0:00:06 > 0:00:09The Great Barrier Reef is huge.
0:00:09 > 0:00:15It stretches for over 2,000km along Australia's north-east coast.
0:00:15 > 0:00:19It's so vast, it's clearly visible from space.
0:00:26 > 0:00:29And it's not simply a collection of coral gardens,
0:00:29 > 0:00:32but a network of very different habitats.
0:00:37 > 0:00:40It means there's a complexity of life here
0:00:40 > 0:00:45on a scale found almost nowhere else in the world,
0:00:45 > 0:00:48and it doesn't exist in isolation.
0:00:50 > 0:00:53Violent storms are unwelcome visitors.
0:00:57 > 0:01:01And ocean voyagers arrive here from many parts of the globe.
0:01:10 > 0:01:13The Great Barrier Reef is such a rich system that
0:01:13 > 0:01:18animals are drawn in from the vast empty spaces of the open ocean,
0:01:18 > 0:01:21from the tiniest plankton to ocean giants.
0:01:41 > 0:01:47It means that the Great Barrier Reef's an international hub,
0:01:47 > 0:01:51home to some of the greatest wildlife spectacles on Earth.
0:02:10 > 0:02:15This green turtle's a summer visitor.
0:02:15 > 0:02:19She's travelled hundreds of kilometres across the ocean
0:02:19 > 0:02:23and she's heading to the very beach where she was born.
0:02:27 > 0:02:31The turtle's come to lay her eggs
0:02:31 > 0:02:33and she's not alone.
0:02:35 > 0:02:37Out here, on the edge of the reef,
0:02:37 > 0:02:40she's joined by thousands more female turtles.
0:02:45 > 0:02:49They're all driven by the same instinct,
0:02:49 > 0:02:52to return home to nest.
0:02:52 > 0:02:57It's the largest breeding population of green sea turtles in the world.
0:03:13 > 0:03:15After her long journey,
0:03:15 > 0:03:18she takes a few days to rest and recover.
0:03:21 > 0:03:24Butterfly fish provide a cleaning service,
0:03:24 > 0:03:27clearing away dead skin and the parasites acquired
0:03:27 > 0:03:29from many months at sea.
0:03:43 > 0:03:46It's always exciting to see
0:03:46 > 0:03:50a large animal in the sea
0:03:50 > 0:03:55and of course the turtle is a very iconic species in the marine world
0:03:55 > 0:03:59and I'm surrounded by them on this dive,
0:03:59 > 0:04:04hundreds and hundreds of turtles in the water column above me,
0:04:04 > 0:04:08passing over the reef crest and out in the blue water.
0:04:16 > 0:04:19The turtles all converge on small islands
0:04:19 > 0:04:23made of coral rock and sand, known as coral cays,
0:04:23 > 0:04:26but not all are suitable for nesting,
0:04:26 > 0:04:33some are sandbanks exposed only at low water,
0:04:33 > 0:04:37and many others have beaches that are swamped at high tide.
0:04:42 > 0:04:44Islands with deep sand
0:04:44 > 0:04:47and a covering of vegetation are more stable
0:04:47 > 0:04:50and one island in particular seems just right.
0:04:52 > 0:04:56In the far north of the Great Barrier Reef is Raine Island.
0:04:58 > 0:05:01It's so wild and so special,
0:05:01 > 0:05:04that few people are permitted to land.
0:05:04 > 0:05:08It's one of the most protected islands on Earth.
0:05:10 > 0:05:15This speck in the ocean is barely a kilometre long,
0:05:15 > 0:05:19yet it attracts thousands of turtles and enormous flocks of seabirds.
0:05:24 > 0:05:28The birds have flown in from New Guinea and Japan to the north,
0:05:28 > 0:05:29Fiji to the east
0:05:29 > 0:05:35and even from the Asian mainland thousands of kilometres away.
0:05:35 > 0:05:39In summer, Raine Island's the most crowded destination on
0:05:39 > 0:05:41the Great Barrier Reef.
0:05:43 > 0:05:46It may look chaotic, but there's some order here.
0:05:47 > 0:05:49Brown boobies are everywhere
0:05:49 > 0:05:53but other species prefer specific nesting sites.
0:05:57 > 0:06:01Red-footed boobies hang out on branches,
0:06:01 > 0:06:04a scarce commodity on the outer reef.
0:06:04 > 0:06:08Caspian terns from Japan nest on the sand.
0:06:10 > 0:06:14Frigate birds find low-growing shrubs
0:06:14 > 0:06:20and red-tail tropic birds hide amongst the limestone rocks.
0:06:28 > 0:06:32The birds, like the turtles, are here to breed.
0:06:33 > 0:06:35But the turtles, unlike the birds,
0:06:35 > 0:06:38are about to face the greatest challenge
0:06:38 > 0:06:41of their visit to Raine Island.
0:06:45 > 0:06:47By late afternoon,
0:06:47 > 0:06:50they move towards the prime-nesting beach that surrounds
0:06:50 > 0:06:52the entire island.
