0:00:20 > 0:00:23A giant roams the world's oceans.
0:00:32 > 0:00:38At more than 12 metres long, the whale shark is the largest fish in the sea.
0:00:41 > 0:00:46Yet for all its size, it remains a total mystery to science.
0:00:53 > 0:00:57For most of the year, they travel the oceans undetected.
0:01:00 > 0:01:05Then, off tropical coasts and coral reefs, the whale sharks suddenly appear.
0:01:10 > 0:01:12Where they come from, nobody knows.
0:01:14 > 0:01:19But just as mysteriously as they arrive, the whale sharks soon disappear.
0:01:22 > 0:01:25Back her off... There it is! Right there!
0:01:26 > 0:01:30Shark expert Dr Mark Meekan is on a quest to find out where they go.
0:01:32 > 0:01:36He believes they're travelling huge distances
0:01:36 > 0:01:40and could even be crossing oceans.
0:01:42 > 0:01:47Over the next year, Mark will attempt to track the whale shark's movements.
0:01:49 > 0:01:54If all goes to plan, he will unravel one of the ocean's great mysteries.
0:02:06 > 0:02:09Ningaloo Reef, Western Australia,
0:02:09 > 0:02:13where the Indian Ocean meets the harsh desert coast.
0:02:17 > 0:02:23Just 100 metres off shore lies one of the world's richest coral reefs,
0:02:23 > 0:02:27where an incredible event is about to take place.
0:02:33 > 0:02:36It's early April, and seven days after the full moon,
0:02:36 > 0:02:40a neap tide calms the great ocean swell.
0:02:48 > 0:02:51Underwater, the reef is stirring.
0:02:52 > 0:02:57Finally the conditions are right for the coral to spawn.
0:03:01 > 0:03:07Over the next 24 hours, several tonnes of sperm and eggs will be released into the water.
0:03:17 > 0:03:21Overnight, the reef explodes into life.
0:03:28 > 0:03:33And soon one of the most mysterious animals in the ocean appears.
0:03:45 > 0:03:49For a few brief months each year, hundreds of whale sharks
0:03:49 > 0:03:51come to cruise the Ningaloo coastline.
0:03:57 > 0:04:00They're the ocean's great wanderers.
0:04:02 > 0:04:06But outside these waters, almost nothing is known about their lives.
0:04:10 > 0:04:15The first time I swam with a whale shark was about ten years ago now,
0:04:15 > 0:04:17out the front of Ningaloo reef.
0:04:17 > 0:04:21Beautiful calm day, jumped in the water,
0:04:21 > 0:04:25and out of the deep blue came this great big shape.
0:04:25 > 0:04:29You've got an animal that hasn't changed, basically,
0:04:29 > 0:04:31since the age of the dinosaurs.
0:04:31 > 0:04:33You're swimming with a dinosaur.
0:04:33 > 0:04:38At the same time, it's just an absolutely beautiful animal.
0:04:40 > 0:04:43Now Mark Meekan is finding worrying evidence
0:04:43 > 0:04:45that whale sharks are on the decline.
0:04:49 > 0:04:54Over the last ten years, the average size of sharks we've been seeing has been getting smaller.
0:04:54 > 0:04:59It's alarming because it's a classic symptom of over-fishing.
0:04:59 > 0:05:01If my kids want to swim with whale sharks,
0:05:01 > 0:05:05there are really two questions that we actually have to answer.
0:05:05 > 0:05:10The first of those is, where do they go after Ningaloo?
0:05:10 > 0:05:14The second one is, who are they interacting with?
0:05:14 > 0:05:17Those are the things we've got to find out.
0:05:19 > 0:05:25The next morning, Mark heads out on the first day of his tagging operation.
0:05:29 > 0:05:33They have just one week to deploy six satellite tags.
0:05:38 > 0:05:41They may be searching for the biggest fish in the ocean,
0:05:41 > 0:05:44but it's still a challenge to find them.
0:05:48 > 0:05:51Norwest Air, this is the Orissa Blue. You receive me?
0:05:51 > 0:05:54'Go ahead Orissa Blue, this is Norwest Air.'
0:05:56 > 0:06:01From the air you can see the sharks much, much better than you can see them on the water.
0:06:01 > 0:06:03These things are brilliantly camouflaged
0:06:03 > 0:06:06but in the air they're silhouetted. The pilot can look down,
0:06:06 > 0:06:09spot the shark and he can lead us straight to it.
0:06:09 > 0:06:14Now the key thing is the whale sharks have to show up and want to play.
0:06:17 > 0:06:20Tim, we'll hang around Black Rock here.
0:06:20 > 0:06:24- If you get a shark we'll have a good run to it.- Roger that.
0:06:28 > 0:06:32Stretching 260 kilometres across the West Coast of Australia,
0:06:32 > 0:06:36Ningaloo is one of the largest fringing reef systems in the world.
0:06:38 > 0:06:41And at this time of year it explodes into life.
0:06:44 > 0:06:49Every creature in the ocean is moving in to feed and breed,
0:06:49 > 0:06:52from the smallest to the largest.
0:06:52 > 0:06:55They've all come to exploit the reef's nutrient-rich waters.
0:07:04 > 0:07:08'Orissa Blue, Orissa Blue, this is Norwest Air - do you copy?'
0:07:08 > 0:07:11Receiving, Tim. What have you got for us?
0:07:11 > 0:07:15'Got a large shark about half a mile...'
0:07:15 > 0:07:16Guys, got a shark.
