Original subtitles

These seas,

thousands of miles from nearest land,

are the most sterile on our planet.

These are marine deserts.

But here live the swiftest and the most powerful

of all ocean hunters.

Simply finding them is an immense challenge,

but we are about to follow them as they search for their food

in this little-known part of the seas--

the open ocean.

Striped marlin.

Voracious predators that can grow to three meters long.

They hunt mainly in daylight,

searching the tropical oceans

from close to the surface down to depths of

a hundred meters or so.

Normally the fish they feed on are widely dispersed,

but sometimes their prey gathers in dense shoals,

like these sardines.

This feast may last for over an hour.

Time enough for other hunters

to reach the scene.

Juvenile tuna join in the feeding frenzy.

The noise attracts a giant-- a sei whale.

It's 14 meters long

and 20 tons in weight, and has an appetite to match.

Soon the only sign that the sardines

ever existed are scales sinking down into the abyss.

Such feasts don't last long.

Within a few short days, waters that once

swarmed with food will have been cleaned out.

The hunters must move elsewhere

and once again start their search of

the seemingly featureless open ocean.

A manta ray, immense.

Five meters across from the tip of one wing-like fin

to the tip of the other.

It's traveling economy,

wasting as little energy as possible

as it glides through the waters of the tropics.

The remora fish that accompany it

travel more economically still, by hitching a lift.

Their host is searching for food--

plankton, the minute fish and invertebrates

that float near the surface.

It needs lots of them and may cruise for days

before it finds a good feeding ground.

Dusk at the edge of a Pacific island

3,000 miles from the nearest continent.

Here surgeonfish have assembled to spawn.

As they perform their nuptial dances,

they discharge clouds of eggs and sperm into the water.

The manta must have known this was about to happen,

for it arrived at exactly this critical moment.

And it is not the only one to do so.

Others are here, too.

Now all they need to do is to sweep the water

into their mouths and sieve out the eggs.

Within an hour, the whole event will be over.

Any surviving eggs will be so dispersed

that they are not worth collecting.

But other perils await them

as they join the clouds of eggs and larvae

and tiny fish that drift through

the surface waters of the open ocean.

These are the eggs of yellowfin tuna.

If the hatchlings survive, it will take them two years

to become adults.

In three years, they could be nearly

two meters long and weigh 200 kilograms.

Perhaps only one in a million will live as long as that.

They and the other animals and microscopic plants

of the plankton constitute the basis of all life

out on the open ocean.

Success is related to standing out, not fitting in.

The day you saw something in me...

my whole life changed.

Cheyenne is a dangerous place.

I got nothing left to lose.

You know, you really don't have to be so dramatic.

Whoo!

I'm moving up in the world.

So, when's the show start?

A storm petrel dancing on the water.

But this is no amiable waltz.

It's a hunt.

As they hover facing into the wind,

they pick out morsels from near the surface,

including eggs.

Only a tiny percentage of the developing eggs

will survive long enough to hatch.

These newly emerged tuna are only three millimeters long,

and although they can swim, they're still very vulnerable.

It will be many weeks before they can swim strongly enough

to make any real headway in the ocean.

After the sun goes down,

other predators rise from the depths

to attack the floating multitude.

Darkness shrouds the arrival of

battalions of dangerous drifting predators.

These shimmering comb jellies-- sea gooseberries--

trap their prey with sticky, net-like webs.

One ill-timed fin-stroke could bring certain death

to a hatchling fish.

There are many kinds of

these comb jellies, all of them very

effective hunters.

By dawn, most of the nocturnal feeders

will have returned to the depths.

The surviving hatchlings, however,

have already started on their travels.

Vast current systems, like immense rivers,

carry them around the ocean basins.

The boundaries between these masses of moving water

form invisible barriers that can trap

both plankton and nutrients carried up from the depths.

So parts of the ocean become rich with food

for days or even weeks at a time.

This attracts vast schools of plankton-feeding fish,

like these sardine.

They take in water through their mouths

and expel it through their gills,

sieving out the plankton, which is then

funneled down their throats.