0:07:01 > 0:07:05All around me it's like the troops are massing,
0:07:05 > 0:07:08the landing force is preparing itself and I can see
0:07:08 > 0:07:12heads popping up, the dark shapes moving in the shallows
0:07:12 > 0:07:16and then a glistening back will appear.
0:07:16 > 0:07:18This is the moment of transition
0:07:18 > 0:07:21when they leave the weightlessness of the sea.
0:07:26 > 0:07:28The bulk of her heavy body
0:07:28 > 0:07:32presses down on all her vital organs.
0:07:33 > 0:07:37She's beautifully adapted to a life at sea
0:07:37 > 0:07:40but ill-equipped to move about on land.
0:07:48 > 0:07:50Her progress is slow
0:07:50 > 0:07:52and probably painful.
0:08:14 > 0:08:17It's quite common for 5,000 turtles
0:08:17 > 0:08:20to emerge in one summer evening
0:08:20 > 0:08:24but tonight is anything but ordinary.
0:08:32 > 0:08:3926,000 turtles are coming at the island from all sides.
0:08:39 > 0:08:41A world record.
0:08:48 > 0:08:50It's going to be a long night.
0:08:53 > 0:08:57These green turtles are only one of the many visitors to the reef.
0:08:57 > 0:09:02Another ocean voyager is heard before it's seen.
0:09:02 > 0:09:04WHALE CALLS
0:09:15 > 0:09:18It's a dwarf minke whale,
0:09:18 > 0:09:21one of the smallest of the great whales.
0:09:21 > 0:09:23The turtles have swum from as far away
0:09:23 > 0:09:25as islands in the South Pacific,
0:09:25 > 0:09:29but the whales have travelled considerably further,
0:09:29 > 0:09:31all the way from the Antarctic.
0:09:31 > 0:09:34They come to the Ribbon Reefs,
0:09:34 > 0:09:35south of Raine Island,
0:09:35 > 0:09:39to calve in the warm, tropical waters
0:09:39 > 0:09:41or to mate.
0:09:46 > 0:09:50Whale watching's become a local tourist attraction
0:09:50 > 0:09:52but some whales turn the tables,
0:09:52 > 0:09:55they go people watching.
0:10:03 > 0:10:06These are adolescent whales,
0:10:06 > 0:10:09and they're extremely inquisitive.
0:10:16 > 0:10:22The moment of the first encounter is extraordinarily intense because
0:10:22 > 0:10:25you see the animal materialise beneath you.
0:10:25 > 0:10:29The first thing you see is the white stripe on the pectoral fin
0:10:29 > 0:10:32and then the water seems to solidify.
0:10:32 > 0:10:35This is a big animal, five or six tonnes,
0:10:35 > 0:10:38and then you gradually see it turn
0:10:38 > 0:10:42and the eye focuses on you and you focus on the eye.
0:10:42 > 0:10:45The animals are plainly studying you
0:10:45 > 0:10:48and gradually getting closer and closer.
0:10:52 > 0:10:55You're an object of curiosity to this whale
0:10:55 > 0:10:57and it is a remarkable sensation.
0:11:05 > 0:11:08By hanging onto the rope, my position is predictable,
0:11:08 > 0:11:10so the whales are quite unafraid.
0:11:21 > 0:11:24The mechanical twang of their call
0:11:24 > 0:11:27is so powerful you feel it rather than hear it.
0:11:35 > 0:11:40Being on nodding terms with a minke whale is a whole new experience.
0:11:44 > 0:11:46And to be here not on ours
0:11:46 > 0:11:50but on their terms is quite amazing,
0:11:50 > 0:11:54to be in the audience of the ultimate underwater ballet.
0:12:31 > 0:12:36More musical sounds announce the arrival of even bigger whales.
0:12:42 > 0:12:44Humpbacks.
0:13:13 > 0:13:17They're one of ten species of whale that visits the reef each year.
0:13:19 > 0:13:22They were once hunted, almost to extinction,
0:13:22 > 0:13:27but numbers here have bounced back to 15,000,
0:13:27 > 0:13:30about half the pre-whaling population.
0:13:33 > 0:13:38Like minkes, humpbacks come up from the Antarctic to mate and to calve.
0:13:40 > 0:13:42This mother gave birth a couple of weeks ago,
0:13:42 > 0:13:45and already her calf weighs over two tonnes
0:13:45 > 0:13:48and is more than six metres long.
0:13:53 > 0:13:55For the turtle mother,
0:13:55 > 0:13:59the final and what may turn out to be the most arduous part
0:13:59 > 0:14:02of her journey, has only just begun.
0:14:31 > 0:14:36With thousands of turtles arriving at the same time,
0:14:36 > 0:14:40their trails criss-cross the sand like tank tracks on a battlefield.
0:14:50 > 0:14:53There are just too many turtles for the space available.