0:07:16 > 0:07:20OK. Going north at about what speed, mate?
0:07:20 > 0:07:23The whale shark has been spotted at the surface
0:07:23 > 0:07:28but it's an unpredictable animal and could disappear at any moment.
0:07:28 > 0:07:32- How big?- It's a seven-metre shark but he's diving.
0:07:32 > 0:07:36- We need to get down there quick. - Toe tag?- Let's get it on.
0:07:40 > 0:07:45- Coming up on the wave now.- Got it? - Good to go.
0:07:45 > 0:07:48- Gun loaded? - The gun's loaded. Safety's on.
0:07:50 > 0:07:54OK. I see him. Go, go, go, go.
0:08:20 > 0:08:23Every shark is photographed for their database.
0:08:27 > 0:08:31The tag is designed to drop off after a year.
0:08:31 > 0:08:34For the shark it will be painless,
0:08:35 > 0:08:38but for Mark it's a dangerous moment.
0:08:39 > 0:08:43An animal this large must be approached with extreme caution.
0:08:43 > 0:08:47A direct hit from its tail could cause serious injury.
0:08:58 > 0:09:01I'll pass you the gun there, Cary.
0:09:01 > 0:09:05- Careful, it's still loaded. Didn't go off.- Got it.
0:09:06 > 0:09:11Got in the water, got up to it, put the tag on the animal
0:09:11 > 0:09:13and the gun didn't go off.
0:09:13 > 0:09:17Ah, you know, occasionally these things happen.
0:09:17 > 0:09:20We're just going to have to work out why it didn't go off.
0:09:25 > 0:09:29Beneath them, the conditions are perfect for whale sharks.
0:09:30 > 0:09:33As the current runs over the reef, it brings with it
0:09:33 > 0:09:37a cloud of minute sea life that turns the water cloudy.
0:09:39 > 0:09:43And soon the ocean's filter feeders are drawn to the feast.
0:09:48 > 0:09:52Whale sharks are the largest of all sharks.
0:09:52 > 0:09:55Fully grown, they're twice the size of a great white.
0:09:57 > 0:10:00And with a mouth up to two metres wide,
0:10:00 > 0:10:02they could easily engulf a human.
0:10:14 > 0:10:16But these sharks are gentle giants,
0:10:16 > 0:10:20and unlike their fearsome relatives, they filter their food.
0:10:21 > 0:10:25Gulping thousands of litres of water every hour,
0:10:25 > 0:10:28they use their huge gills to sieve out small particles of life.
0:10:36 > 0:10:40OK, we've got a whale shark just off the bow.
0:10:40 > 0:10:43With the gun fixed, Mark is back in action.
0:10:44 > 0:10:47Go, go, go!
0:11:13 > 0:11:15As Mark approaches,
0:11:15 > 0:11:18the whale shark turns its back in a defensive posture.
0:11:21 > 0:11:24Here its skin grows up to 14 cm thick -
0:11:24 > 0:11:28thicker and tougher than any animal in the world.
0:11:51 > 0:11:53Breath-holding to 15 metres,
0:11:53 > 0:11:57Mark's first successful tag shows just how dangerous it can be.
0:11:59 > 0:12:02This animal's swimming towards you at about two knots.
0:12:02 > 0:12:07It's like a bus coming past and you're trying to whack something on the side of a bus as it goes past.
0:12:07 > 0:12:09It's not a simple matter.
0:12:09 > 0:12:13The animal's big. You can't stop it, you get one shot at it.
0:12:24 > 0:12:27As the afternoon progresses,
0:12:27 > 0:12:31more sharks are tagged and the project gets off to a good start.
0:12:43 > 0:12:45You know what I was doing today?
0:12:45 > 0:12:48I tagged three whale sharks today.
0:12:48 > 0:12:50Three!
0:12:50 > 0:12:52Three great big ones.
0:12:52 > 0:12:54'You know, it's my son,'
0:12:54 > 0:12:56he's five years old and he's got a...
0:12:56 > 0:12:59Well, he loves whale sharks
0:12:59 > 0:13:02and I was hoping maybe we could get some names
0:13:02 > 0:13:05for some of the whale sharks we tagged today.
0:13:05 > 0:13:07You know, he came up with some names.
0:13:07 > 0:13:11Hamish and Jack, two of his school friends.
0:13:18 > 0:13:23As the moon rises over Ningaloo, it's time to input the day's data.
0:13:27 > 0:13:29Over the last ten years,
0:13:29 > 0:13:34the team has built up an impressive database of whale shark sightings.
0:13:34 > 0:13:38Each animal has a unique pattern of spots that allows Mark
0:13:38 > 0:13:41to identify and cross-reference every individual.
0:13:41 > 0:13:45OK, lets see if we've got any matches from today.
0:13:45 > 0:13:50The first shark to be analysed is a six-metre male he calls Hamish.
0:13:52 > 0:13:56The next two are smaller adolescent males named Jack and Sammy.
0:13:59 > 0:14:03Have they ever been seen in these waters before?
0:14:03 > 0:14:07The photo ID results show they're new to the database.
0:14:08 > 0:14:11But just like the other sharks at Ningaloo,
0:14:11 > 0:14:13they share one thing in common.
0:14:14 > 0:14:19The population here at Ningaloo, they're all young males. They're all juveniles, really.
0:14:19 > 0:14:23Although they're quite large, five to six metres in size,
0:14:23 > 0:14:27they're still babies really. They haven't become mature.
0:14:29 > 0:14:33By the end of the week, a total of six whale sharks are tagged.