The immense schools travel along the boundaries of the currents,

seeking the spots where the plankton is thickest.

As the position of the current boundaries

changes constantly, so does both the supply

of plankton and the numbers of fish.

A small pod of Pacific spotted dolphin

20 miles from the coast of Panama.

Like all predators, they seek parts of the ocean

where their food is thickest.

They cover as much as a hundred miles in a day.

And while they travel, they play.

They have detected the sound of schooling fish

from hundreds of meters away and start to

track down the shoals using sonar,

leaving their toys behind them.

For the hunted, there are few places to hide.

Schooling mackerel.

They have already sensed the sonar beams

of approaching dolphin.

Their only defense is to gather into a ball.

Any individual that stayed

out of the shoal would be quickly picked off.

Within it, there is at least

some chance of survival.

The noise of the attack alerts another predator,

a sailfish, one of the fastest fish in the ocean.

It has detected rapid vibrations in the water

and is searching for the cause.

Sailfish rely on eyesight for their final approach,

so they hunt mainly in daylight.

When sailfish become excited, they change color,

lighting up with bright blue stripes.

Since mackerel eyes are especially sensitive

to blue and ultraviolet, these colors confuse them,

making them easier to catch.

Far below, a blue shark returns from

a squid-hunting trip in the cold darkness

300 meters down.

It's heading for the surface to reheat in the warmer water.

As it ascends, it detects the smell of

oils and proteins shed into the water

by the panicked mackerel.

The trail leads both the shark and its attendant pilot fish

towards an easy meal.

Scraps and casualties

float in the wake of the passing mackerel school.

Throughout the ocean, predators and prey

are locked in a deadly three-dimensional contest

of hide and seek played out over immense distances.

To survive, they must travel.

The huge, four-meter long bluefin tuna

has special blood vessels

that enable it to keep its body temperature

significantly warmer than the surrounding water.

As a result, they can survive in much colder conditions

than any other tuna, and they travel

thousands of miles away from their spawning grounds

in the tropics to hunt in cold seas,

where the food supply is richest.

Ocean travelers come in many guises,

and few are stranger than this.

A crab that spends much of its life afloat.

It's a worrying passer-by for booby birds with delicate toes.

Many floaters are little more than jelly

enclosed in membranes,

but they may drift for vast distances.

And turtles, like these olive ridleys,

migrate thousands of miles every year.

The ocean is full of such wanderers

riding the currents and doing their best

to avoid enemies while they search for food

and a safe place to breed.

Which is exactly what these rays are doing--

forming the two-mile-high-club, gathering together

for courtship on the wing

far above the ocean floor.

More nomads-- flying fish.

They seem to be

on every large predator's menu,

so their whole life is spent on the run

in the open ocean.

They don't scatter their eggs,

but lay them on pieces of flotsam,

like this palm frond.

If the quality of water is right,

they will attach their eggs to the frond,

which will then serve as a kind of life raft

for their offspring.

But it's not only flying fish

that seek nurseries.

Any piece of floating debris can serve as a shelter

under which baby fish can hide.

The only drawback is that

predators like this wahoo

always check up on who's hanging about

in the shadows.

The wahoo may trail the flotsam for weeks.

Few bits of flotsam are without

their quota of lodgers.

Even man-made junk attracts them, and some,

like this oceanic triggerfish,

defend their squatters' rights

with vigor.

The triggers, in fact,

tend to claim all the prime residences.

Out here, even discarded netting

can provide valuable shelter, so,

in a bizarre twist, a wrecked trawl net like this

can end up as a sanctuary for fish

until such time as it finally sinks.

Indeed, a single

large piece of flotsam can be the reason why

several square miles of open ocean,

instead of being empty,

will support a fish population of hundreds of tons.

This huge clump of seaweed is doing exactly that.

It is a giant kelp plant ripped from the rocks

off the coast of California.

Now it's floating above thousands of meters of water,

held up by its

gas-filled floats.

Clouds of young rockfish are growing up

in the safety of its shadow.

Giants also seek out this algal flotsam.

This is a sunfish.

It can measure as much as four meters

from fin tip to fin tip.

