Open Ocean 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 100m 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 14m 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.

In a few days, waters that 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 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 on 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 would happen,

for it arrived at exactly this moment

and it's 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 eggs left will be so dispersed that they're not worth collecting.

But other perils await them as they join the eggs, 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 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.

They hover, facing into the wind, picking out morsels near the surface,

including eggs.

Only a tiny percentage of the eggs will survive long enough to hatch.

These newly emerged tuna are only three millimetres long.

Although they can swim, they're still very vulnerable.

It will be many weeks before they swim strongly enough to make any 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 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 the mouth 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 where 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 up to 100 miles in a day.

And while they travel,

they play.

They have detected 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.

Individuals that stayed out of the shoal would be quickly picked off.

Within it, there is at least some chance of survival.

The noise 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 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 the shark and the 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 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.

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...

..A 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 what these rays are doing, forming a 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 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.

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.

Even the very best of health clinics can only trade on a temporary basis.

The seaweed rafts rot

and eventually lose their buoyancy.

Then their lodgers will have to find a new home.

If they can't, they will be eaten and 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'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 by a lucky current.

Now they're growing up in the reeds growing around the island's fringes.

More plankton and juvenile fish are being swept towards the island.

But now, there's a welcoming committee.

Schools of trevally fish and blue maomao patrol 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, like these 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 -

sea mounts whose summits do not break the surface.

Juvenile fish, for their first few months,

would do well to avoid such places.

These yellowfin tuna are six months old.

They are 40cm long - big enough 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 are deflected up its towering walls.

The water from the depths

carries plankton and nutrients to the surface.

Reef fish take up residence, feeding where the plankton's most dense.

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

there is a strange shimmering effect, a sign the currents are strong.

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,

but not in order to eat them.

They, like the sunfish, are looking for cleaners

to rid them of their parasites.

White-tipped reef sharks gather here, too. They DO eat reef fish.

They 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.

They spend much of their time

in open water, but they've come to the sea mount to spawn.

Trigger eggs are good food.

The plankton feeders gather what they can before they're swept away.

This community is only here due to the nutrients

the sea mount deflected into the water.

But ocean-going hunters

are never far away.

Silky sharks pick off injured fish

and 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 becomes a soup of plankton.

At such times, hunters gather in astonishing numbers.

Bonito, smaller relatives of the tuna,

are searching for smaller plankton feeders attracted by the bloom.

So are these jacks. Their prey is nearby.

A school of anchoveta has strayed to the surface

even though it's broad daylight and hunters are on the prowl.

They can already feel the vibrations of the approaching predators.

Swimming at speed, they form a ball and wait for whatever comes.

They've been rumbled.

At first, the scale of the bait ball seems to daunt the predator.

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 the attack is so great

that 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 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 third year of life have sufficient stamina

to keep up with the fast-moving spinners.

This is 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 hunt in small family groups, but in midsummer they head for traditional socialising grounds,

where they will assemble in super-herds, several hundred strong.

Already, two families have joined together.

Males are starting to compete for 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 midday, they're nearing the Islands of the Azores -

900 miles west of the Portuguese coast.

Other hunters are already here - corys shearwaters.

500,000 of these birds breed on the Azores every year and scour the 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 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, following the dolphins' every turn.

The mackerel are still some metres down.

When the baitfish come sufficiently close,

the airborne division makes its move.

Far from being mere bystanders, the shearwaters can now become 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 yellowfin 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. The shearwaters battle among themselves.

As long as predatory fish or dolphin remain at the scene,

the mackerel can't escape.

But when the skipjack tuna start to move away,

the bait ball begins to sink into the depths towards safety.

The shearwaters follow it down

to the limit of their breath-holding ability, maybe 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,

in 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, seeking another feeding opportunity -

the next pulse of life in the distant reaches of the open ocean.

The open ocean is so vast

that finding anything in this big blue arena is difficult.

The Blue Planet team invested over 400 days

on often unsuccessful trips far out to sea,

and many days were spent looking and waiting.

Somewhere in this marine desert

there were amazing animal dramas played out every day.

Their reward for finding them was to gain a unique insight

into some of the least-known animals on our planet.

The day is young - a little after four in the afternoon.

Historically this is the time that the predator hour starts up.

'The open ocean, which I call liquid space,

'is the most difficult area to work in the ocean.

'People wonder how we film out there. Do you throw the camera in?

'There's hours of boredom. Nothing goes on.

'A little voice says, "Stay focused."'

Patience, patience. Looking, looking.

After weeks of searching,

the crew finally caught up with a pod of spinner dolphins.

I keep my eye on areas of the herd where there's an open spot,

an area that allows spinning. It happens in a minute or two.

An adrenaline rush and it's over.

The action is over almost as quickly as it started.

Then there is nothing to do but keep scouring mile after mile of empty ocean.

Our crew spent six weeks off Panama searching for tuna and the smaller fish they attack.

They look for signs of feeding activity at the surface, and when it finally happens,

the crew must work hard to make the best of the action.

The prey fish crowd together in a bait ball - there is some protection for an individual in numbers.

Diving into this frenzy requires a great deal of care.

One thing when filming bait balls, particularly sardines, that you must be careful of is that

when you're in the water, sardines will often swarm you to try to seek shelter, to hide from predators.

And through the camera viewfinder, I could see a piece of bait ball break off and come directly at me.

Fish were on me; the tuna knew it and were rocketing out of the deep.

Just for a second, I felt like I'd become part of this feeding frenzy,

like a big sardine in the middle of a bait ball.

Finally, when it was all over, all that remained

was just a shower of fish scales slowly sinking into the deep...

and me.

Clues to what's happening underwater are circling birds or moving dolphins

and Taco the sea-dog had spotted some action.

I should probably try to get in the water here, huh?

Why don't you just go on by 'em here and we'll slow up, mellow out...

Get my grey!

Hundred yards!

'You're pumped up, you're hunting, and you're hunting with a camera.

'You have to filter out an awful lot of what's going on around you.

'Get specific, get in there where the action is.'

'A lot of our success is based on intuition, so it's called sixth sense - that we follow down trails

'and sometimes they lead us nowhere, and other times you just have that feeling something's going to happen.

'I can't put a handle on when you get that feeling, but you know it's special and you better be ready.'

Good stuff, lots of marlin.

OK, Gary, let's do it again please.

Getting these shots of marlin was the culmination of over four years of persistence and many failed trips.

It was a thrilling but somewhat dangerous moment - these underwater javelins can swim at 70mph.

All the marlin activity attracted other predators...

tuna...

..and then an extraordinary bonus - a sei whale.

'Doug and Dave went into the water. I stayed on deck to try to link the shot

'between the underwater world and the surface world.

'Here we were at the right time, we followed groups of whales feeding

'and we put cameramen in the water

'at the same time we were filming from the surface, so we could get proper coverage.'

Subtitles by BBC Subtitling, 2001

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