0:14:35 > 0:14:38Now every time Hamish and the others surface,
0:14:38 > 0:14:41a satellite deep in space will record their every move.
0:14:45 > 0:14:48But unlike a true whale, these sharks don't need to come up for air
0:14:48 > 0:14:52and could spend several days deep underwater.
0:15:03 > 0:15:10With the tags on, Mark can now focus on another whale shark mystery.
0:15:10 > 0:15:12Beyond a few brief encounters at the surface,
0:15:12 > 0:15:16almost nothing is known about their day-to-day lives.
0:15:18 > 0:15:22We're going to try and attach this camera - it's more than a camera.
0:15:22 > 0:15:25It's a mini laboratory, it's a dive computer.
0:15:25 > 0:15:28This has never been done successfully before, but if we get it right
0:15:28 > 0:15:32it's going to tell us about what these animals are feeding on,
0:15:32 > 0:15:37how they get down there and what they're actually doing on the bottom.
0:15:37 > 0:15:39We can't go down there. This can,
0:15:39 > 0:15:43and when it pops back up to the surface we get that information back.
0:15:53 > 0:15:56The camera is attached by a time-sensitive fuse
0:15:56 > 0:15:59and will release at 4am tomorrow morning.
0:16:04 > 0:16:07Riding on top of its back the camera will capture
0:16:07 > 0:16:10a shark's eye view of Ningaloo Reef.
0:16:12 > 0:16:17Offering for the first time a unique window into the whale shark's world.
0:16:19 > 0:16:21What secrets will it uncover?
0:16:27 > 0:16:33- 22...- 22 degrees, 40.211 minutes. - 40.211.
0:16:33 > 0:16:37- The next morning, the team head out early.- 113 degrees.
0:16:37 > 0:16:39We've got the lat longs of the camera.
0:16:39 > 0:16:43It's come off the animal, floating in the ocean and we've got to go and get it.
0:16:44 > 0:16:46The coordinates will get them close,
0:16:46 > 0:16:50but from here on they must rely on a VHF receiver
0:16:50 > 0:16:51to pick up the camera's signal.
0:16:53 > 0:16:56With 40,000 of equipment floating in the open ocean,
0:16:56 > 0:16:59there's a lot riding on its safe return.
0:17:03 > 0:17:07- One o'clock, straight ahead.- Yeah, there it is, there it is. Got it.
0:17:08 > 0:17:10Keep coming, Terry.
0:17:10 > 0:17:14- One o'clock, Terry. - About four boat lengths.
0:17:14 > 0:17:18- Keep the same position. - Coming down the side.
0:17:18 > 0:17:20Well done!
0:17:23 > 0:17:25Hup! He's got it.
0:17:27 > 0:17:29Woo-hoo!
0:17:29 > 0:17:31Hey, you caught a fish!
0:17:31 > 0:17:33An expensive fish.
0:17:33 > 0:17:36A very expensive fish, look at that!
0:17:36 > 0:17:39Great, fantastic, we got it back.
0:17:39 > 0:17:41We got it back!
0:17:43 > 0:17:46Well, hopefully it's full of data, that's the next question.
0:17:46 > 0:17:50Did the camera work, did the instruments work, that's the thing.
0:17:50 > 0:17:54If they did it's fantastic, it's a huge amount of data in there,
0:17:54 > 0:17:58that no-one's ever managed to get from whale sharks before.
0:18:02 > 0:18:04While the sharkcam data is being analysed,
0:18:04 > 0:18:08information starts to come in from the other tagged sharks.
0:18:10 > 0:18:14Will they finally discover where Ningaloo's whale sharks are going?
0:18:18 > 0:18:19In the weeks that follow,
0:18:19 > 0:18:22Mark tracks their progress up and down the coastline
0:18:22 > 0:18:26as they continue to feed on Ningaloo's rich currents.
0:18:31 > 0:18:35By mid-May one of the sharks, the six-metre male called Hamish,
0:18:35 > 0:18:36breaks away from the group.
0:18:38 > 0:18:42Moving offshore, he leaves the security of the Ningaloo Marine Park
0:18:42 > 0:18:46travelling 800 kilometres into the Indian Ocean.
0:18:49 > 0:18:52It's an exciting development.
0:18:52 > 0:18:56It confirms Mark's suspicions that Ningaloo's whale sharks
0:18:56 > 0:19:00are not only leaving Australia, but travelling huge distances.
0:19:07 > 0:19:11But it's here, entering the busy shipping lanes of Asia,
0:19:11 > 0:19:13that Hamish is most vulnerable.
0:19:21 > 0:19:24Every year, hundreds of whale sharks
0:19:24 > 0:19:29are killed by boat strikes, fishing, and even pollution.
0:19:29 > 0:19:33It's having a devastating effect on the Ningaloo population.
0:19:37 > 0:19:41Ten years ago whale sharks averaged seven metres in size.
0:19:41 > 0:19:45Today the whale sharks we see out there are three to four metres in size.
0:19:45 > 0:19:48That's a drop of two metres, it's big.
0:19:48 > 0:19:52There's also been a worrying decline in abundance.
0:19:52 > 0:19:55Now those are classic signs of a population in trouble.
0:19:55 > 0:19:59It makes it all the more critical that we find out where these sharks are going to.
0:19:59 > 0:20:03You have to know where they're going and who you have to talk to
0:20:03 > 0:20:06if you're going to do anything about the problem.
0:20:08 > 0:20:13Protecting an animal that moves through international waters isn't easy.