0:14:56 > 0:14:58She spends much of the night
0:14:58 > 0:15:02heaving her bulk back and forth across the sand,
0:15:02 > 0:15:05searching for a vacant nest site.
0:15:07 > 0:15:09Such is their enthusiasm for digging,
0:15:09 > 0:15:14neighbours are in real danger of being buried alive.
0:15:16 > 0:15:19Some even dig up eggs that have already been laid.
0:15:44 > 0:15:46After several hours searching,
0:15:46 > 0:15:49the female finds a suitable place,
0:15:49 > 0:15:51where the sand is still moist.
0:15:57 > 0:16:00Her flippers may be a liability for moving on land,
0:16:00 > 0:16:03but now they come into their own.
0:16:03 > 0:16:05Using her front flippers,
0:16:05 > 0:16:08she first digs a protective hollow for herself.
0:16:12 > 0:16:15Then, with her back flippers,
0:16:15 > 0:16:18she delicately scoops out a deep pit.
0:16:35 > 0:16:38Each one of her clutch of 100 eggs is
0:16:38 > 0:16:41the size of a ping-pong ball.
0:16:45 > 0:16:49A soft shell prevents them from breaking as they drop into the hole.
0:16:55 > 0:16:58When she's finished, she'll cover the nest
0:16:58 > 0:17:02and over the coming weeks her eggs will incubate in the warm sand.
0:17:04 > 0:17:07This is a deeply private moment for this turtle
0:17:07 > 0:17:09and I do feel I'm rather intruding.
0:17:16 > 0:17:19You can see she is flicking sand to fill the hole
0:17:19 > 0:17:24and very successfully flicking it straight in my face as well.
0:17:24 > 0:17:27But this is a huge physiological effort for this animal
0:17:27 > 0:17:32you will see the turtle... Oh! Good one, right in my eye.
0:17:35 > 0:17:39That was right on the button.
0:17:39 > 0:17:43I think I'm going to take the subtle hint
0:17:43 > 0:17:46that I should leave her alone to get on with it.
0:17:54 > 0:17:59By morning, she joins the mass evacuation of the island.
0:18:10 > 0:18:16Most of the exhausted turtles head back to the sea at the same time,
0:18:16 > 0:18:19so there's an even bigger pile-up at the water's edge
0:18:19 > 0:18:21than when they arrived.
0:18:24 > 0:18:27But quite a few stragglers are left behind.
0:18:31 > 0:18:34It's been an absolutely exhausting night
0:18:34 > 0:18:37for any of the turtles that you can see behind me.
0:18:37 > 0:18:41In fact, you can make out this old girl here is absolutely shattered.
0:18:41 > 0:18:44She's completely spent and she is desperately trying
0:18:44 > 0:18:48to get back into the sea before the heat of the sun kicks in.
0:18:51 > 0:18:55For the last to leave, it's a race against time.
0:18:55 > 0:18:59In a couple of hours the temperature on the sand will soar.
0:19:08 > 0:19:12And, on one part of the island, there's a major obstacle that
0:19:12 > 0:19:15wasn't there when they arrived.
0:19:23 > 0:19:26Rocks exposed at low tide
0:19:26 > 0:19:30make the return to water anything but easy.
0:19:34 > 0:19:36Being reptiles,
0:19:36 > 0:19:39sea turtles have little control over their body temperature.
0:19:44 > 0:19:48There's no shade anywhere,
0:19:48 > 0:19:52so those left on the beach risk being cooked alive.
0:20:15 > 0:20:16For the unlucky few
0:20:16 > 0:20:20this is the last journey they'll ever make.
0:20:30 > 0:20:33Every turtle that leaves the sanctuary of the ocean
0:20:33 > 0:20:35is taking a gamble.
0:20:35 > 0:20:38And it's a knife edge whether they will live or die
0:20:38 > 0:20:41and obviously for this turtle that gamble didn't pay off.
0:20:41 > 0:20:44There's a number of factors that can kill them,
0:20:44 > 0:20:46it can be exhaustion, it could be overheating
0:20:46 > 0:20:50or it could be being buried, which may have happened in this case,
0:20:50 > 0:20:53by other turtles laying their eggs.
0:20:59 > 0:21:03If turtles trapped by the rock wall can make it to a pool,
0:21:03 > 0:21:05they might survive.
0:21:07 > 0:21:10The seawater cools their bodies.
0:21:25 > 0:21:29All they have to do is wait for the incoming tide
0:21:29 > 0:21:32and whatever that will bring.
0:21:51 > 0:21:55Raine Island is part of the outer barrier reef,
0:21:55 > 0:21:58so it's right next to the open ocean.
0:22:00 > 0:22:05Here the mottled hues of the shallow reef meet the dark blue of deep sea.
0:22:10 > 0:22:13The reef wall plunges down vertically
0:22:13 > 0:22:16to the ocean floor 1,000 metres below.
0:22:31 > 0:22:36It's here that reef life and creatures of the deep coexist.
