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[indistinct speech over radio]

[David Attenborough] Just 50 years ago,

we finally ventured to the moon.

For the very first time, we looked back at our own planet.

Since then, the human population  has more than doubled.

This series will celebrate  the natural wonders that remain

and reveal what we must preserve

to ensure people and nature thrive.

Immense shoals of fish  throng our shallow seas.

These are anchovies.

Small fish, in turn, sustain bigger ones.

Hunters, such as giant trevally...

and mobula rays.

When hunters such as these work together,  they become extremely efficient.

The rich coastal seas  are the fishing grounds of our planet

and can provide an abundance of food  for wildlife and humanity.

The seas fringing land make up  less than a tenth of the world's oceans,

yet, astonishingly,  90 percent of all marine creatures

live in these coastal waters.

This superabundance  is due to the fact that the seafloor here

is within reach of sunlight.

Everglades National Park  in southern Florida.

Fields of seagrass  carpet the shallow tropical waters.

Like savannas on land,

these lush marine meadows  support a great variety of animal life.

Stingrays search for prey hiding in the seagrass.

The Everglades  are also the hunting grounds

of the coasts' most ingenious fishermen.

[dolphins clicking]

Bottlenose dolphins.

They search for food using echolocation, a type of sonar.

[dolphins clicking]

Ahead of them, a shoal of mullet.

These particular dolphins

have developed their own special way  of catching their prey.

They carefully herd the fish  into just the right place.

Then one dolphin stirs up a ring of mud  that encircles the shoal.

The fish panic and take to the air.

Most escape.

Catching flying fish isn't easy.

But the dolphins move on  and take another helping.

People aren't allowed to fish commercially within the national park,

so there's plenty for the dolphins.

Only from the air can we really appreciate the industry of these master fishermen.

The shallow seas are vitally important in the fight against climate change.

Seagrass absorb 35 times as much carbon dioxide

as the same area of rainforest,

and that reduces the damage  caused by the recent warming of our seas.

Mangroves often border  the seagrass meadows.

These remarkable trees are the only ones that can cope

with the varying saltiness  of coastal waters.

They not only protect our coasts  from the destructive forces of hurricanes

but also, like seagrass,  capture carbon dioxide.

Their tangled roots  create safe nurseries for young fish,

which will eventually leave the mangroves

and make their homes in the tropical seas' most magical wonderlands,

coral reefs.

These reefs cover  less than one percent of the seafloor,

yet they are home  to a quarter of all marine species.

None of these creatures would be here  were it not for the coral.

Corals create the structures  that provide food and shelter,

on which the entire community depends.

Every resident has a role to play in maintaining the health of the reef.

Unfortunately, few reefs  are as pristine as they once were,

and most are now missing

one extremely important member  of the community.

Sharks.

Relentless overfishing has reduced  shark populations around the world

by over 90 percent.

Today they only thrive in anything like their former numbers

on the very remotest of reefs.

French Polynesia,  in the middle of the Pacific Ocean.

Here, at least, sharks are fully protected.

By day, gray reef sharks  hang in the current.

They let little wrasse  pick their teeth clean

in readiness for the night's hunt ahead.

These sharks prefer to hunt in darkness,

when their acute senses  give them an advantage over their prey.

Nightfall.

Time for small fish to take refuge  within the nooks and crannies of the reef,

beyond the reach of the gray reef sharks.

The sharks can detect the slightest movement.

Breaking cover is dangerous.

Stay motionless  and the sharks may not detect you.

A little fish may outmaneuver  a single shark,

but with so many hunters  there is little chance of escape.

Sheltering within the coral branches is usually the safest option,

but the arrival of a second hunter  changes everything.

Whitetip reef sharks.

Slender and more agile,

the whitetips can get into hiding places  other hunters can't reach.

Those that are not caught by the whitetips

are flushed out  to the waiting gray reef sharks.

With both kinds of sharks  hunting in partnership,

there is little respite,  even within the reef.

More and more sharks arrive,  attracted by the commotion.

Such numbers of sharks were once common  on reefs around the world.

And while so many predators  may seem damaging,

sharks, in fact, assist in maintaining the health of the reef.

They help to keep a balance  in the fish community

by hunting the predators  that feed on small grazing fish.

The grazers, in their turn,

keep the corals free of seaweeds and parasites

that would otherwise overgrow the reef.

A balanced community,  with sharks as top predators,

gives a coral reef much greater resilience in the face of damage and disaster.

But today, even healthy coral reefs  are facing a greater threat.

With climate change, our seas are warming.

Microscopic plants  that live within the tissues of the corals

give them both their color  and most of their nourishment.

But if seas temperatures rise  by just a degree or two,

the corals expel their plant partners.

So they lose their main source of food...

and turn white.

Here, on Australia's Great Barrier Reef, unusually warm seas

have caused many of the reefs  to bleach in this way.