0:20:14 > 0:20:17But thankfully, Hamish's journey doesn't end there.
0:20:17 > 0:20:21Whilst the other sharks stay around Ningaloo,
0:20:21 > 0:20:24Hamish continues west at a rate of 30 kilometres a day.
0:20:31 > 0:20:33But before more satellite data comes in,
0:20:33 > 0:20:38the sharkcam starts yielding some interesting results.
0:20:38 > 0:20:42We put the sharkcam on the sharks to really, for the first time,
0:20:42 > 0:20:46get a view of what the shark is actually doing
0:20:46 > 0:20:48down on the bottom at Ningaloo,
0:20:48 > 0:20:51and it's an amazing bit of footage we've recovered.
0:20:51 > 0:20:56On the surface we can see the shark gulping and feeding at things.
0:20:56 > 0:21:01But there are plenty of strange behaviours I can't explain.
0:21:01 > 0:21:04There's some interesting behaviour where the shark is literally
0:21:04 > 0:21:07standing on its tail and sinking down from the surface.
0:21:07 > 0:21:11What it's doing then I have no idea.
0:21:13 > 0:21:18And more surprises are being revealed by the 3-D dive profile.
0:21:19 > 0:21:22These animals are a lot like marine mammals.
0:21:22 > 0:21:25They go down and come back up again. Why?
0:21:25 > 0:21:26Well, one, it's a search for food.
0:21:26 > 0:21:30The water is in layers. Smells travel really well through those layers
0:21:30 > 0:21:34and if you're a shark you go down, you go up, you pass through all those layers
0:21:34 > 0:21:37and you get a good sample of what's around to feed on.
0:21:37 > 0:21:41The other thing is that, even though these are big animals,
0:21:41 > 0:21:44the water down at the bottom is colder than it is at the top
0:21:44 > 0:21:48so it may well be that when the shark spends a lot of time at the bottom
0:21:48 > 0:21:51it actually has to come back up to the surface to reheat.
0:21:51 > 0:21:53But we need a lot more footage like this
0:21:53 > 0:21:57to really understand the reasons behind that diving behaviour.
0:22:04 > 0:22:07It's two months since the tags were deployed
0:22:07 > 0:22:10and Hamish has travelled 1,000 kilometres.
0:22:12 > 0:22:16Where he is heading, nobody knows.
0:22:16 > 0:22:19Mark has a hunch that Hamish could be following
0:22:19 > 0:22:22seasonal food pulses around the Indian Ocean.
0:22:23 > 0:22:27We know that these animals turn up at Ningaloo
0:22:27 > 0:22:31for these mass feeding events and there are other places
0:22:31 > 0:22:35around the world's oceans where this happens as well.
0:22:35 > 0:22:39In the Indian Ocean they also turn up periodically to feed off
0:22:39 > 0:22:45Christmas Island, India, Kenya, Mozambique and the Seychelles.
0:22:47 > 0:22:53The real question is, are they the same animals that are participating in all these events?
0:22:53 > 0:22:55Are they travelling between them?
0:22:55 > 0:22:59The DNA evidence shows they're all closely related
0:22:59 > 0:23:02and could be part of one large inter-breeding population.
0:23:06 > 0:23:10So could Hamish be travelling from Australia to Africa?
0:23:16 > 0:23:21As he continues west, now almost 1,000 kilometres from Ningaloo,
0:23:21 > 0:23:23some disappointing news comes in.
0:23:26 > 0:23:29The tags are off the animals, they're floating.
0:23:29 > 0:23:32How they came off, I don't know. It's just one of those things.
0:23:32 > 0:23:36We have to expect a high failure rate for tags, so we just have to
0:23:36 > 0:23:40get more out there, get more on the sharks I think is the only way forward.
0:23:50 > 0:23:55Following his hunch, Mark flies 5,000km across the Indian Ocean
0:23:55 > 0:23:58to the tropical islands of the Seychelles
0:23:58 > 0:24:02for a working holiday with his family and the next leg of the study.
0:24:04 > 0:24:07To meet them is long-time collaborator, David Rowatt,
0:24:07 > 0:24:11who runs the largest whale shark monitoring programme in the world.
0:24:13 > 0:24:15Hiya. How are you two?
0:24:17 > 0:24:19So are you getting good sharks, David?
0:24:19 > 0:24:22Yeah, sharks arrived about a day or two ago.
0:24:22 > 0:24:25We're getting reasonable numbers at the moment.
0:24:25 > 0:24:28It's taken 18 hours to get here from Australia
0:24:28 > 0:24:31and now Mark and David have just one week to find out
0:24:31 > 0:24:33if the whale sharks are making a similar journey.
0:24:33 > 0:24:37Basically the boats are going out every day, so yeah, ready to roll.
0:24:46 > 0:24:50Perched on the edge of the African continent, the granite islands
0:24:50 > 0:24:54of the Seychelles rise out of the Indian Ocean.
0:24:56 > 0:24:58It's the start of the whale shark season
0:24:58 > 0:25:00and for the next three months,
0:25:00 > 0:25:04seasonal upwellings of nutrient-rich waters
0:25:04 > 0:25:07will attract whale sharks to these shores.
0:25:07 > 0:25:11Real time, real time, I have sighted a shark...
0:25:11 > 0:25:14- It's just south under me. - We've got a visual.
0:25:14 > 0:25:20During this period, every encounter is photographed, measured and recorded.
0:25:24 > 0:25:27Go when you're ready. That's it... Go, go.
