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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 metres long.

They hunt mainly in daylight

searching the tropical oceans from close

to the surface down to depths of a hundred metres 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 fourteen metres long and twenty tonnes 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 metres

across from the tip of one wing-like fin to the tip of the other.

It's travelling 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

three thousand miles from the nearest continent.

Here surgeon fish 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 yellow fin tuna.

If the hatchlings survive, it will take them two years to become adults.

In three years, they could be nearly two metres long

and weigh two hundred 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.

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 millimetres 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 multitudes.

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 funnelled 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

twenty 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 hundred miles in a day.

And while they travel, they play.

They have detected the sound of schooling fish from hundreds of metres 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 defence 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 colour

lighting up with bright blue stripes.

Since mackerel eyes are especially sensitive

to blue and ultraviolet, these colours confuse them

making them easier to catch.

Far below, a blue shark returns from a squid-hunting trip in the

cold darkness three hundred metres 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 metre long blue-fin 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 travellers come in many guises

and few are stranger than this...

crab that spends much of its life afloat.

It is 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 draw back, 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 trigger fish,

defend their squatters rights with vigour.

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 tonnes.

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 metres 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 metres 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 is 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.

Half moon fish.

The sunfish form up in an orderly queue.

They have a problem.

Their skin is covered in parasites.

The hungry half-moons will help.

The sunfish turn their heads towards

the surface as a clear invitation to their personal hygienists.

The half-moons nip off - and eat

every parasite they can find.

If the half moons 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 seafloor and it is still growing.

It has formed an island some seventy 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 is 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 twenty three thousand...

elands as well as countless other submerged mountains

sea mounts - whose summits do not break the surface.

Juvenile fish, for their first few months

would do well to avoid such places.

These yellow-fin tuna, however,

are now more than six months old and forty centimetres long

big enough to be able to eat fry,

so sea mounts 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 sea mount 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.

White-tip reef sharks gather here too.

They do eat reef fish.

But they prefer to 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 specialise 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 anchovetta has strayed up near the surface,

even though it is 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 yellow fin tuna move in.

The speed of this attack is so great

that gradually groups of anchovetta 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.

Yellow-fin 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 socialising 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 favours 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

nine hundred 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 is 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 metres down.

But when the baitfish 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 metres.

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 yellow-fin tuna.

These are giants - two metres long.

They are 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 in to the depths towards safety.

The shearwaters follow it down to the very limit of their breath-holding ability

perhaps as deep as fifteen metres.

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.

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