0:22:43 > 0:22:47For migration to the reef is not only from across the ocean,
0:22:47 > 0:22:50it's also up from the depths.
0:22:52 > 0:22:56As a diver I can explore the first 100 metres or so.
0:22:56 > 0:23:01It's a very contrasting face
0:23:01 > 0:23:07to the gloriously kaleidoscopic world of the upper reef
0:23:07 > 0:23:11and the dark, cold, echoing world of deeper water.
0:23:13 > 0:23:17So, to see what's living down there,
0:23:17 > 0:23:20we need a remotely operated vehicle, an ROV.
0:23:37 > 0:23:40It enters an alien world,
0:23:40 > 0:23:44pitch black, with crushing water pressures.
0:23:47 > 0:23:49And extremely cold.
0:23:53 > 0:23:56At four degrees, it's the same temperature
0:23:56 > 0:23:58as the sea in the Antarctic.
0:24:01 > 0:24:04The ROV reaches a depth of 800 metres,
0:24:04 > 0:24:08not quite at the bottom, but not far off.
0:24:08 > 0:24:10A pile of coral sand at the base of the reef wall
0:24:10 > 0:24:14slopes gently into the abyss,
0:24:14 > 0:24:18and here we find signs of real deep-sea creatures.
0:24:20 > 0:24:23Some, like this sea anemone, are familiar.
0:24:26 > 0:24:31Others are less well known, like this chambered nautilus.
0:24:34 > 0:24:38It's an ancient relative of octopus and squid,
0:24:38 > 0:24:42a living fossil, the last survivor of a group of animals
0:24:42 > 0:24:47that dominated the world's oceans 500,000,000 years ago.
0:24:49 > 0:24:52It moves around by jet propulsion,
0:24:52 > 0:24:56squirting water backwards, in order to go forwards.
0:25:07 > 0:25:11At night, it's the nautilus's turn to migrate.
0:25:11 > 0:25:13It swims up towards the surface
0:25:13 > 0:25:16to feed on shrimps beside the reef wall,
0:25:16 > 0:25:19returning back down during the day.
0:25:22 > 0:25:25This is a baby nautilus,
0:25:25 > 0:25:28the first time one's been filmed in the wild.
0:25:30 > 0:25:33It's no bigger than a two-pound coin,
0:25:33 > 0:25:36yet it makes the same daily up and down journey
0:25:36 > 0:25:39as its plate-sized parents.
0:25:41 > 0:25:45But that pales into insignificance when compared
0:25:45 > 0:25:47to the daily vertical migration
0:25:47 > 0:25:52of these microscopic animals called zooplankton.
0:25:57 > 0:25:59At sunset,
0:25:59 > 0:26:02all of these tiny creatures swim upwards,
0:26:02 > 0:26:04and, under cover of darkness,
0:26:04 > 0:26:07they graze on floating algae close to the surface.
0:26:25 > 0:26:28Many of them are fish larvae.
0:26:28 > 0:26:32In fact, almost every fish species on the Great Barrier Reef
0:26:32 > 0:26:34starts life in the plankton.
0:26:34 > 0:26:38There are billions upon billions of them
0:26:38 > 0:26:43making the roundtrip, the greatest daily migration on Earth.
0:26:43 > 0:26:46They all travel an extraordinary distance,
0:26:46 > 0:26:51size for size, it would be like me running a marathon twice a day.
0:26:53 > 0:26:56That's if they're not caught on the way up.
0:26:58 > 0:27:01On the reef wall, at about 150 metres deep,
0:27:01 > 0:27:04are huge sea fans.
0:27:04 > 0:27:07They look like plants, but they're colonies of animals,
0:27:07 > 0:27:10whose branching arms capture the rising plankton.
0:27:11 > 0:27:15Hidden amongst the branches is a pygmy seahorse.
0:27:15 > 0:27:19It's a tiny fish that also feeds on plankton.
0:27:28 > 0:27:31At little more than a centimetre long, fully grown,
0:27:31 > 0:27:35it's one of the world's smallest fish.
0:27:51 > 0:27:55Contrast that with this monster,
0:27:55 > 0:27:56a tiger shark.
0:28:01 > 0:28:03Like the tiniest marine life,
0:28:03 > 0:28:06it too rises up from the ocean's depths,
0:28:06 > 0:28:10but, unlike the plankton, it's planning to stay a while.
0:28:18 > 0:28:23Tiger sharks travel over 800km to reach Raine Island,
0:28:23 > 0:28:26and each year they show up at exactly the same time,
0:28:26 > 0:28:29the time when turtles are nesting.
0:28:46 > 0:28:51Turtles trapped in rock pools begin to refloat on the incoming tide.
0:28:59 > 0:29:02For some, it's a second chance.
0:29:02 > 0:29:07Now all this female must do is cross the lagoon to reach the reef edge
0:29:07 > 0:29:10and the safety of the open ocean.
0:29:14 > 0:29:19But swimming in with the incoming tide is her number-one predator.
