Nearly three-quarters of our planet lies underwater.
Now, venture beneath the waves, and enter an astonishing world.
The sea is a realm ruled by giants,
stalked by hunters;
an arena for epic battles.
This is the kingdom of the oceans.
***** subs by HC *****
Off the Pacific coast of Central America,
an enormous school of lantern fish.
They are trapped near the surface.
Spinner dolphins block their retreat.
And, the spinners won't pass up this windfall.
Other predators are eager to claim their share.
Frigate birds rarely dive for fish...
but these fish are just below the surface.
Yellowfin tuna rush to the scene.
Hunting in close formation, yellowfins overwhelm their prey.
The dolphins don't mind the company.
There's plenty to go around.
This feast is far from over.
Devil rays are just arriving.
Sleek acrobats, the rays seem to fly, instead of swim.
Their mouths are framed with a pair of fins that create a funnel.
Ideal for scooping up prey.
Even with all these attackers,
the school is too big to be wiped out.
The survivors can finally escape,
and the satisfied hunters take their leave.
Food is less abundant farther from shore,
and hungry creatures must travel a lot farther to find it.
They depend on currents,
or on their mysterious powers of navigation,
as they journey over the trackless deep.
But, they have to cover a lot of ground.
A mako shark can swim 2000 kilometres a month as it searches for prey.
The swordfish, too, is a lonely hunter.
A green turtle undertakes her epic journey,
not for food, but to reproduce.
She seeks the distant beach where she was hatched,
and where she'll return to lay her own eggs.
Here, where prey is hard to find,
a great white shark stays on the move.
Its keen senses can pick up an odor, a vibration;
even the electrical field generated by its next meal.
One of the strangest of these voyagers is the ocean sunfish.
It's about the size of a pickup truck,
and it seems to defy the rules of marine biology.
The sunfish can swim either upright, or on its side.
It's equally at home basking on the surface,
or diving to depths of nearly 500 metres.
When it comes to deep-water giants,
nothing compares to the blue whale.
Stretching up to 30 metres, and weighing in at more than 100 tons,
the blue whale is the largest animal that's ever lived.
But, it's sustained by one of the smallest: ....
Krill.
These shrimp-like crustaceans, each the size of a jelly-bean,
gather in enormous schools for mutual protection.
Swarms of krill can stretch for several kilometres, ...
but they can't avoid being preyed on.
Not just by whales, but by, almost, any creature bigger than they are.
In the dense swarm, it's hard for small fish to find a target.
Each krill seeks safety by trying to reach the centre.
This causes the whole school to twist and turn.
Forming a school is a common survival strategy in the ocean,
but it's no defence against a really big predator.
The blue whale doesn't need teeth to feed on krill.
She just opens wide.
In a single gulp, she can take in more than 80000 litres of water,
along with countless krill.
Given the chance, a blue whale will gorge on krill,
eating nearly 4 tons a day.
And, no wonder.
This is a creature whose heart weighs nearly 500 kilograms.
Her tongue is the size of an elephant.
It takes a lot of krill to satisfy her appetite,
but no one knows exactly how
blue whales locate their food in the vastness of the ocean.
Finding a mate is an even bigger challenge.
During the last century,
whalers killed off 99% of the population.
Today, there may be as few as 5000 blue whales.
Blue whales were once prized by human hunters,
but their greatest value may be to the ocean, itself.
The whales' waste is rich in iron from the krill in their diet.
Dissolved in sea water, combined with sunlight,
the iron fertilises blooms of marine algae.
The algae, in turn, nourish tiny creatures called plankton.
They're even smaller than krill.
Plankton are crucial for life in the oceans.
They form the bottom of the food chain.
In the barren, mid-ocean realm, food is scarce.
A blue shark always keeps his eyes open for opportunities.
A mass of kelp has drifted far from the coast, where it normally grows.
It could be harbouring a meal, but it's too tangled,
and the shark can't make its way inside to find out.
Frustrated, he moves on.
But, the shark was right.
Out here, there aren't many places for small fish to hide.
