Original subtitles

A Perfect Planet.

All life in the oceans depends on the continuous movement of water.

There are not five separate oceans on Earth,

but just one...

whose parts are linked by powerful, unceasing currents.

Every drop of seawater on Earth rides these currents,

taking a thousand years to complete a single circuit.

And where there are currents...

there is life.

Off the coast of South Africa,

dolphins are on the hunt.

They have found a cold-water current

and are now travelling along it looking for food.

Gannets follow them.

They know that doing so is the fastest way to a meal.

A shoal of mackerel -

just what the dolphins have been looking for.

They encircle the fish, driving them into a bait ball,

and then trap them against the surface

to prevent them from escaping to deeper water.

Now the fish are within range of the dive-bombing gannets,

who hit the water at 50 miles an hour.

A sudden gathering of thousands of predators brought together

by the flow of currents.

Last to the feast are sharks.

In these vast open waters,

finding food would be all but impossible without currents,

the highways of the seas that bring this life together.

When the bait ball has been dispersed,

all that is left are scales drifting downwards.

They are part of a slow, never-ending blizzard of organic waste

that eventually settles on the sea floor.

But it doesn't stay here for ever.

The currents sweep it back up into the sunlit surface waters...

where it nourishes clouds of phytoplankton...

simple microscopic plants that are the pastures of the seas.

There are thousands of different kinds,

and together they produce half of all the oxygen in the atmosphere...

more than all our forests and jungles combined.

And by absorbing carbon,

they are our greatest ally in combating climate change.

Plankton are the foundation of almost all life in the ocean,

for in those places where the currents bring nutrients to the surface,

they multiply in astonishing numbers...

turning the ocean green.

The currents travelling through our oceans bring life to seas

that would otherwise be marine deserts.

The Galépagos Islands lie in the path of one of them,

the deep-flowing Cromwell Current

that runs for 6,000 miles across the Pacific.

As it approaches Fernandina Island,

it rises and delivers nutrients into its shallows.

And it also brings life to this otherwise barren island.

Iguanas.

There are thousands of them.

And yet there's nothing on the island for these vegetarians to eat.

Or...

almost nothing.

Cormorants bring seaweed ashore with which to make their nests.

But what is building material for a cormorant

is food for an iguana.

Both these species evolved here,

but that doesn't necessarily make them good neighbours.

No matter. He knows where there's more elsewhere.

He's a marine iguana...

the only lizard in the world

that gets its food from the sea.

The seaweed on which he totally relies

only grows in abundance here

because of the nutrients brought by the Cromwell Current.

Once in the water, he has just 30 minutes to find food.

Any longer than that,

and his muscles will seize up and he'll drown...

for, like most reptiles, he can't handle the cold.

Chilly water isn't a problem for a warm-blooded cormorant.

She can swim in it all day,

but can only hold her breath for a few minutes.

He, on the other hand,

completes his whole half-hour trip

on one single breath.

His flat face and sharp teeth

make him an efficient seaweed-cropping machine,

but with the clock ticking, he must eat fast.

The cormorant, having caught its fish,

goes back to the surface.

One last mouthful,

and it's also time for the iguana to head for home.

But to stop his muscles from seizing up in the cold water,

he must get back quickly.

So he could do without the attentions of an inquisitive sea lion.

Dry land is now just 30 metres away,

but the biggest hurdle is still to come.

The surging water now fights against him.

He's out,

but he stayed in the cold so long that he's lost his strength.

And he's made it.

Few reptiles on the planet have to work harder for a meal than he does.

And tomorrow,

he'll have to do it all over again...

unless next time

he can outwit his neighbour.

Over 100,000 marine iguanas live on Fernandina...

and each owes its existence to the Cromwell Current

that brings nutrients to these shores.

But there is another, much bigger current

which carries water from the Pacific into the Indian Ocean.

On this great journey, it travels through the islands of Indonesia,

bringing together life from both oceans.

A third of all the world's reef fish live here.

Some call it the Coral Triangle,

the most diverse marine region on Earth.

The variety here is dazzling, not just of coral,

but of animals of all kinds.

Few are stranger than the flamboyant cuttlefish.

This is a male, just five centimetres long.

Swimming against the current isn't easy when you're small,

so, instead, he prefers to walk...

very, very slowly.

He's a master of camouflage.

But right now he wants to be noticed.

He's looking for a mate.

His potential partner is a giant by comparison,

four times his size.

When it comes to courtship,

being flamboyant isn't enough.

To win her over, he must dazzle.

His aim is to deposit a packet of sperm

inside her mouth.

Close...

but no cigar.

He'll have to turn up the dazzle.

Take two.

Bingo!

His job is done.

Now she must find somewhere to lay their eggs.

An old shell will do nicely

if she can slip past the present occupant.

She fastens her eggs to the underside of the shell,

where they'll be safe from predators.

