Original subtitles

The open ocean.

It covers more than half the surface of our planet.

Yet, for the most part, it's a watery desert,

empty of life.

Hunters here spend their lives in a constant search

for scarce and elusive prey.

Remarkably, this seemingly barren wilderness

is home to the largest hunter of them all...

..the blue whale.

Weighing 200 tonnes

and 30 metres long,

these are the biggest animals ever to have lived.

Despite their immense size,

blue whales are one of the most streamlined

and energy-efficient of all swimmers.

Their food is so scarce and widely spread

that blue whales must journey across whole oceans

just to find a single meal.

They can travel over 100 miles a day for weeks at a time.

The ocean's largest animal feeds on one of its smallest.

Krill - small shrimp-like crustaceans.

Opening its gigantic mouth takes so much effort

that they only do so

when the swarms of krill are rich and concentrated.

The krill here is too scattered -

not even worth slowing down for.

This blue whale's lonely search for food

must go on.

Out here, feeding opportunities are always few and far between.

And they never last long.

Prey is devoured within minutes.

When it's all over,

the hunters must resume their endless search.

Frigatebirds -

the pirates of the high seas.

Soaring effortlessly on the gentle trade winds,

they can scan vast tracts of ocean for food.

Frigates must be so lightweight

that they can't afford the heavy oils

that waterproof the plumage of other sea birds.

So getting wet would be lethal.

This may seem an impossible limitation for a seafaring hunter.

But frigatebirds overcome this handicap

with help from others.

Dorado.

One of the fastest and most voracious of ocean predators.

They patrol close to the surface, searching for prey.

Little fish try to hide amidst the undulating swell of the ocean -

the only cover there is.

It's a game of hide and seek

played out amongst the waves.

Their cover blown...

..escape seems impossible.

But these particular fish have a unique ability.

They're flying fish.

With an extra thrust from their tails,

the flying fish get airborne once more.

With a good wind,

they can glide for hundreds of metres.

But this is just what the frigatebirds

have been waiting for.

When frigates join the hunt,

the flying fish are literally caught between the devil...

..and the deep blue sea.

If the flying fish get too much lift

they become easy prey for the frigates.

If they dive to evade attack from above...

..they could fall into the mouths of the dorado.

With the help of the dorado,

the wily frigatebird has become a flying fish specialist...

..and without getting a single feather wet.

Not all open-ocean hunters are able to travel

in search of their food.

Some have no choice but wait for a meal to come to them.

A mat of sargassum weed drifts in the middle of the Atlantic.

Sargassum is the only seaweed to live entirely at the surface.

It never attaches to the sea floor.

This floating tangle of fronds

is home to a surprising open-ocean predator -

the sargassum fish.

Every part of his body mimics the weed.

His fins are more suited to walking than swimming.

In fact, he can barely swim at all.

He will spend his entire life marooned on this weedy raft.

This sargassum fish must lie in wait

for those seeking shelter amongst the weed.

Unfortunately his mat is empty for now.

But at least he's not wasting valuable energy

searching for food.

In the featureless ocean,

these mats are much-sought-after sanctuary for juvenile fish.

At last -

his first opportunity for weeks.

He must get closer.

He can only strike when he's within a few centimetres of his prey.

Trusting his perfect camouflage,

he hides in the weed.

Patience.

Still not close enough.

Hunger is clearly getting the better of him.

Surely this time.

Blown it.

It may be weeks before he gets another chance.

The open ocean is so vast

that some hunters can only find enough prey

by searching as a team.

Dolphins live in highly sophisticated social groups.

Working together,

they can cover a huge area of ocean.

These are spinner dolphins.

Why they make these twisting leaps is still debatable.

Perhaps it's a form of communication...

..or perhaps it's just fun.

Small groups sometimes come together,

forming superpods 5,000 strong.

And these are on the hunt.

SONAR CLICKS

Spinners are the most vocal of all the dolphins.

CLICKING AND CRACKLING

They use echolocation, a kind of sonar, to find their prey.

Each hunter sends out a series of clicks

and then listens for returning echoes...

..allowing them to scan for distant prey,

hundreds of metres away.

CLICKS CONTINUE

The superpod spreads out into a wide hunting line,

up to a mile across...

..producing a wall of sound.

CONSTANT CLICKING

They're searching for their favourite prey.

Lanternfish.

They are the most numerous fish on the planet.

But these small fish spend most of their time

down in the deep ocean, way beyond the reach of dolphins.

It's only when they come up to the surface to feed

that they become prey.

