce an acoustic barcode

at different moments of the day and the lunar cycle.

Cousteau's "Silent World" is actually really noisy.

I'm sure Cousteau knew it too.

Can you pass me the triggerfish?

What we observe in the pass is that day and night

have different sounds.

That's understandable.

Fish can't spend 24 hours a day yelling,

they have other things to do.

We noticed straightaway

that the fish have specific periods

during which they make sounds.

It might be afternoon for some, nightfall for others,

or between 10 o'clock and midday.

It's cool there's so much sound

because we can study

what happens by day and by night and with the tides and the moon.

But we find ourselves with a big problem:

Who is saying what?

We can compare stress sounds with the sounds

a fish makes when hunted by a shark.

-Tell me if you hear anything. -OK.

From an acoustic point of view,

we can try to detect the attacks of different sharks

on the groupers.

The long term objective

is to be able to just clip a microphone in the water

and describe what's happening.

Identify the species present, understand what they're saying

and monitor the evolution of the pass at Fakarava.

One possibility,

which is what we're starting here,

is a sort of sound library,

that could be used 5 or 10 years from now

to compare biodiversity in the pass.

Financially, it works out a lot cheaper

than putting divers in the water

at various depths.

You just send down a microphone,

record for 24 hours, or at a certain time,

then use comparisons to make an inventory

of the health of the pass.

Sharks are mute

and swim silently, making them stealthy predators.

Eric hopes to detect and count the sharks indirectly,

by the quantity and nature of the sounds they provoke in fish.

Briefing with Laurent at the HQ.

Eric has isolated the stress sound of the Myripristis

or soldier fish.

A soldier fish in this position would be making a sound.

Yes, and, unfortunately for him,

in the frequency detected by sharks.

Sharks can feel sound but not hear it.

If you go to a concert and stand by the speakers,

you feel the sound and you hear it.

If you move further back, you only hear it.

It only works at a certain distance.

Exactly.

Like all fish, sharks have a lateral line

the length of their body

which ends at their snout.

They're pressure sensors.

Pressure and movement sensors, good for hunting.

Sharks are mute.

Nobody has ever recorded

sharks producing sounds to communicate.

When you eat, when you chew, you make sounds.

That‘s a consequence not a produced sound.

There's a thin line between the two.

A woman in high heels walks in a room, all the men turn to look.

Did she send a message?

She hasn‘t spoken.

But we know she's there.

Sharks don't produce sounds but they do make

hydrodynamic sounds.

You displace water, you make a wave.

Outside HQ,

the last instrument to be installed needs no light

to take ultrasounds of the pass.

Laurent wants to know if their lights affect the sharks.

Belgian biologist Damien Sonny

observes what happens before and after the divers arrive.

I'm using the ultrasound camera to observe

the middle of the pass, right by the wall of sharks.

Using this camera, I can see, like a gynecologist,

a shark gynecologist.

I'm not disturbing the animals, but I can see their echoes.

I can even see their shapes

and distinguish the sharks from other fish.

with his acoustic camera.

Here come the divers.

The sharks are all around them.

The sharks are not afraid of the arch.

They continue hunting around us.

Whether while resting by day,

or attacking by night,

it’s becoming clear they operate in pairs.

Accomplices swimming together.

The arch finally pays off,

showing what happens at the heart of an attack.

The beater on the right,

the recipient on the left.

Slow motion shows these hunting duos in action.

As well as the general organization of the school,

another level of cooperation arises from fleeting common interests.

The prey can get away once, maybe twice,

but not forever.

This behavior is a mixture of cooperation and competition.

Watching, more than helping each other.

If one loses out, the other profits.

Johann '3 latest count

reveals nearly 600 sharks.

The predators are closing ranks.

They fascinate us.

How many sharks could there be

in this one photo

that is brimming with them?

We don't dive amongst the pack.

-We are the pack! -True.

-We're part of the pack. -That‘s how it felt.

When you're in the middle... you're with them.

You want to go like this and hang out with them.

You want to be with them, they're your mates.

I think I might have a dorsal fin growing on my back.

They were all over us.

There are more every year.

-Lots of youngsters. —Yes, lots about this big.

-I think one called you daddy! -I wasn't going to mention it.

It's now we‘re in the middle that we have perspective.

-l‘m writing that down. -Go for it!

He's going to say that tomorrow, or in two days.

He‘ll be like: "Damn, I'm clever."

Hi, Damien. What does the pass gynecologist say?

-You went in the water at 01:55. -Yes.

This is at 02:05.

Still no light affecting the area.

The moving shapes are the fish, the prey.

As you see, they're fairly active.

This second sequence is from 02:13.

Light is edging into my area.

Prey still quite active.

Guess what happens next.

-The fish disappear. -It goes quiet.

In one minute, there are almost no prey.

Wow! No more fish.

They don't leave, they just disappear from the screen.

In the next sequence, you move away,

and we gradually see the prey reappear.

The storm has passed.

It confirms a hunch we had that when we switched off our lights,

we saw the fish reappear.