For horse mackerel and half-moon fish,
the floating kelp is a welcome retreat.
The raft harbours other little travellers.
Coastal residents that came along for the ride. Rockfish, ...
and even pipefish, a cousin of the seahorse.
But, they didn't evolve for survival on the high seas.
If their kelp island breaks up and sinks,
they will likely perish, along with it.
'Til then, they make the most of what they find.
A ring of salp, a kind of barrel-shaped plankton, will suffice for dinner.
This whale shark doesn't need to visit a spa.
The world's largest fish, up to 10 metres long,
he travels with his own skin-care specialists.
The little hitchhikers are remoras,
They attach themselves to their host using suckers on the tops of their heads.
They do the whale shark a favour, by eating any parasites they find.
In return, they get to travel, and see the world.
Their destination? Anywhere the whale shark wants to go.
There's plenty of hospitality on the high seas.
A huge jellyfish provides a safe haven for dozens of young fry.
The little fish are immune to the jelly's toxic sting,
so, this is a good place for them to hang out,
until they're big enough to fend for themselves.
The whale shark, meanwhile, has a new entourage:
a school of sardines.
He barely notices them.
Still, it's wise to stay well back of the giant's jaws,
just in case.
A hungry mahi-mahi is much more interested in the sardines.
When schools of small fish face an attack,
they tend to react in the same way.
At first, they panic and scatter.
Then, they cluster together for safety, all swimming in the same direction.
That forms a spear, called a bait ball.
Some predators respond with a direct approach,
strafing the ball to break it apart.
Others use a different strategy.
They push the bait ball up against the surface, where it begins to unravel.
For a small school of fish, a bait ball doesn't provide much cover.
Their only defence is flight.
But sailfish are too fast, and too agile.
Unfurling their dorsal fins make them look bigger,
and helps them herd prey.
Skilled swordsmen, they pick off their victims, one by one.
The dazed survivors flee towards the surface,
only to face a new threat.
Alert sea birds are not about to pass up the chance for an easy meal.
Of all the ocean's hunters,
few are as well-equipped as dolphins.
Off the South African coast,
long-beaked common dolphins put their talents to the test
during one of nature's greatest spectacles.
Thousands of dolphins come together here, to form super-pods,
but they only do it when food is especially abundant.
It's the southern winter,
when sardines gather by the billions to spawn.
Then, they surge north,
riding the cold currents that hug the coast.
And dolphins give chase.
The dolphins aren't the only ones taking advantage of the bounty.
Cape gannets begin their dives from 20 or 30 metres up.
They pull in their wings at the last instant
to pierce the surface like a guided missile,
at speeds of up to 120 kilometres an hour.
For the sardines, under assault from above and below,
there is no defence.
Copper sharks join the feast.
Sharks; dolphins are usually bitter rivals.
It's rare to see them hunt together,
but today, there's plenty for everybody.
They eat until they're full,
and, still, countless sardines survive, to carry on their migration.
The same cannot be said for human hunters.
Nets can take every last fish in the school.
The nets are impenetrable, and unavoidable.
As they sweep through the seas, they envelop everything in their path.
Nets catch a lot more than the little fish they target.
It's called an incidental kill,
but the impact on ocean life is devastating.
Staying alive is often a matter of luck.
Being in the right place at the right time.
A crowd is gathering in mid-ocean,
and inquisitive dolphins are eager to find out why.
This tree trunk drifts far from the beach where it once grew.
Out here, even a wooden island is a haven for predators.
And a coconut palm will stay afloat for a long time.
It can also turn creatures of the land into reluctant mariners.
The monitor lizard may get lucky, and land safely on a distant shore.
In the barren stretches of mid-ocean,
any object carried by the currents can serve as refuge.
An old tire is a magnet for small creatures,
and for the hunters that prey on them.
A discarded net still catches fish,
only, now, they come to it willingly.
Currents transport our trash throughout the seas.
To us, it's junk, but to fish, it's a place to hide.
Many of the ocean's greatest journeys are undertaken by its biggest creatures,
like the basking shark.
At 10 metres long, the basking shark is a gentle giant.