Once they've found a shoal,

the dolphins use their sonar in a different way.

SONAR BLASTS

They stun the fish with loud blasts,

then simply gather them up.

As they feed, the dolphins work the underside of the shoal

to stop their prey from escaping back into the safety of the deep.

Within a few minutes,

all that's left is a shower of scales

drifting downwards to the ocean depths.

The deep ocean is by far

the largest habitat for life on Earth...

..and home to some of the most bizarre hunters of all.

Down here, food is much scarcer than at the surface,

so deep-sea predators must do all they can to save precious energy.

Waiting patiently, a viperfish.

Special light-producing organs on its head

entice prey towards fearsome teeth.

Strange yet deadly jelly hunters also live here.

Most simply drift,

trailing tentacles loaded with lethal stings.

Others, propelled by lines of beating hairs,

glide gently through the darkness.

Beroe -

the top deep-sea jelly predator.

They actively hunt other jellies...

..like this ctenophore.

To grasp its gelatinous prey,

Beroe has special teeth-like spikes in its mouth.

Many deep-sea hunters just hang in the abyss,

saving their energy,

luring their prey to come to them.

Chiroteuthis.

This deep-sea squid fishes for prey using long, sticky tentacles.

Each has a glowing lure,

pulsing to attract passing prey.

A gentle twitch adds to the temptation.

Down here in the darkness, this meal is a rare bonanza.

The open ocean may be a vast blue desert,

but like all deserts, it has oases.

Scattered widely across this endless space

are thousands of small islands.

These are the summits of underwater mountains,

which rise up from the sea floor many miles below.

The seamounts deflect deep-ocean currents upwards,

forcing nutrient-laden water to the surface.

A busy oasis in the emptiness of the big blue.

For potential prey, there's plenty of shelter amongst the coral...

..and in the caves that are hidden beneath the reef itself.

Small fish take refuge here,

out of the reach of most of their predators.

But not all.

Lionfish.

They're not built for speed -

success here depends on delicate manoeuvring.

Their strategy is to hide in plain sight,

lulling their prey into a false sense of security.

The lionfish's stripes are visually confusing,

making it difficult for their prey to judge how close it is.

Using its extravagant fins to hide slow and deliberate tail movements,

it edges ever closer.

It must get to within a few centimetres,

close enough for a sudden strike.

Got one.

The deep-water currents that sustain so many residents

also attract visitors to these oases.

Silky sharks.

They journey hundreds of miles between seamounts,

using them as gathering places in the featureless ocean.

They're joined by hammerheads.

Both these sharks constantly travel

between the Galapagos and other isolated seamounts

in the Eastern Pacific.

No-one knows for sure why they gather in such numbers,

but some certainly come to these oases in search of food.

A school of resident silversides cloaks the seamount.

If these little fish stay close to the coral,

the sharks won't be able to get at them.

Other, more agile visitors are attracted by the potential feast.

Striped bonito...

..and golden trevalley.

To get a meal, they'll need to drive their prey up and away from the reef

into open water.

As long as the silversides stick close to the sea floor,

they should evade their predators.

This time, the frustrated hunters will have to search elsewhere -

there's never an easy meal in the open ocean.

The Southern Ocean encircling Antarctica.

The wildest seas on our planet.

Here, it's the storm-tossed waters that bring nutrients to the surface,

creating isolated patches of richness.

Far from the calm tropics,

this weather-beaten ocean is home to the albatross.

Black-browed albatross are the same size as frigatebirds

but three times as heavy,

and so they need a totally different flying technique.

Albatross have the longest wingspan of any bird,

and that enables them to exploit the power

of the Southern Ocean winds.

First, they glide into the wind,

harnessing its energy to give them lift.

Then they turn and descend downwind, picking up speed.

Soaring on wind fronts like this,

an albatross can travel

hundreds of miles of ocean in a day...

..barely beating its wings.

They often spend weeks at sea,

searching for prey without ever returning to land.

Food at last - a patch of krill close to the surface.

Because the winds are so strong here,

albatrosses can afford the extra weight

of waterproofing oils on their feathers.

They can duck-dive to no more than a metre,

so they rely on the churning of the Southern Ocean

to bring their prey up into range.

Like all birds, albatross have to breed on land,

but suitable islands are so few in the South Atlantic

that most are heavily overcrowded.

Steeple Jason - one of the largest albatross colonies in the world.

ALBATROSS SQUAWK

Nearly half a million come back here each year to raise their young.

Adults share parenting duties,

returning every few days to feed their chick.