The current that brings so much life to the Coral Triangle

now washes the eggs with clean, oxygenated water.

After just three weeks,

they start to hatch.

Smaller than a human fingernail,

the hatchlings are now carried by the current

to other parts of the reef.

And in just a few months,

this young male will be ready to find a female of his own.

By a stroke of cosmic good fortune,

the Earth has a satellite,

the moon,

which orbits our planet every 27 days.

Its gravitational pull drags our oceans across the planet...

and so gives us the tides.

Unlike currents that stir the open ocean,

the tides have their greatest impact on the coasts,

flushing them with nutrients from both sea and land.

And nowhere are they more violent and dramatic than here...

Norway's Saltstraumen strait.

Every six hours,

nearly half a billion tonnes of water

are forced through a channel

just 150 metres wide.

Its very narrowness accelerates the water...

making this the strongest tidal pull in the world.

Most animals caught here would be swept away.

But not these tidal specialists.

Eiders are one of the few ducks

that depend totally on the ocean

for their survival.

And they're the only kind strong enough

to live permanently in these racing waters.

But there is food here, and in great quantity,

for any that can gather it...

mussels.

They filter out particles of food brought to them by the tide.

And eider ducks love mussels.

The challenge is reaching them.

Eiders seem to be the only creatures

that can hold their own in the fast-flowing water...

so they have the mussels all to themselves.

They swallow them whole, shell and all.

Each eider duck eats hundreds of mussels a day...

a year-round feast

that no others can reach.

The tides here owe their power

to the unique geography of the coastline.

But elsewhere in our oceans,

the lay of the land influences tides

in a very different way.

Here in the Bahamas,

wide, shallow sandbanks mean the tide moves gently

over the sea floor...

turning what would be a sandy desert

into a rich underwater habitat.

This is the home of garden eels and razorfish.

And fresh food arrives for them from deeper waters twice a day.

Life seems unhurried and gentle...

but there is trouble in paradise.

These bottlenose dolphins eat razorfish,

and they're not so easily fooled by vanishing tricks.

They scan the sand with echolocating clicks

to discover exactly where the razorfish are hiding.

But knowing where they are is not the same as catching them.

The more the dolphins dig,

the deeper the razorfish burrow.

But it's clearly not deep enough.

Blowing jets of water into the sand

exposes even the most hard-to-reach razorfish.

Before long, the dolphins have had enough and they move on.

It looks as if they have picked the sand clean...

but here, at least,

there really are plenty more fish in the sea.

Closer to the land, the same tides bring nourishment

to one of the most threatened of coastal habitats...

mangrove forests...

part land...

part sea.

Mangroves are the only trees

capable of surviving in salt water

and are specially adapted to it coming and going twice every day.

As seawater floods in, fish come with it.

Here, in the flooded forests,

they can find both food and shelter.

Stingrays ride on the incoming tide.

Other commuters follow.

Young lemon sharks, still far from full-grown,

are looking for food.

When the tide is at its highest,

even adult lemon sharks

can get into the mangroves.

A three-metre female moves cautiously into the shallows.

She can't stay here for long,

but then she hasn't come here to hunt.

She's come to give birth...

returning to the very place where she was born.

She has nourished the pups inside her body with a placenta,

as we do.

The mangroves provide an ideal nursery for them,

and placing them here gives them an excellent start,

but that is the end of her parental care.

She has to return to deeper water

before the tide goes out.

Her young must now fend for themselves.

The pups instinctively take refuge among the roots of the mangroves.

They're so small they can swim deep into this tangled labyrinth.

With the tide fast receding,

even they need to find a place

where they won't be left high and dry...

a place like this...

a permanent pool in the heart of the mangrove forest.

Only the smallest sharks can get here,

and only at the highest tides.

The pups will spend the next two years here,

perfecting the skills that make them one of the ocean's top hunters.

And it seems...

that there's a lot to learn.

Got one!

All life at the coasts has to move to the daily rhythm of the tides,

but tides are not the same throughout the year.

Every month, when our planet, the moon and the sun are all aligned,

the increased gravitational pull produces particularly high tides.

And this triggers a truly extraordinary event

on one particular reef in the central Pacific.

Thousands of resident surgeonfish

begin to assemble on these high tides.

And they are being followed by one of the largest fish in the sea...

manta rays.

The rays spend their year moving between coral islands.

But it's only now, when the tide is at its highest

and the surgeonfish have gathered,

that they appear on this particular reef.

Their timing is so perfect

that they rarely have to wait more than an hour

for the event to begin.

At the precise moment when the tide is at its highest,

the surgeonfish begin to spawn.

They release billions of eggs and sperm into the water.

Breeding in this way gives their fertilised eggs

the best chance of being carried on the tide

away from predators that haunt the reef...

all except one.

The mantas move in.

They gorge on the eggs,

filtering them out using specially adapted gills.

