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 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 100 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 14 metres long and 20 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 search 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,
3,000 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.
0thers are here, too.
Now, they just need 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 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.
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.
0nly 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.
0ne 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 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,
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 100 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 fish that stayed out of the shoal would be quickly picked off.
Within it, there is at least S0ME 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 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
300 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 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.
0cean travellers 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
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.
0ut 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 it finally sinks.
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's a giant kelp plant,
ripped from rocks off the coast of California.
Now, it's floating above thousands of metres of water,
held up by its gas-filled floats.
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'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.
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
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 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
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 0cean, however,
is peppered with over 23,000 islands,
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 40 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 sea mount.
As currents sweep towards it,
they're 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,
feeding where the currents are strongest
and the plankton most dense.
Where the cold water mixes with warmer water at the surface,
there's 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 D0 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've come to the sea mount to spawn.
Trigger eggs are good food,
and the planton feeders gather what they can
before the current sweeps them away.
This community is here because of the nutrients and plankton
the sea mount deflected into the surface waters.
But ocean-going hunters
are never far away.
Silky sharks specialise in picking off injured fish
and constantly check over the residents around the sea mount.
At some times of the year,
seasonal changes make the currents especially rich in nutrients,
and the ocean around the sea mount 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'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've 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 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 competing 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 many hours.
By midday, they're nearing the islands of the Azores,
900 miles west of the Portuguese coast.
0ther 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.
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.
0ut to sea, the dolphin have found prey.
They are driving a shoal of small mackerel up towards the surface.
The shearwaters crowd the skies,
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 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 sinks into the depths towards safety.
The shearwaters follow it down
to the very limit of their breath-holding ability,
perhaps as deep as 15 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.