Feeding done, it's time to head out to sea.

They need to make their way to the edge of the packed colony

where there's more room for takeoff.

CHICK CRIES

Albatross are so heavy that they can only get airborne

in places where the wind is strong enough.

Using a special runway...

..with a good headwind...

..she's off.

From the air, the ocean may appear featureless,

but beneath the surface,

a network of powerful currents is constantly on the move.

It's these currents, more than any other force,

that determine the distribution of life out here.

A whole community of ocean drifters

hitches rides on these rivers in the sea.

Pelagic red crabs.

They've gathered to feed on tiny floating plants and animals,

a bloom of plankton fuelled by the currents.

Fine hairs on their legs slow their descent...

..and then, with a few flicks of the tail,

they swim back up to continue feeding.

The currents that carry these wandering crabs

also serve as highways for the ocean's larger predators.

Striped marlin.

Beautifully streamlined,

they can travel huge distances with minimum effort.

These hunters patrol the boundaries between ocean currents,

where their prey often gathers.

Each predator has an incredible sense of smell,

able to detect faint trails left by their prey.

Somewhere out here is the big prize...

..and hunters of all kinds are looking for it.

Great shoals of fish are attracted to a plankton bloom.

A single school of sardines can be many miles long.

The fish swim tightly together - there's safety in numbers.

Their defence relies on coordination.

When attacked, the sardines move as one.

Each fish instantly matches the movements of its neighbour...

..and the whole shoal moves in synchrony.

A lone sea lion can't keep up with their rapid reactions.

Even when more sea lions arrive,

they can't seem to break down the sardines' coordinated defences.

With a shoal this big, the sea lions need to isolate

a smaller, more manageable group of fish.

But with so few predators, the fish still have the advantage.

All the sea lions can do is keep the sardines at the surface

and wait for others to join them.

Tuna.

Their arrival changes everything.

Tuna attack from below,

cutting off the sardines' escape route down to deeper water.

Next to appear, shearwaters -

excellent fliers, but also surprisingly agile underwater.

With so many predators attacking from all sides,

the advantage starts to shift away from the sardines.

As the fish pack ever tighter,

their shoaling strategy now makes it easier for the hunters.

Copper sharks.

They've scented blood in the water.

Surprisingly, perhaps, the predators never attack one another.

They work together to corral the ball of fish,

taking turns to grab a mouthful.

Common dolphins.

As the shoal gets ever smaller,

each sardine scrambles desperately to hide in the middle.

But now, there's no escape.

A Bryde's whale finishes off the feast -

tonnes of sardines devoured in less than an hour.

The predators melt away into the blue...

..going their separate ways once more.

This blue whale is still searching for a meal

to satisfy its giant hunger.

Being so large,

it must catch an average of four tonnes of food a day.

But many days may pass without feeding at all.

It is their great size that enables blue whales to travel the furthest,

roaming every ocean from the tropics to the poles.

BLUE WHALE SPOUTS

Trapped against the surface by fish,

a dense patch of krill.

This blue has finally found

what it's been searching for for so long.

A meal big enough to make opening its massive mouth worthwhile.

The krill swarm is hundreds of metres across

and packed tight.

The whale lines up on its prey,

targeting the densest part of the shoal.

It takes so much effort to swim with a fully extended throat

that the whale virtually comes to a standstill.

The whale uses its tongue to force the water out of its mouth,

trapping the krill on plates of hairy bristles that line its jaw.

But it takes time to sieve so much water.

And that gives more nimble hunters their chance.

Blue whales may not be as agile as other hunters,

but they don't need to be.

In one giant mouthful,

they can swallow whole swarms of krill.

No other predator is better suited

to exploit the scattered riches that the open ocean can provide.

The blue whale -

the greatest hunter in all the world's oceans.

The open ocean created many challenges for the Hunt team.

But none came bigger than trying to film blue whales underwater.

Achieving this was to turn into a two-year mission.

The crew teamed up with John Calambokidis,

the world's foremost blue whale scientist.

John gets crucial information from these harmless tags.

But he can only observe blue whales for the brief time they surface.

Well, part of this research and part of the reason

that, er, we're working with film-makers

is it gives us an unique chance to get scientific information

we wouldn't be able to otherwise.

So we are doing this under our research permit

to get an insight into what they're doing underwater -

how they're diving, how they're swimming, how they're feeding.

Pictures of blue whales are rare.

They're the biggest animal ever to live on the planet,

but there are only a few underwater pictures that exist today.

A few where you can actually see the animal.