If the mantas had arrived just an hour later,

there would have been nothing here for them to eat.

No-one knows how the mantas are so perfectly in tune

with the rhythm of the tides.

But they appear without fail whenever the surgeonfish spawn.

Most of the eggs, however,

are carried out into the open ocean

before the mantas are able to eat them all.

The rhythms of coastal life are influenced by another ocean force.

Winds blowing over the sea so batter the surface

that it begins to rise and fall.

These swells may travel far

and reach the shores of even the most sheltered bays.

As they approach shallower water,

they turn into waves.

A shoal of hardyheads,

close to the beach of Australia's Lizard Island.

The clearness of these glassy waters shows that they lack nutrients.

But the gentle waves expose food hidden in the sand,

and that's what the hardyheads are looking for.

But... they must beware.

Packs of trevally are on the hunt.

The hardyheads stick together.

There's safety in numbers.

But they're vulnerable nonetheless.

They're so small, they can swim in the shallowest waters...

even in the body of the waves themselves,

out of the reach of their enemies.

But trevally aren't their only concern.

Blacktip reef sharks.

They are bigger and more powerful than trevally...

but not as fast or as agile.

The hardyheads are well aware of them,

but so long as they stay just out of reach, they have little to fear.

But now the sharks and the trevally join forces.

Together, they enter the shallows,

each looking for a chance to attack.

The trevally make the first move...

and the hardyheads take refuge again in the waves.

And this is what the sharks have been waiting for.

Surging forwards, they chase the hardyheads out of the water...

beaching themselves in a daring bid to hoover up their prey.

The hardyheads that escape the sharks

swim back out to deeper water...

but into the mouths of the trevally.

Now the receding waves help to pull the sharks back

into deeper water.

In the chaos, the seabirds get their chance.

It's a feeding frenzy

in only ten centimetres of water.

The power of waves is dramatically evident

when they crash onto our shores.

But the biggest of all start far away from land, out at sea.

Great storms blowing over the surface of the ocean

raise towering walls of water.

Such giant swells can travel for thousands of miles.

As they approach land,

the shallowing sea floor begins to drag on their undersides,

and they topple forward...

and break.

This stirring of the ocean produces great riches.

The Falkland Islands are surrounded by some of the stormiest waters on Earth...

ideal hunting grounds for rockhopper penguins.

It's the breeding season,

and for the last two weeks, the males have been incubating the eggs by themselves.

They're confined to the nest with nothing to eat,

while the females are out at sea collecting food.

All across the colony, eggs are starting to hatch.

This male now has two youngsters to care for.

But he has no food to give them, and he can't leave them unprotected.

He can do nothing but wait.

The females, after weeks fishing in the stormy seas,

are now heading for home with food in their crops.

There's just one problem.

The colony sits at the top of huge cliffs.

The waves that make feeding so good here

have now become major obstacles.

Timing is vital.

Go too early...

and they could be smashed against the rocks.

Too late...

and they will be carried back out to sea.

Hooked claws now help to get purchase

on the slippery rocks.

But they're not out of trouble yet.

Success depends on both judgment...

and luck.

Time and again, the waves drag her back in.

She has to persevere.

The lives of her chicks depend on her safe return.

Finally...

she's made it.

They're not called rockhoppers for nothing.

With one more jump, she's home.

And just in time.

Her chicks are desperately hungry.

This is their first proper meal.

The oceans have sustained life on our planet for millions of years.

But today, there's growing evidence that this is changing.

As our climate warms,

polar ice sheets are melting at an alarming rate.

In the Arctic alone, 14,000 tonnes of fresh water

are emptying into the sea every second.

This is slowing the flow of currents around the globe.

And if the atmosphere continues to warm,

ocean circulation could eventually stop altogether.

Our seas would then stagnate,

threatening the life within them.

And there are places in the oceans today

where this is already beginning to happen.

The Gulf of Thailand.

Eden's whales have lived here for generations.

But the world around them is changing.

Today, agricultural pollution flowing from the land...

is beginning to suffocate this sea.

Many fish now stay closer to the surface,

where the waters still contain enough oxygen to survive.

Eden's whales depend on these fish.

They swallow huge quantities of water

before filtering out their prey.

It takes a lot of energy to drive their 15-tonne bulk through the water.

And with so few fish,

the rewards from feeding like this are barely worth it.

So, to survive here,

the whales have developed a new hunting technique...

one that requires almost no effort.

They simply open their mouths...

and wait.

The panicked fish jump right in.

Swimming alongside,

another whale scares even more into the open jaws.

With this ingenious new technique,

Eden's whales have found a way to survive the pressures they now face.

All across the planet,

animals are having to adapt

to a changing world.

But the speed of these changes

will be too fast for many.

If we could only halt our unrestrained plunder of the ocean,

its habitats and species would recover.

And at a time when our overexploited lands are already failing us,

this has never been more important for humanity.