It contradicts the common belief

that lights attract fish.

Is it the light that makes them hide,

or is it the light attracting sharks that scares the fish?

They anticipate the threat.

Even before the sharks arrive, they hide.

I don't think the amount of light matters.

It's about being able to see.

Diving at full moon, you don't need lights.

We add lighting for our cameras.

But to find your way and see what's around you,

the light of the full moon is enough.

When you dive with a full moon,

you don't see fish in open water, they're hiding.

So maybe our artificial lighting

sometimes recreates full moon conditions.

We don‘t attract the sharks, they're already there.

In that one little spot

we make an ordinary night like a full moon.

We give them a full moon every night.

This is great. It's so clear.

Fifty nights of diving.

Tonight is the full moon.

The most important night for the sharks and for us.

Everyone is busy, quietly working,

as if a month here and 3 years of planning wasn't enough.

The surveillance cameras alert us.

Like last year, the groupers arrive en-masse.

Solitary all year, they group together for their annual spawning.

They always know the date. The full moon in June.

No need to check the sky, all the females are full of eggs.

This collective spawning presages a peak in the number of sharks.

Johann ’3 latest count approaches the 700 of three years ago.

The predations will increase

allowing us to observe more

and test our latest hypotheses about the school's behavior.

The attack will come tonight from inside the pass.

The countdown to the massacre has begun.

The groupers are packed in the pass.

A one year wait for 30 minutes of spawning.

What is the sharks' battle plan?

-The groupers are here. -On the right.

A few days before, this area empties out

and they're all concentrated here.

Right at the entrance to the pass.

During the day, there are fewer sharks than this.

They're by the drop-off.

I counted 100 there last time. Not that many.

Right on the edge.

And at night?

At night, they come over here.

All dispersed.

Night falls...

What if it is actually like this before we arrive,

and when we go in the water,

it goes like this?

-No, I don't believe that. -Why not?

Sometimes, you go in the water

and you think your lights are causing the predation,

then you go back the next night

and they only hunt outside the areas you light up.

In the end, it feels like us being there

doesn't always influence them.

In three hours, we’ll join the pack

for the long-awaited full moon.

50 nights of diving has taken its toll.

Our faces are drawn

but our spirit is resolute.

We know that tonight is the last.

Slaves to their instinct,

the groupers dare to spawn here once a year.

While they wait,

they must hide.

To do othenivise would be suicide.

At night, the school comes alive with ravenous energy.

The sharks form

a deadly spiral around their prey.

Tornadoes of teeth producing clouds of scales.

The voracious vortex feeds

again and again, throughout the night.

One grouper manages to evade three sets of jaws,

but not four.

A half-asleep parrot fish bumps into this shark.

The parrot fish escapes,

and the grey shark bites the reef shark.

Encumbered by her swollen belly,

this female grouper cannot get away.

Her eggs are strewn around and lost forever.

The full moon is a paradox.

At its midnight zenith,

it lights up the seabed,

but at five in the morning, it no longer penetrates.

The sharks are in darkness,

tired, almost blind, they ease off,

and the groupers react.

The time has come for them to spawn.

In the darkness, the sharks are less reactive.

The lights for our cameras don 't change a thing.

Sharks have evolved

over generations to be more efficient.

But so have their prey.

Groupers use speed and perfect timing

to spawn at just the right moment.

These clever adaptations show that we had a false idea

of both prey and predator.

Both have strengths and weaknesses.

Groupers are not as vulnerable,

and sharks are not as fearsome as we thought.

The spectacle ends at daybreak.

The moon continues its course

and the groupers and sharks will meet again next year.

That this wild spectacle

still exists

frightens us, dazzles us and reassures us.

After four years of observations,

this miracle of existence has emerged from the shadows.

Five months after the expedition,

the electronic tags reveal their secrets.

Despite its wild nature, the school follows rules.

The sharks departed in three different groups.

The behavior of the 40 tagged sharks is extrapolated to the school.

By day, they circle around, resting.

There are pairs who swim together.

When night falls, they come together to hunt

and new associations are formed, like sub-units of the school.

These associations are a new discovery.

They produce spectacular spirals,

but, over time, the results reveal something even more remarkable.

The sharks' behavior depends on the lunar cycle.

More than the quantity of prey,

it is the amount of moonlight that affects how intense/y they hunt.

It is the first time research has shown this.

Each morning, the sharks dutifully return to their respective "walls",

except for at least two,

who left the pass and have yet to return.

The electronic tags will transmit for one year.

We will also return next year.

These 700 sharks are not so unpredictable.

They do not have a leader,

but they obey the moon and march to the rhythm of pass.

Understanding their routines was our adventure.

We spent 3,000 hours undenivater

to witness and then prove

that the shark pack is more efficient than the wolf pack.

For the first time, we can see that their hunts

are actually a sort of collective effort,

macabre dances which are orchestrated

by a need to survive

and the existence of a special place:

the pass at Fakarava.

Translation: Daniel Murray Subtitles: Diane Bardinet