Rather surprisingly, it has the record as

the heaviest bony fish in the sea.

Sunfish spend much of their time at depth

where they feed on jellyfish,

but it's cold and dark down there,

so from time to time,

they seek a little rest and recuperation

and warm up near the surface.

They too are looking for floating kelp plants.

Not for shelter, but because here

they can find a particular

kind of fish that only lives in such places--

halfmoon fish.

The sunfish

form up in an orderly queue.

They have a problem.

Their skin is covered in parasites.

The hungry halfmoons will help.

The sunfish turn their heads towards the surface

as a clear invitation to their personal hygienists.

The halfmoons nip off and eat every parasite they can find.

If the halfmoons don't do the job,

there is another rather drastic treatment available here.

Gulls rest on the floating kelp.

And if the sunfish send the right signals,

the gulls will investigate.

Their beaks can dig out the most stubborn parasites.

But even the very best of health clinics

can only trade on a temporary basis.

The seaweed rafts are rotting and will

eventually lose their buoyancy.

Then their lodgers

will have to find a new home.

If they can't, they will be eaten

or die and sink down into the abyss.

But the open ocean is not

entirely devoid of permanent shelter.

A volcano is erupting from the sea floor,

and it's still growing.

It has formed an island some 70 miles

from the coast of New Zealand.

Some juvenile reef fish have already arrived,

carried here by a lucky current.

Now they are growing up in the shelter

of the weeds growing around the island's fringes.

More plankton and juvenile fish are being

swept by currents straight towards the island.

But now, there's a welcoming committee.

Schools of trevally and blue maomao

are patrolling the surface water.

All are in search of a meal.

These one-kilo fish

snap up every morsel of plankton they find.

At times, the currents sweeping in from the open ocean

bring with them all kinds of

small creatures in dense concentrations.

These are mysid shrimps.

Very little that is edible is left after such feasts.

Islands are far from being safe havens for plankton.

The Pacific Ocean, however,

is peppered with over 23,000 islands,

as well as countless other submerged mountains--

seamounts whose summits do not break the surface.

Juvenile fish, for their first few months,

would do well to avoid such places.

These yellowfin tuna, however, are now more than

six months old and 40 centimeters long,

big enough to be able to eat fry.

So seamounts, for them, are promising feeding grounds

where they may hunt for several months.

The base of a seamount.

As currents sweep towards it,

they are deflected up its towering walls.

The water coming from the depths

carries both nutrients and plankton to the surface.

Numerous reef fish take up permanent residence here,

feeding where the currents are strongest

and the plankton most dense.

Where the cold water mixes with warmer water at the surface,

there is a strange shimmering effect,

a clear sign that the currents are running strongly.

But these currents attract more than just coastal fish.

Giants come here from the open ocean.

Hammerhead sharks, and in great numbers.

During the day, they circle the seamount,

looking for small fish at the reef edges,

but not in order to eat them.

They, like the sunfish, are looking for cleaners

to rid them of their parasites.

Whitetip reef sharks gather here, too.

They do eat reef fish.

But they prefer hunt at night,

when the reef fish are sleepy and easier to catch.

Far better to rest by day

and allow the cleaners to do their work.

Even swarms of breeding trigger aren't a serious temptation.

These triggers spend

much of their time in open water,

but they have come to the seamount to spawn.

Trigger eggs are good food,

and the plankton-feeders gather what they can

before the current sweeps them away.

This whole community is only here

because of the nutrients and plankton that

the seamount deflected into the surface waters.

But ocean-going hunters

are never far away.

Silky shark specialize in picking off injured fish,

and constantly check over

the residents around the seamount.

At some times of the year, seasonal changes

make the currents especially rich in nutrients,

and then the ocean around the seamount

becomes a virtual soup of plankton.

At such times,

hunters gather in astonishing numbers.

Bonito, smaller relatives of the tuna.

They are searching for still-smaller plankton feeders

that have been attracted by the bloom.

So are these Jacks, and their prey is nearby.

A school of anchoveta has strayed up

near the surface, even though it's

broad daylight and hunters are on the prowl.