0:25:28 > 0:25:31And what's so special about this research
0:25:31 > 0:25:35is that almost all the data is collected by tourists.
0:25:35 > 0:25:37We thank you for coming along,
0:25:37 > 0:25:41because it's only with your assistance and support
0:25:41 > 0:25:43that we can run this programme.
0:25:43 > 0:25:49It's all part of David Rowatt's plan to make the whale sharks more valuable to the island.
0:25:50 > 0:25:54And by offering hundreds of tourists the chance to swim with them,
0:25:54 > 0:25:58it means that David can run his research on an epic scale.
0:26:01 > 0:26:07It's this data that could tell Mark if Ningaloo's sharks are visiting the Seychelles.
0:26:07 > 0:26:10That long line there - it's not the same shark.
0:26:10 > 0:26:12Both David and I have very large
0:26:12 > 0:26:14photo-identification libraries of whale sharks,
0:26:14 > 0:26:19for me in Ningaloo and David in the Seychelles and what we've been doing
0:26:19 > 0:26:22is using a computer programme to try and match those photographs.
0:26:22 > 0:26:28- So we've done the fingerprint, we've done our three points in.- Yeah.
0:26:28 > 0:26:32If Hamish and other Ningaloo sharks really are crossing the Indian Ocean
0:26:32 > 0:26:34they would expect to find some matches.
0:26:38 > 0:26:40But with over 1,000 images to compare,
0:26:40 > 0:26:43it will be another month before they will know the result.
0:26:46 > 0:26:50In the meantime, Mark and David have more tags to deploy.
0:26:56 > 0:27:00Real time, real time, I've sighted a shark.
0:27:00 > 0:27:04It's just south. We're gonna go down and have a look.
0:27:07 > 0:27:12Real time, if you come right 45 degrees, at 500 metres from you,
0:27:12 > 0:27:1545 degrees, 500 metres.
0:27:28 > 0:27:30Go, Dave, go!
0:27:36 > 0:27:39Every year more than 200 sharks visit these islands,
0:27:39 > 0:27:43but when the food runs out, almost all of them disappear.
0:27:46 > 0:27:51By tagging more whale sharks, Mark and David hope to find out where they're going.
0:27:54 > 0:27:57Could they be heading back to Australia
0:27:57 > 0:27:59and the rich waters of Ningaloo?
0:28:09 > 0:28:12After a successful start, it's time for Mark to head out
0:28:12 > 0:28:15for some fun with his six-year-old son, Sammy.
0:28:19 > 0:28:23We've promised him he can go swimming with a whale shark.
0:28:23 > 0:28:26I think I'm more delighted than he is, actually.
0:28:26 > 0:28:30It's been such a fondly held dream of mine for so long,
0:28:30 > 0:28:32and finally we did it, you know?
0:28:32 > 0:28:36- OK, it's just here, Mark, close to the boat.- We'll get there.
0:28:36 > 0:28:39Here we go, are you ready, mate?
0:28:50 > 0:28:53It really was a contrast between the two of them.
0:28:53 > 0:28:56I mean, there's Sammy, he's just over a metre tall,
0:28:56 > 0:28:59and that shark, five or six times his size,
0:28:59 > 0:29:02and he's not afraid of it at all.
0:29:02 > 0:29:05Perfectly happy to paddle along with the shark.
0:29:20 > 0:29:24Missing most of its dorsal fin, this shark is typical of the population
0:29:24 > 0:29:29in the Seychelles, where some 30% carry the scars of boat strikes.
0:29:33 > 0:29:37It's a sobering reminder of just how vulnerable these sharks are.
0:29:39 > 0:29:43It was important for me to have Sammy swim with the whale shark
0:29:43 > 0:29:46and share that joy I get out of it with my kids,
0:29:46 > 0:29:52and important to do it now too, because who knows, mate,
0:29:52 > 0:29:55when he's my age they may not be around.
0:30:01 > 0:30:06As the week progresses, the team collect data on ten more whale sharks.
0:30:07 > 0:30:10And as Mark's trip draws to a close,
0:30:10 > 0:30:14they attach a second satellite tag to a four-metre male named Joe.
0:30:22 > 0:30:26Soon, data comes in showing him moving north along the main island.
0:30:28 > 0:30:32But as Mark returns home, the airwaves go silent.
0:30:35 > 0:30:38And as weeks turn into months, it becomes all too apparent
0:30:38 > 0:30:42that the satellite has lost contact with the sharks.
0:30:44 > 0:30:50You know, pretty disappointed that another tag design has failed.
0:30:50 > 0:30:54But, look, there are some real positives that came out of the Seychelles.
0:30:54 > 0:30:57One of those was the photo ID study.
0:30:57 > 0:31:00We found no matches between the Seychelles and Ningaloo.
0:31:00 > 0:31:04That's really significant because it tells us the animals
0:31:04 > 0:31:08probably aren't travelling between Ningaloo and the Seychelles or vice versa.
0:31:08 > 0:31:11What I think it means is that we have to look closer to home
0:31:11 > 0:31:14for some of the answers about where these animals are going.
0:31:17 > 0:31:21If Ningaloo's whale sharks aren't crossing the Indian Ocean,
0:31:21 > 0:31:23then where are they heading?
0:31:27 > 0:31:31Mark re-examines the data from Hamish,
0:31:31 > 0:31:34the adolescent male that lost his tag mid-ocean.
0:31:37 > 0:31:4016,000 kilometres from Ningaloo
0:31:40 > 0:31:43lies the tiny Australian outpost of Christmas Island,
0:31:43 > 0:31:46where there are reports of whale sharks gathering.