0:29:19 > 0:29:23It's crossed the reef and is heading towards the beach.
0:29:32 > 0:29:36A tiger shark could dismember this turtle
0:29:36 > 0:29:39and can even saw through her shell.
0:29:39 > 0:29:41She's on high alert.
0:30:15 > 0:30:18She turns and tilts rapidly,
0:30:18 > 0:30:20presenting her widest profile.
0:30:23 > 0:30:28It's too much of a mouthful for the shark,
0:30:29 > 0:30:33but the tiger shark doesn't give chase.
0:30:41 > 0:30:43It can't be bothered.
0:30:43 > 0:30:46There's a much easier way to get a meal.
0:30:55 > 0:30:58It's been waiting for fresh turtle carcasses
0:30:58 > 0:31:01to float out on the rising tide.
0:31:01 > 0:31:05To predict such an event is an amazing thing for a shark to do.
0:31:10 > 0:31:15This is no mindless killer, this shark is smart.
0:31:15 > 0:31:19It pitches up at the peak of the turtle nesting season
0:31:19 > 0:31:23and simply waits for the tide to deliver its food.
0:31:25 > 0:31:27Ah, look at that.
0:31:31 > 0:31:33So distinctive,
0:31:33 > 0:31:36right under the boat.
0:31:36 > 0:31:39Tigers have been found with all matter of interesting things
0:31:39 > 0:31:42in their stomachs and, as an opportunistic predator,
0:31:42 > 0:31:45she's come in and had a look at the boat a little look at me.
0:31:45 > 0:31:48Just a nudge of the boat. She's using the nose,
0:31:48 > 0:31:51all those senses packed into the nose, to try and figure out
0:31:51 > 0:31:56what we are and what I am. And now she's heading back to the carcass.
0:31:56 > 0:31:57I hope.
0:32:03 > 0:32:05Watch this bite.
0:32:05 > 0:32:11Good grief! What she is doing now is sawing using the weight of her body.
0:32:11 > 0:32:14These are huge, bulky animals
0:32:14 > 0:32:18and you get a lot of torsion with that weight.
0:32:18 > 0:32:22And with that torsion she'll clamp the jaws on to the flipper or head
0:32:22 > 0:32:26and just rip from side to side and the mechanical action
0:32:26 > 0:32:30and the cutting action of the teeth will tear lumps of flesh off.
0:32:33 > 0:32:38Vibrations from the commotion and the odour of mashed turtle flesh
0:32:38 > 0:32:43are carried on the currents, attracting more sharks to the feast.
0:32:47 > 0:32:50But, this is no feeding frenzy.
0:32:50 > 0:32:53Sharks of this size could do each other real damage,
0:32:53 > 0:32:56so instead they take it in turns,
0:32:56 > 0:33:01with smaller sharks deferring to the larger ones.
0:34:18 > 0:34:21Tiger sharks are generally solitary,
0:34:21 > 0:34:24so this gathering of 16 is extremely unusual.
0:34:25 > 0:34:30This is the largest number of tiger sharks seen in one place
0:34:30 > 0:34:31at any one time.
0:34:38 > 0:34:42When you hunt the dead, there's no need to hurry.
0:34:44 > 0:34:48It means the living can slip away, for now.
0:34:52 > 0:34:55She'll not go far, for she'll be back again,
0:34:55 > 0:34:58up to eight times during a single breeding season
0:34:58 > 0:35:01to deposit up to 100 eggs on each visit.
0:35:08 > 0:35:12Only then can she head back to her feeding grounds,
0:35:12 > 0:35:16where who knows what will happen to her.
0:35:17 > 0:35:22Away from the Great Barrier Reef, sea turtles are caught to eat.
0:35:22 > 0:35:25And sharks aren't immune either.
0:35:27 > 0:35:31These tiger sharks bear the scars of hooks from long-line fishing.
0:35:34 > 0:35:37It's not surprising then
0:35:37 > 0:35:41that the older sharks are conspicuous by their absence.
0:35:43 > 0:35:48While at Raine Island, the sharks and turtles are in a sanctuary,
0:35:48 > 0:35:50but even sanctuaries can come under threat.
0:35:50 > 0:35:53Although not necessarily from us.
0:35:57 > 0:36:01Brief thunderstorms are a welcome break from the heat and humidity,
0:36:01 > 0:36:05but they can also build into the mother of all storms.
0:36:08 > 0:36:10The summer heats up the ocean,
0:36:10 > 0:36:15creating tropical storms that spiral in from the Coral Sea.
0:36:15 > 0:36:19In America, storms of this intensity are known as hurricanes,
0:36:19 > 0:36:21and in Japan they're called typhoons.
0:36:21 > 0:36:24But here they're known as cyclones.
0:36:29 > 0:36:33A cyclone can be over 500km across,
0:36:34 > 0:36:39with winds swirling around the eye at 300km per hour.
0:36:39 > 0:36:44It's the most destructive force the Great Barrier Reef must face.