These small fish can already feel the vibrations

of the approaching predators.

Swimming at speed, they have formed into a ball,

and now they must wait for whatever comes.

They're been detected.

At first, the sheer scale of the bait ball

seems to daunt the predators.

But now

the bonito arrive and launch the first attack.

Still the bait ball holds together.

The young yellowfin tuna move in.

The speed of this attack is so great

that gradually, groups of anchoveta

are splintered from the main fish ball.

Before long, the currents will shift,

and the ocean will become once more a blue tropical desert,

plankton-free,

and the hunters will have to move on.

Spinner dolphins, still searching

for food.

Their twisting leaps are apparently purely

social displays.

Since the hunting has been good,

many hundred have gathered together

in this exuberant super-pod.

But now the spinners are starting to hunt once more.

Their skill in tracking food is not a secret.

Yellowfin tuna must be aware of it,

for they regularly follow them.

But only adult tuna

in their second or third year of life

have sufficient stamina to keep up with

the fast-moving spinners.

These are another kind-- common dolphin.

They too are on the move.

As they travel, ever inquisitive,

they pay a call on

one of their larger relations, a pilot whale.

The whale is not hunting.

It's on its way

to its breeding grounds in the Mediterranean.

Pilot whales normally hunt in small family groups,

but in mid-summer, they head for traditional

socializing grounds, where they will assemble

in super herds several hundred strong.

Already, two families have joined together.

The males are starting to compete

for the favors of the females.

As the weeks pass by, these group rubbing sessions

will become more overtly sexual.

But now, it's just flirting in the sun.

Timing in the ocean can be crucial.

In summer, the northern Atlantic waters

are beginning to warm.

The hunting is good here and by June,

predators from southern waters are heading towards the Azores.

These are more common dolphin.

Like most oceanic dolphins,

they too often travel in huge herds

containing many different families.

There is seldom enough prey

in any one place to feed such numbers.

So small groups leave the super pod

and set off on hunting expeditions.

This group will be away from the main herd for several hours.

By mid-day, they're nearing the islands of the Azores,

900 miles west of the Portuguese coast.

Other hunters are already here-- Cory's shearwaters.

Half a million of these birds breed on the Azores every year

and scour the nearby ocean for food.

Right now there's insufficient wind

to support gliding flight, and since flapping

is a waste of energy, they sit out the calm

clustered in rafts and riding the gentle swells.

By mid-afternoon,

the dolphin are starting to hunt in earnest.

As the sea breeze picks up,

the shearwaters take to the air once more.

Out to sea,

the dolphin have found prey.

They are driving a shoal of small mackerel

up towards the surface.

The shearwaters crowd the skies above,

following the dolphins' every turn.

The mackerel are still some meters down.

But when the bait fish come sufficiently close,

the airborne division makes its move.

Far from being

mere bystanders, the shearwaters can

now become underwater predators themselves.

Incredibly, they can dive down to

depths of several meters.

The hunting dolphin prevent the mackerel

from escaping downwards, and both predators

gorge themselves.

Soon the diving birds

outnumber the dolphin, and even drive them away

from their meal.

But another squadron of predators

arrives to replace the dolphin--

adult yellowfin tuna.

These are giants two meters long.

They're heading directly for the bait ball.

Despite the arrival of the giant fish,

the shearwater continue to press home their attack unfazed.

Eventually the tuna move on,

leaving the shearwaters to battle among themselves.

As long as a predatory fish

or a dolphin remains at the scene,

the mackerel can't escape to safety.

But when the little skipjack tuna

start to move away, gradually the bait ball

begins to sink into the depths towards safety.

The shearwaters follow it down to

the very limit of their breath-holding ability,

perhaps as deep as 15 meters.

At last, even they are forced to leave their quarry.

However good or bad this summer's feeding may be,

within three months, winter will be on its way

and the temperature of these waters

will drop by a few degrees.

Then the ocean hunters will abandon the Azores once more.

As ever, they will move on,

searching for another feeding opportunity,

the next pulse of life

in the distant reaches of the open ocean.