To feed, it simply opens its cavernous jaws,
taking in plankton, crustaceans, little fish; whatever is out there.
Basking sharks swim slowly,
but they journey thousands of kilometres every year,
crossing entire oceans.
They've been known to summer off Cape Cod, and winter near Brazil.
Whales are legendary for their long-distance voyages.
Many whales feed exclusively in cold waters,
and breed in warm waters.
Humpbacks journey farther than any other migratory mammal:
16000 kilometres round-trip, from the tropics to the poles.
But, we still know little about their habits.
What triggers their migrations?
How do they find their way through featureless seas?
And, there's another mystery.
In spite of their ultra-marathon swims,
humpbacks almost never cross the equator.
Humpbacks in the northern hemisphere feed in the arctic.
Those in the southern hemisphere head to the antarctic.
Only rarely do the two populations mix and mingle.
The whale shark, and its remora hitchhiker,
have travelled far through empty ocean.
Now, they're in for a surprse, and plenty of company.
They reach a bustling oasis,
an undersea mountain, in the eastern Pacific.
The sea mount deflects currents, producing an upwelling of nutrients,
and a constant supply of food.
Unlike the whale shark, and other migrants,
many of these fish are permanent residents.
The mountain provides food and shelter, so, why leave?
A manta ray makes an appearance here, too, with its own remoras attached.
Hungry angel fish welcome them,
eager to clean the visitors' skin of parasites.
A group of hammerheads arrives to have their skin cleaned, too.
Butterfly fish will take care of them.
Unlike most sharks, hammerheads travel in schools,
but a gathering this large is rare.
An amazing spectacle.
Silvertip sharks happily take up residence here, where the feeding is easy.
But, hammerheads are always on the move, roaming from one oasis to the next.
The sea mount's resident population is here to stay.
Countless generations of fish live, and feed, and breed here.
For the nomads of the deep sea, reproduction requires a journey.
These sea turtles swim thousands of kilometers
to their breeding ground in the western Pacific.
Humpback whales gather in the tropics, too.
They haven't eaten since they left Antarctica.
A lonely voyage of 5000 kilometres.
But, they didn't come here to feed.
It's winter in the southern hemisphere.
A time, simply, to enjoy the company of their own kind.
This is a place to dance and sing;
to court, and give birth.
For these nimble giants,
every gesture, whether below or above the surface, is invested with meaning.
But, their choreography and their culture are beyond our understanding.
At this very moment, it's summer in the far north.
The humpbacks of the northern hemisphere have completed their fast.
Now, they're returning from the tropics to these cold waters off Alaska to feed.
Close to shore, a humpback whale hunts alone.
Swimming on its side, manoeuvring with its long pectoral fins,
it encircles a school of herring.
The fish are trapped within a net of bubbles,
and, in a single gulp, ...
the herring disappear.
But, the whales don't always work solo.
Southeastern Alaska is one of just a few places on Earth
where humpbacks hunt as a team.
Some of the whales blast a school of herring
with a series of feeding calls,
to herd them toward a wall of bubbles.
The others close the trap,
and they all lunge to the surface at once.
For the herring, there is no escape,
and, for the whales, there's a sumptuous meal.
They'll spend the rest of the summer here,
building up their strength for the next long jouney across the open sea.
Not far away, in a freshwater stream,
one of the ocean's greatest journeys nears its end.
Sockeye salmon have travelled thousands of kilometres at sea,
fighting tides and currents, to return to this river.
Now, they must conquer gravity.
An irresistible impulse to reproduce drives them up-river,
and pushes them to the point of exhaustion.
Instinct compels them to seek out the very same rivers where they were spawned.
How they do it? No one knows.
There is no time to rest.
There are more waterfalls to climb.
Even the strongest among them cannot overcome one obstacle.
Still, many salmon will complete their journey, and their mission.
They bridge the gap between two worlds,
between saltwater, and fresh; sea, and land.
And, their river bridges that gap, as well.
Conveying its waters, its energy, its essence, to the oceans.
***** subs by HC *****