0:36:50 > 0:36:53And in February 2011,
0:36:53 > 0:36:55this part of the reef was hammered
0:36:55 > 0:36:59by the biggest storm in living memory,
0:37:02 > 0:37:04Cyclone Yazi.
0:37:05 > 0:37:09'This is a special broadcast of 9 News with Peter Overton.
0:37:09 > 0:37:11'Live in the cyclone...'
0:37:11 > 0:37:14'Good evening and welcome to a special edition of 9 News,
0:37:14 > 0:37:16'live from Airlie Beach.
0:37:16 > 0:37:19'The Cyclone Yazi bears down on North Queensland.
0:37:19 > 0:37:22'These are the latest satellite images of the biggest cyclone
0:37:22 > 0:37:25'Australia has experienced in more than 100 years.
0:37:25 > 0:37:29'Estimated to have the same intensity as Hurricane Katrina,
0:37:29 > 0:37:32'it's a category 5, you can't get anything more powerful.'
0:37:37 > 0:37:43Daybreak exposed the ferocity of Cyclone Yazi.
0:37:43 > 0:37:46'We are expecting to wake up tomorrow morning
0:37:46 > 0:37:49'to scenes of devastation and heartbreak, that's unprecedented,
0:37:49 > 0:37:53'not only in Queensland, but Australia's history.'
0:37:55 > 0:37:58Island resorts were destroyed.
0:38:02 > 0:38:05And a massive storm surge smashed into marinas,
0:38:05 > 0:38:07demolishing everything it touched.
0:38:14 > 0:38:17In the direct path of the cyclone,
0:38:17 > 0:38:21waves pulverised the reef
0:38:21 > 0:38:25and huge swells seriously damaged corals 500km
0:38:25 > 0:38:28from the eye of the storm.
0:38:29 > 0:38:34Cyclones form when humidity and air temperature build,
0:38:34 > 0:38:38but that's not the only time high temperatures directly affect
0:38:38 > 0:38:40the Great Barrier Reef.
0:38:41 > 0:38:46In summer, the air temperature can soar.
0:38:55 > 0:38:59Close to the ground, heat reflected by the sand compounds the problem.
0:39:03 > 0:39:09On Raine Island, the turtle eggs are incubating safely below ground
0:39:09 > 0:39:14but for the seabird chicks, the cay becomes a searing furnace.
0:39:14 > 0:39:18Birds must find shelter wherever they can.
0:39:18 > 0:39:23Bizarrely, one option is to shade your head with your own rear end.
0:39:27 > 0:39:29But with global warming,
0:39:29 > 0:39:32temperatures are increasingly higher than the norm,
0:39:32 > 0:39:35and then an unusually high water temperature
0:39:35 > 0:39:39can be just as destructive as a storm.
0:39:39 > 0:39:42An ominous white glow along the edge of the reef
0:39:42 > 0:39:44indicates it's under stress.
0:39:47 > 0:39:50The corals have lost their colour.
0:39:53 > 0:39:55This bleaching occurs
0:39:55 > 0:39:58because corals can only live in a narrow temperature range.
0:39:58 > 0:40:02Healthy corals get their colour from microscopic algae
0:40:02 > 0:40:05living in their tissues.
0:40:06 > 0:40:10These manufacture food for the corals by photosynthesis
0:40:10 > 0:40:13but when the temperature rises
0:40:13 > 0:40:17just two degrees above the normal summer maximum,
0:40:17 > 0:40:19the algal cells are expelled
0:40:19 > 0:40:22because they no longer benefit the coral.
0:40:26 > 0:40:29The bleaching effect is the white chalky skeleton
0:40:29 > 0:40:33showing through the coral's transparent tissues.
0:40:35 > 0:40:38But they're not dead. Not yet.
0:40:38 > 0:40:43They can survive in this bleached state for several weeks.
0:40:43 > 0:40:45If the temperature drops,
0:40:45 > 0:40:50the corals acquire new algae from plankton floating by.
0:40:53 > 0:40:57But, if the warm water persists, the coral dies.
0:41:05 > 0:41:09Coral bleaching hadn't been seen on the Great Barrier Reef
0:41:09 > 0:41:11before the 1980s.
0:41:12 > 0:41:17Due to global warming, bleaching's now more common
0:41:17 > 0:41:20and cyclones are likely to be more frequent too.
0:41:21 > 0:41:25And there's something even more insidious.
0:41:27 > 0:41:31Temperatures are rising because more and more carbon dioxide
0:41:31 > 0:41:34from human activity enters the atmosphere.
0:41:34 > 0:41:38This dissolves in seawater turning it weakly acidic,
0:41:38 > 0:41:41which can stop coral growth.
0:41:41 > 0:41:43If they can't build their chalky skeletons,
0:41:43 > 0:41:46reefs will start to crumble.
0:41:52 > 0:41:56With such threats, it's a wonder the reef has any future at all
0:41:56 > 0:41:59but it does have a chance,
0:41:59 > 0:42:02for the reef has a neat way to help itself recover
0:42:02 > 0:42:06and it's evident for just one week in late spring.