0:31:55 > 0:31:59It's a tiny little green speck out in the middle of the Indian Ocean
0:31:59 > 0:32:03but to whale sharks they see it from a different perspective.
0:32:03 > 0:32:09It's a giant mountain rising out of the sea floor and the whale sharks turn up there predictably every year.
0:32:11 > 0:32:13Arriving on the tail end of a storm,
0:32:13 > 0:32:16Mark has just two weeks to find some answers.
0:32:19 > 0:32:22But his first lead doesn't point to the ocean.
0:32:22 > 0:32:27Instead, it takes him inland to the lush tropical forests that cover Christmas Island.
0:32:30 > 0:32:32Here, beneath a pristine canopy,
0:32:32 > 0:32:36one of the greatest events of the natural world is under way.
0:32:39 > 0:32:41Hey, look, crabs, crabs on the road.
0:32:41 > 0:32:44Yeah, there's a lot on the side of the road.
0:32:44 > 0:32:46OK, look out, don't run them over.
0:32:46 > 0:32:50Stop, stop, we're going to drive over those ones. Yeah, I'll get out.
0:32:59 > 0:33:01Look at that.
0:33:01 > 0:33:05It's a red land crab. It's land crab breeding season
0:33:05 > 0:33:10and these guys are all moving from the forest down to the beach.
0:33:10 > 0:33:14Each one of these crabs is spawning about 100,000 eggs
0:33:14 > 0:33:18and you've got to figure that with about 15 million females on the island
0:33:18 > 0:33:21we're talking about a billion eggs being released into the ocean.
0:33:21 > 0:33:25We don't know if the whale sharks are actually eating them or not
0:33:25 > 0:33:27but you'd have to figure that it would make sense.
0:33:29 > 0:33:31The crabs only spawn on one night of the year.
0:33:33 > 0:33:36Is it just coincidence that the whale sharks arrive at the same time?
0:33:49 > 0:33:52It's now a month since they left their forest homes,
0:33:52 > 0:33:55and their arduous migration is almost over.
0:34:00 > 0:34:02Now heavily laden with eggs,
0:34:02 > 0:34:05the females navigate the final stretch to the ocean.
0:34:16 > 0:34:18In a few hours' time,
0:34:18 > 0:34:23100 million new lives will be launched into the water.
0:34:34 > 0:34:36Look at that! That's amazing!
0:34:36 > 0:34:40Crabs on top of crabs on top of crabs.
0:34:40 > 0:34:41That's incredible, look.
0:34:41 > 0:34:47At 4am, on the turn of a high tide, the action begins.
0:34:47 > 0:34:49Like their marine ancestors,
0:34:49 > 0:34:52the land crabs are still tied to the ocean
0:34:52 > 0:34:55and their eggs can only hatch when they reach the water.
0:34:58 > 0:35:01That is solid crabs, look at it.
0:35:03 > 0:35:04They're flapping, look at that!
0:35:04 > 0:35:08There's the jig as they release the eggs.
0:35:08 > 0:35:11As the eggs hit the water they hatch immediately
0:35:11 > 0:35:14and turn into these tiny little crab larvae.
0:35:14 > 0:35:17They'll swim around for about 27 days out on the water
0:35:17 > 0:35:20and then come back in a stage called the megalope -
0:35:20 > 0:35:23it looks just like a baby, juvenile crab -
0:35:23 > 0:35:27and climb up the cliffs and head back into the forest.
0:35:27 > 0:35:29Look at the water - it's brown with eggs.
0:35:29 > 0:35:33you've got to figure that this is a great reason
0:35:33 > 0:35:36for the whale sharks to come to Christmas Island.
0:35:39 > 0:35:43No-one has ever witnessed sharks feeding on red crab larvae.
0:35:43 > 0:35:47It's just a theory, but if Mark can prove it, it will be a first.
0:35:54 > 0:35:58But the next day gets off to a slow start.
0:36:00 > 0:36:04We're very keen to get out and see if we can find some whale sharks.
0:36:04 > 0:36:09One problem is though that our plane, our aerial support, is out there on a supply ship.
0:36:09 > 0:36:14The weather's been so bad they've been unable to bring the boat against the wharf here.
0:36:14 > 0:36:17Without that plane it just means we're going to have to get out there
0:36:17 > 0:36:21and maybe do an exploratory dive and see what we can see.
0:36:23 > 0:36:26With no indication of when the boat might come in,
0:36:26 > 0:36:29Mark's best chance of finding a whale shark
0:36:29 > 0:36:33is to start surveying the island's 80 kilometres of coastline.
0:36:51 > 0:36:55Surrounded by a narrow ledge of tropical reef, the sea bed
0:36:55 > 0:36:59quickly drops away to the abyssal depths of the Java Trench.
0:37:01 > 0:37:03Four kilometres below the surface,
0:37:03 > 0:37:06it's one of the deepest points in the Indian Ocean.
0:37:08 > 0:37:13And it's along these walls that the whale sharks are reported to cruise.
0:37:16 > 0:37:18As you went over the edge of the drop-off,
0:37:18 > 0:37:22you lean forward and it was like you were flying a plane into the depths.
0:37:28 > 0:37:30An absolutely fantastic feeling.
0:37:32 > 0:37:35It truly is the edge of the unknown.
0:37:35 > 0:37:40You feel like you're clinging to the last vestiges of the land,
0:37:40 > 0:37:44and somewhere out there are these giant creatures cruising up and down.
