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  • [beep]

  • [static]

  • [beep]

  • [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 togetherthey 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 seagrasscapture 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 Polynesiain 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 respiteeven within the reef.

  • More and more sharks arriveattracted 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 communitywith 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 tropicsin 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 seasgiant 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 shellthere'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 uncheckedthey 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.