The Blue Planet

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.