Our Planet

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.

Bottlenose dolphins.

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

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.