0:37:48 > 0:37:51But until they can get up in the air,
0:37:51 > 0:37:53the sharks remain as elusive as ever.
0:38:03 > 0:38:07Then, on the fifth day, the container ship finally comes in,
0:38:07 > 0:38:13and with just one week to go Mark's study can at last take off.
0:38:17 > 0:38:20The team's first task is to locate the whale sharks
0:38:20 > 0:38:22and establish where they are congregating.
0:38:22 > 0:38:25What a great morning for flying.
0:38:25 > 0:38:27- Yeah, it's awesome, isn't it? - Beautiful.
0:38:27 > 0:38:30You get a great view from up here, don't you?
0:38:31 > 0:38:35It's the first time a whale shark survey has ever been conducted
0:38:35 > 0:38:40on Christmas Island, and Mark has no idea what he's going to find.
0:38:40 > 0:38:43OK, that's deep. Let's just swing around.
0:38:43 > 0:38:48Yeah, that's a shark. OK, that's a big tadpole.
0:38:48 > 0:38:52Look at that, they look just like big tadpoles from the air.
0:38:52 > 0:38:54A great big, fat head
0:38:54 > 0:38:56and a skinny tail at the back.
0:38:56 > 0:38:59- It's great, isn't it? - Oh, we lost them.
0:39:04 > 0:39:07Well, we saw two whale sharks, they're both on the north coast.
0:39:07 > 0:39:10What it means for the population we're not really sure.
0:39:10 > 0:39:15We need to do a few more flights and spend a more time out there to find out if there are more of them.
0:39:15 > 0:39:19Maybe it's the same two whale sharks out there every day. Who knows.
0:39:19 > 0:39:23It's fewer sharks than they were expecting, but it's enough to get started.
0:39:26 > 0:39:29Mark has just six days left to find out
0:39:29 > 0:39:32if Hamish and the other whale sharks from Ningaloo
0:39:32 > 0:39:34are visiting Christmas Island.
0:39:37 > 0:39:42To do this they plan to find, photograph and tag as many of them as possible.
0:39:44 > 0:39:48The spotter plane is now finding many more sharks.
0:39:48 > 0:39:50It's right here, by the boat.
0:39:53 > 0:39:58But, frustratingly, as soon as they get close, the animals are diving.
0:40:03 > 0:40:05And as the days go by,
0:40:05 > 0:40:10the difficulties of studying such an elusive fish are all too evident.
0:40:11 > 0:40:15- That's getting ridiculous, don't you think?- Yes.
0:40:18 > 0:40:21Same old story, I'm afraid - get on the shark,
0:40:21 > 0:40:25right up close to it just off the bow, within distance to swim to it,
0:40:25 > 0:40:29the thing dives straight away. I didn't see it.
0:40:29 > 0:40:34It was on the surface but it jumped in. It hit the water and was gone.
0:40:34 > 0:40:38With one day left, it looks like they will go home empty-handed.
0:40:39 > 0:40:43- I'm over it.- It's hard to tag them when you can't see them. - I'm over it.
0:40:45 > 0:40:47Bye, Mark!
0:40:49 > 0:40:5210 o'clock...
0:40:52 > 0:40:54Two only, back her off, back her off.
0:40:54 > 0:40:56There it is, right there!
0:40:56 > 0:40:59Then on the very last day, just hours before their flight,
0:40:59 > 0:41:02a final opportunity presents itself.
0:41:02 > 0:41:04We have a visual, stay on it, small shark.
0:41:04 > 0:41:07They're coming round on the back, OK?
0:41:07 > 0:41:09In... go, Cary, go! Go!
0:41:09 > 0:41:11Get it, go!
0:41:11 > 0:41:13We're looking at it.
0:41:13 > 0:41:16We're not waiting here, we're going.
0:41:19 > 0:41:21It's a friendly shark!
0:41:21 > 0:41:23Good God, I didn't know they existed.
0:41:35 > 0:41:38HE SHOUTS AND CHEERS
0:41:42 > 0:41:46Christmas Island was nothing if not dramatic.
0:41:46 > 0:41:51We spent two weeks there fruitlessly searching for whale sharks
0:41:51 > 0:41:55and in the final day we finally got some.
0:41:55 > 0:41:57In fact, we got a lot.
0:41:57 > 0:42:00We got seven tags out.
0:42:02 > 0:42:06It was an absolutely fantastic end to the trip.
0:42:10 > 0:42:15And remarkably, we even got a whale shark that managed to do a poo for us.
0:42:15 > 0:42:17It's such a rare event.
0:42:17 > 0:42:19I've only ever seen it twice
0:42:19 > 0:42:22with the hundreds of whale sharks I've swum with.
0:42:22 > 0:42:27It's so significant because the DNA in that sample is worth gold.
0:42:32 > 0:42:35Here it is - whale shark poo.
0:42:36 > 0:42:39Tip it out and there it is.
0:42:39 > 0:42:43That little, reddy brown soupy mixture there,
0:42:43 > 0:42:46if we can extract the DNA out of this, it's gonna tell us something
0:42:46 > 0:42:50about why the whale sharks are coming to Christmas Island.
0:42:54 > 0:42:59And when the result come in, it proves Mark's theory.
0:42:59 > 0:43:02The sample is packed with red crab DNA.
0:43:04 > 0:43:07The results of the poo analysis are pretty exciting.
0:43:07 > 0:43:12They tell us that the migration of these sharks from Ningaloo to the north is not some random thing.