It may look hauntingly beautiful,

but if temperatures remain high  for a few weeks,

the corals will starve  and eventually die.

The carbon dioxide  that causes global warming

is also making the seas more acidic.

No reefs can survive both changes.

And without living corals,

many of the reef's residents will also perish.

During 2016 and 2017,

over a thousand kilometers  of the Great Barrier Reef bleached.

Six months later,

and many of the corals are dead.

The community destroyed.

Worldwide, half of all shallow coral reefs have already died.

The rest could be gone  over the next few decades.

Away from the tropics,  in higher latitudes,

the seas are much cooler.

Storm-lashed waters bring essential nutrients to the surface.

The enriched water, combined with long summer days,

make these  the most productive seas on the planet.

Fur seals thrive and even have time to ride the waves.

Spring off the coast of California

and fronds of golden kelp  floating on the surface

hint at the riches hidden beneath.

A magnificent submarine forest.

These stands of giant kelp are as important to the oceans

as trees are to the land.

Like a rainforest,

the dense canopy provides food and shelter for an abundant community.

In the cool, rich seas,  giant kelp can grow 50 meters high.

Air-filled floats lift the fronds  towards the sunlit surface.

Nestling in this canopy,

a sea otter.

His thick fur keeps him warm  in the chilly water.

Blowing air into it  gives him extra insulation.

Sea otters  have the densest fur of any animal,

and such a luxuriant coat  requires a great deal of attention.

They need to eat a lot to keep warm

and consume up to a quarter  of their body weight every day.

Sea urchins are a favorite food.

There's one.

And a second.

And then the otter is out of breath  and must return to the surface.

An urchin  looks like a rather prickly meal,

but if you can break the shell,  there's a feast inside.

Sea otters are crucial  to the health of the kelp forest

because they keep down  the number of sea urchins.

The urchins graze on kelp,

munching through their tough stems.

Left unchecked, they spread  across the seafloor like a plague.

Fortunately, the otters have a helper  in the fight against these spiny grazers.

Sheephead wrasse.

They have powerful teeth

and can make short work  of the smaller purple urchins.

The larger black urchins  are more of a challenge,

especially for smaller wrasse.

Only the very largest sheephead  can tackle them.

But it still has to get past  those long spines.

The urchin has to be flipped over  to expose its vulnerable underside.

Not easy when you have no hands.

Success at last.

And everybody benefits.

These fish and otters,  living within marine protected areas,

keep the kelp grazers under control.

But just outside these sanctuaries, the urchins graze unchecked...

and fell the kelp forest.

All those that depend on the kelp have disappeared.

In California, kelp forests  grow abundantly in protected areas

where the whole marine community  can live without pressure from people.

Here, at least, sea otters  can chill out Californian style.

[bird caws]

Three thousand kilometers further north,  in Alaska,

spring arrives much later in the year.

After a cold, hard winter,

the first rays of the sun  trigger a great annual event.

Immense shoals of herring  rise up from the depths

and move into shallower waters.

They're gathering to breed.

But predators are waiting for them.

Steller's sea lions.

So intent on their mission, the herring make easy prey.

Sea lions drive the fish to the surface, within range of aerial hunters.

Bald eagles, normally solitary birds, have assembled in numbers for the feast.

In places, the herrings reach shore,

and there the eagles  don't even need to take to the air.

Humpback whales.

[whale lows]

They have come  all the way from the tropics,

hoping to get their first meal in months.

These whales have a remarkable way  of collecting this seasonal bounty. 

They work as a team.

The lead whale dives first...

and blows a curtain of bubbles  to concentrate the fish.

[whale calling out]

A call synchronizes the attack.

[whale song increases in volume]

By cooperating in this way,  each whale can eat a ton of fish a day.

Yet the herring are so abundant

that, in spite of the whales, most managed to reach the shallows.

And here they spawn.

The females lay their eggs  on the seagrass.

The males follow,  fertilizing the eggs with clouds of sperm.

Once, herring thrived here in an abundance we can barely imagine today.

They used to turn shorelines white   with spawn,

right across the North Pacific  from Japan to California.

With industrialized fishing techniques

it's all too easy  to overexploit these breeding stocks.

This is one of the few places where a fishery still survives.

Pacific herring populations  are greatly reduced.

A mere fraction of what they once were.

This sorry tale of unsustainable fishing

has been repeated time and again  around the world.

Many of the world's fish stocks  are now in serious decline.

A third of them have collapsed.

In coastal seas, 

jellyfish are taking over waters  that were once dominated by fish.

Compass jellyfish have an ethereal beauty,

but they provide very little sustenance  for other wildlife or us.

These immense swarms  are becoming increasingly common.

A worrying sign of a serious imbalance  in our shallow seas.

Unless we start to fish sustainably, our seas, once so bountiful,

will be filled with little more  than vast swarms... of jellyfish.