0:42:10 > 0:42:13A few days after a full moon,
0:42:13 > 0:42:17each hard coral species, along the entire reef,
0:42:17 > 0:42:21spawns at the same time, on the same night.
0:42:51 > 0:42:56Eggs and sperm unite to form free-swimming larvae.
0:43:03 > 0:43:08Smaller than a pinhead, a coral larva is a like a space capsule.
0:43:13 > 0:43:18It floats away on the current and seeks a new place to grow.
0:43:28 > 0:43:32Some larvae travel no more than a few metres
0:43:32 > 0:43:36others drift thousands of kilometres across the ocean,
0:43:36 > 0:43:39depending on where the current takes them.
0:43:45 > 0:43:50Attracted to settle by the sounds made by reef life,
0:43:50 > 0:43:53like fish, shrimps or even sea urchins,
0:43:53 > 0:43:57the larvae searches for a spot to call home.
0:43:58 > 0:44:01It transforms into a coral polyp,
0:44:01 > 0:44:05like a miniature sea anemone anchored to the seabed
0:44:09 > 0:44:14and here it starts a brand new colony of coral.
0:44:27 > 0:44:30This constant process of re-seeding
0:44:30 > 0:44:33may help ailing reefs to recover,
0:44:33 > 0:44:36as long the damage is not too severe or too frequent.
0:44:46 > 0:44:51Sunrise on Raine Island marks another mass movement of wildlife.
0:45:02 > 0:45:06The first light of dawn is just touching the horizon.
0:45:06 > 0:45:09And there's an exodus taking place from the island.
0:45:09 > 0:45:14The seabirds behind me just massing prior to leaving the island
0:45:14 > 0:45:16and heading out to the open sea to hunt.
0:45:19 > 0:45:22Both parents normally take turns to search for food,
0:45:22 > 0:45:26but as the chicks grow and become more demanding
0:45:26 > 0:45:30they both go to sea, leaving their offspring on its own.
0:45:35 > 0:45:37Along with other predators,
0:45:37 > 0:45:42the parents search far offshore for dense shoals of fish.
0:45:42 > 0:45:46Here, sharks, tuna, and seabirds are competing for
0:45:46 > 0:45:50a tight ball of fish that'll last just a few minutes.
0:46:00 > 0:46:04The birds rely on sharks and tuna
0:46:04 > 0:46:08to drive the smaller fish closer to the surface.
0:46:13 > 0:46:17Their chicks' very survival depends on their success.
0:46:29 > 0:46:33A hungry young booby waits patiently for its parents to return.
0:46:38 > 0:46:40This time it's lucky.
0:46:40 > 0:46:43They've flown in with plenty of food.
0:46:51 > 0:46:53By late summer,
0:46:53 > 0:46:57Raine Island is the largest nursery on the Great Barrier Reef,
0:46:57 > 0:46:59with seabird chicks growing on top of the sand
0:46:59 > 0:47:02and turtle eggs developing underneath.
0:47:13 > 0:47:17But Raine is not the only island with nesting seabirds,
0:47:17 > 0:47:21on Heron Island, shearwater parents return not at sunset,
0:47:21 > 0:47:23but after dark.
0:47:25 > 0:47:27It's a hangover from times
0:47:27 > 0:47:31when these birds nested on islands with predators.
0:47:31 > 0:47:35Flying in and out at night was one way to avoid them.
0:47:43 > 0:47:48Parents find each other in the dark by their raucous calls.
0:47:53 > 0:47:56The pair reaffirms its bond before the returning bird
0:47:56 > 0:47:59enters the underground nest.
0:48:07 > 0:48:12Although there's a little housekeeping to be done first.
0:48:24 > 0:48:26Both parents share nursery duties,
0:48:26 > 0:48:31and they take turns to fly great distances in search of food.
0:48:31 > 0:48:34They may be at sea for several days.
0:48:51 > 0:48:54They leave as they arrived, in the dark.
0:49:02 > 0:49:04Well before sunrise,
0:49:04 > 0:49:08they line up in clearings like aircraft taxiing for takeoff.
0:49:29 > 0:49:31And then it's away out to sea.
0:49:55 > 0:49:59Back on Raine Island, the very last birds to nest
0:49:59 > 0:50:01are rufous night herons.
0:50:04 > 0:50:06They haven't travelled far,
0:50:06 > 0:50:09just out from the mainland
0:50:09 > 0:50:12and they surely have the most unattractive chicks on the reef.
0:50:17 > 0:50:21They've hatched late in the season for a very good reason,
0:50:21 > 0:50:26because their food is not out at sea, but right on the doorstep.
0:50:31 > 0:50:36In late afternoon, the parent birds take their positions.
0:50:39 > 0:50:43They scan the sand, alert to any movements.
0:50:48 > 0:50:53At sunset, the temperature change triggers the start.