0:43:12 > 0:43:16These sharks are actually directing their movements.
0:43:16 > 0:43:18They're going to Christmas Island for a reason.
0:43:18 > 0:43:23There's a whole pile of food up there and these sharks are feeding on it.
0:43:23 > 0:43:26So the next question is, where do they go after Christmas Island?
0:43:26 > 0:43:29Do they come back to Ningaloo or do they go somewhere else?
0:43:35 > 0:43:41In the weeks that follow, all seven sharks remain around the island.
0:43:41 > 0:43:46But after a month, only one tag is still transmitting.
0:43:48 > 0:43:51It's attached to a five-metre male named Fin,
0:43:51 > 0:43:54and to Mark's delight, he's now on the move.
0:44:00 > 0:44:02First, he travels north,
0:44:02 > 0:44:05before heading south-east on a direct course back to Australia.
0:44:09 > 0:44:12It's an exciting development.
0:44:12 > 0:44:14With Ningaloo's coral spawn imminent,
0:44:14 > 0:44:17could Fin be returning on an annual migration?
0:44:22 > 0:44:27But then, to everyone's surprise, he changes direction.
0:44:27 > 0:44:30In two months he travels 2,000 kilometres,
0:44:30 > 0:44:33before disappearing off the radar.
0:44:36 > 0:44:39It looks like there's a problem with Fin's tag.
0:44:42 > 0:44:46But two weeks later, he reappears in the Banda Sea.
0:44:47 > 0:44:52The dive data reveals he travelled 800 kilometres,
0:44:52 > 0:44:55rarely coming close to the surface.
0:44:55 > 0:45:01Mark wonders whether he was feeding on an unknown food source deep underwater.
0:45:01 > 0:45:04Or perhaps he was avoiding the strong surface currents
0:45:04 > 0:45:07that run between these islands.
0:45:07 > 0:45:10Then Fin turns around and heads back to West Timor.
0:45:14 > 0:45:19It's here, four months and 4,000 kilometres later,
0:45:19 > 0:45:21that the tag finally falls off.
0:45:26 > 0:45:32It washed up on a beach and it was found by a guy searching for turtle eggs.
0:45:32 > 0:45:36The tag was still transmitting so I could actually see the guy
0:45:36 > 0:45:38and I could see his house where he was storing it.
0:45:38 > 0:45:41So at that point I got one of our PhD students
0:45:41 > 0:45:44to go and recover the tag, which he did.
0:45:44 > 0:45:49And here it is. And when we got the tag back I think we got
0:45:49 > 0:45:53some pretty interesting clues as to why it came off in the first place.
0:45:53 > 0:45:56See this? Shark bite marks,
0:45:56 > 0:45:58and it's been a pretty big shark too
0:45:58 > 0:46:02judging by the distance between those scrape marks.
0:46:02 > 0:46:05I think a couple of hundred kilos of biting shark
0:46:05 > 0:46:07hanging off the end of this
0:46:07 > 0:46:10has been the reason the tag came off the whale shark.
0:46:10 > 0:46:14And it could be the reason behind many of the tags failing.
0:46:20 > 0:46:24As Fin's journey ends, Mark is left to draw his conclusions
0:46:24 > 0:46:27from the remarkable data he's provided.
0:46:28 > 0:46:33But 12 months and 20 satellite tags later, is Mark any closer
0:46:33 > 0:46:39to unravelling the mystery of where Ningaloo's whale sharks are going?
0:46:43 > 0:46:46What I thought we'd see at the start of this study
0:46:46 > 0:46:49is we'd tag sharks in Ningaloo and we'd see at least some
0:46:49 > 0:46:51go across the Indian Ocean to the Seychelles.
0:46:51 > 0:46:55And that's certainly what the genetic data was suggesting.
0:46:55 > 0:46:58But that's not what happened.
0:46:58 > 0:47:01Moreover, when we looked at the photo identification data,
0:47:01 > 0:47:04there were no matches between sharks at Ningaloo
0:47:04 > 0:47:06or sharks in the Seychelles.
0:47:06 > 0:47:10Hundred of sharks, thousands of photos, not one shark in common.
0:47:10 > 0:47:13So I think this is basically wrong.
0:47:13 > 0:47:15I suspect what's actually going on
0:47:15 > 0:47:18is the sharks that are over here on this side of the Indian Ocean
0:47:18 > 0:47:21stay within the same area, and that's certainly what we saw
0:47:21 > 0:47:24for the sharks we tagged at Christmas Island.
0:47:24 > 0:47:27What does that mean for the sharks at Ningaloo?
0:47:27 > 0:47:29Well, actually, I think it's good news.
0:47:29 > 0:47:33Look if the sharks are really truly moving all this way over to Africa,
0:47:33 > 0:47:36conservation of them is a much more complex question,
0:47:36 > 0:47:39but if they're actually just staying over here,
0:47:39 > 0:47:42we only have to deal with our nearest neighbours.
0:47:44 > 0:47:45Since this study,
0:47:45 > 0:47:50Mark has started an ecotourism project in Western Timor.
0:47:50 > 0:47:54And now the prospects for Ningaloo's giant fish look a little brighter.
0:48:00 > 0:48:04But whilst the whale sharks have given up some of their secrets,
0:48:04 > 0:48:07there are still many more questions that will need to be answered
0:48:07 > 0:48:10before their future can be truly secured.
0:48:24 > 0:48:27Subtitles by Red Bee Media Ltd
0:48:27 > 0:48:31E-mail subtitling@bbc.co.uk