First, they have to find a blue whale.

Well, I always feel really embarrassed, you know,

because how can you lose the biggest animal that's ever lived, you know?

But...er, while the whale is big, the ocean is even bigger.

Just over here.

The team's success will depend on good water visibility.

Too murky, and the whale literally disappears into the gloom.

No shot.

The first year was blighted by poor visibility

and the shoot ended in failure.

Year two, and the sea conditions are much better.

Oh, here he is, right here.

Blue whales only surface for two to three minutes at a time

before diving for up to 15 minutes.

It's a narrow window for everything to come together.

There's a bit of tricky manoeuvring here for John.

He's got to try and get the boat

ahead of the whale but not in front of it.

Then David has to swim out

and hope the whale passes close enough to get a shot.

It's too far.

It's a little too far that time.

This year, the water is exceptionally clear...

..but getting David in the right position still takes time.

He went by, I definitely got a shot.

It's not one of our best.

Missed it.

After weeks of effort,

all the elements finally come together,

giving David the chance of a lifetime.

Like, I got down to about 25 feet. I knew that whale was out there.

But, er...he just came out of the blue, right to me.

I could see his eye, the details of his mouth,

every scratch on his skin.

And he cruised on by - it took forever.

You know, they're so big - it just went by like a freight train.

I saw his tail slide by.

And he slipped back into the blue.

It...it was awesome.

That is awesome - I have never, of all my years of doing this,

I've never got a shot like that.

That was amazing.

Images of blue whales underwater are so rare

that this shot of one simply swimming by is a major success.

But the crew need more.

This series is all about hunting,

and so, nice though it is to have that shot, to make a sequence,

we've got to get shots of blue whales eating krill.

And what we've got to wait for

is when the krill actually comes to the surface.

It might happen once, maybe twice a month.

The crew come across an encouraging sign,

from a rather unsavoury source.

That's some whale faeces.

So we've found a big whale poo in the water.

Um, it's a good sign for us, we know that they're feeding here,

so maybe they'll stick around.

Finally, the crew find what they've been looking for -

krill at the surface.

Look at those birds in the water.

So there's a small krill ball.

We're going to go take a look at it.

All right - remember, don't put me right on top of it.

All right, ready? OK, the ball's right here.

- Right here, right here. - Is it on the surface? - Yeah.

- Good red? - Up at the top.

- Got it? - Yeah, got it.

No-one was prepared for what happened next.

Right behind you! Hey, Hugh, here he comes!

Oh!

Massive surfacing.

Oh, God - that's a shot. I bet you he got it.

I bet you he got it.

Oh!

- MAN ON BOAT: - God, they're beautiful, aren't they?

- What did you guys get? What did you guys get? - Oh, my God.

Oh, my God.

I still... I can't actually quite get my head around what I just saw.

We were down there, Dave was filming the krill balls,

and I just noticed, literally between his fins,

this massive great whale just loomed out.

David saw him, panned down.

I don't know where it came from.

Well, when I first looked down and I saw this whale,

I was a little bit stressed out,

because 95 feet of blue whale is going between my fins.

He hooks around, comes up and just grabs a big mouthful.

They're called blue whales for a reason,

you just see them underwater,

this bright, iridescent cobalt blue

just pops out of the blue of the ocean.

Finally, I get round to the other side,

I go "OK, he left," we're like, "Phew! That was pretty intense."

So I'm up trying to get just another shot of the krill

and sure enough, wide open, he comes through one last time.

I was kind of in the wrong spot,

I had to do some serious evasive manoeuvres.

But he comes through, closes his mouth...

It was just amazing.

The whole thing probably lasted ten minutes

but it was hands down, without a doubt, no questions,

the most intense, amazing thing that I've ever seen.

To share the water and to look eye-to-eye with a blue whale

is something I will never, ever forget.

These shots gives John a unique insight

into the feeding behaviour of blue whales.

Let's just look at the side of his mouth there.

So, basically, the water flow, you'd expect,

would be coming out the back there.

Yeah, look at that little fold there,

that is really interesting.

That's a great view.

That's fantastic.

Notice the full rotation there. Oh, that is...

And the full inversion.

OK - that's, again, a unique chance

to see a perspective we don't get to see.

With this close collaboration,

the team have filmed blue whales as never before

and at last have started to reveal

the secret life of the ocean's greatest hunter.

Next time, the hunt is on out in the open.

On the deserts and grasslands...

..where hunters and hunted have nowhere to hide.

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..and follow the links to the Open University.