0:50:53 > 0:50:56It's what the herons have been waiting for.
0:51:00 > 0:51:02The clutch of turtle eggs has been incubating
0:51:02 > 0:51:04under the sand for two months.
0:51:08 > 0:51:14At the right moment, the hatchlings all emerge together.
0:51:25 > 0:51:28They must reach the water in the shortest possible time.
0:51:57 > 0:52:00Baby turtles are food for baby herons,
0:52:00 > 0:52:03so the heron parents have timed the peak of their nesting
0:52:03 > 0:52:06to coincide with this mass emergence.
0:52:13 > 0:52:15Even ripples in the sand
0:52:15 > 0:52:18slow the hatchling's headlong rush to the sea.
0:52:18 > 0:52:22It could mean the difference between life and death.
0:53:04 > 0:53:07But the herons have had their fill.
0:53:07 > 0:53:11They simply couldn't eat another baby turtle.
0:53:19 > 0:53:21It's been a narrow escape for this one.
0:53:27 > 0:53:31The first wave of hatchlings has taken the brunt of the attacks,
0:53:31 > 0:53:34but the sacrifice of a few hundred ensures
0:53:34 > 0:53:38the following thousands have a better chance to get to the sea
0:53:38 > 0:53:42but some babies go the wrong way, just like their mothers.
0:53:52 > 0:53:56The wall of rock's an even bigger obstacle for the hatchling
0:53:56 > 0:53:58than it was for the adult.
0:54:08 > 0:54:12And there's something even more sinister up ahead.
0:54:17 > 0:54:19Rock crabs,
0:54:19 > 0:54:22they like baby turtles too.
0:54:33 > 0:54:37Their powerful pincers can tear a hatchling limb from limb.
0:55:18 > 0:55:22Another lucky escape but there's still a way to go yet.
0:55:26 > 0:55:28Come on, little fella! Keep going, keep going!
0:55:28 > 0:55:31It's a delicious little package of protein.
0:55:31 > 0:55:34It's made the difficult and dangerous journey
0:55:34 > 0:55:37from the dunes there and it's still going and,
0:55:37 > 0:55:40of course, this transition into the marine environment
0:55:40 > 0:55:42doesn't mean this hatchling is safe.
0:55:42 > 0:55:44It faces a whole new set of hazards
0:55:44 > 0:55:46as it tries to swim out, over the reef top.
0:55:46 > 0:55:49Everything out there is waiting for these hatchlings.
0:55:49 > 0:55:54He's nearly there, that tiny, tiny, little turtle
0:55:54 > 0:55:58and that's a huge expanse of ocean.
0:56:07 > 0:56:11Exactly where this little hatchling goes is a mystery.
0:56:11 > 0:56:15But she'll be at sea and she'll not return to Raine Island
0:56:15 > 0:56:17before her 30th birthday,
0:56:17 > 0:56:20when she'll come back to lay eggs on the same beach
0:56:20 > 0:56:22that she's leaving today.
0:56:22 > 0:56:26That's if she survives in an uncertain and often hostile world.
0:56:35 > 0:56:38Raine Island has the biggest concentration of wildlife
0:56:38 > 0:56:41on the Barrier Reef, but many of its animals are visitors
0:56:41 > 0:56:45and at departure time they leave behind the sanctuary
0:56:45 > 0:56:48of one of the world's largest marine parks.
0:56:50 > 0:56:53The migrants cross international borders
0:56:53 > 0:56:57travelling to places where animals are not protected.
0:57:00 > 0:57:04It means their survival is linked very much to events in other
0:57:04 > 0:57:07parts of the world.
0:57:10 > 0:57:14The Great Barrier Reef is still an amazing place.
0:57:17 > 0:57:21It's a magical, underwater world, stunningly beautiful
0:57:21 > 0:57:24and a never-ending source of wonder.
0:57:25 > 0:57:28But how will it be when our turtles return?
0:57:31 > 0:57:35The reef has proved to be resilient in the past,
0:57:35 > 0:57:38surviving great natural changes.
0:57:39 > 0:57:45But nothing like the pace of man-made change today,
0:57:45 > 0:57:48especially the pace of climate change.
0:57:53 > 0:57:55We've seen how the Great Barrier Reef
0:57:55 > 0:57:59is connected to the rest of the world, in many ways.
0:57:59 > 0:58:04It means we're all, no matter how remote, involved in its future.
0:58:06 > 0:58:09Now, only we can decide what that future will be.
0:58:09 > 0:58:13Whether it remains the glorious marine spectacle of today,
0:58:13 > 0:58:17one of the richest and most diverse of all environments
0:58:17 > 0:58:20and the largest biological structure on the planet
0:58:20 > 0:58:23or whether it become something much poorer.
0:58:23 > 0:58:27It's a future that is entirely in our hands.
0:58:32 > 0:58:35Subtitles by Red Bee Media Ltd
0:58:35 > 0:58:38E-mail subtitling@bbc.co.uk