0:00:30 > 0:00:37There is a force sufficiently powerful to move the oceans of this world.
0:01:04 > 0:01:07It is a force not of this earth.
0:01:10 > 0:01:14The moon is big enough to generate gravity -
0:01:14 > 0:01:19and with sufficient force to pull on the earth 230,000 miles away.
0:01:21 > 0:01:24As the moon orbits the earth,
0:01:24 > 0:01:29its gravity sweeps across the face of our planet.
0:01:29 > 0:01:34Its power drags a great bulge of oceanic water in its wake.
0:01:40 > 0:01:42The rising tide.
0:01:46 > 0:01:50The River Amazon in Brazil.
0:01:50 > 0:01:53On some special days,
0:01:53 > 0:01:59the gravitational forces of the moon and the sun pull together to extraordinary effect.
0:02:06 > 0:02:11A growing tidal wave from the ocean is being forced 200 miles inland.
0:02:13 > 0:02:16This is a "tidal bore".
0:02:52 > 0:02:55Fortunately, tidal bores are rare.
0:02:55 > 0:02:58but the moon does create strong tides
0:02:58 > 0:03:03out in the world's oceans on every day of the year.
0:03:09 > 0:03:12The Bay of Fundy in Nova Scotia.
0:03:14 > 0:03:18The tides here are the largest in the world,
0:03:18 > 0:03:22and have a profound effect on marine life,
0:03:22 > 0:03:25creating a rich feeding ground...
0:03:28 > 0:03:34..a feast that attracts some of the largest diners on the planet.
0:03:42 > 0:03:44Humpback whales.
0:03:45 > 0:03:50But they are not the biggest threat to the herring.
0:04:05 > 0:04:10These are finback whales, at 70 tons, the second-largest animal on earth -
0:04:10 > 0:04:13but so beautifully streamlined
0:04:13 > 0:04:17that it is the fastest of the great whales.
0:04:18 > 0:04:23This combination of speed and immense size
0:04:23 > 0:04:27makes the finback a voracious hunter of schooling fish.
0:04:27 > 0:04:31The Bay of Fundy can attract so many fish
0:04:31 > 0:04:38that during the summer as many as 500 of these magnificent whales hunt here every day.
0:04:40 > 0:04:45The feeding is best where the tides run strongly.
0:04:45 > 0:04:49So the whales move further into the bay,
0:04:49 > 0:04:53following tidal rips and searching for fish.
0:05:00 > 0:05:06Their movements are closely watched by flocks of Cory shearwaters.
0:05:09 > 0:05:16As the whales dive down towards the fish, more and more birds gather, anxious to pick up scraps.
0:05:28 > 0:05:31The flowing tide may provide a feast,
0:05:31 > 0:05:34but before long, it will turn.
0:05:38 > 0:05:42In just six hours, 100 BILLION tons of water
0:05:42 > 0:05:45will flow out of the bay -
0:05:45 > 0:05:50the sea level falling by as much as 15 metres
0:05:50 > 0:05:54and exposing vast tracts of mud and sand...
0:05:55 > 0:06:00..at first sight, a barren place entirely devoid of life.
0:06:03 > 0:06:10In fact, the damp sand is packed with microscopic life, the meiofauna,
0:06:10 > 0:06:12feeding in a sandy underworld
0:06:12 > 0:06:17quite unaffected by the departure of the sea.
0:06:18 > 0:06:23But life is not all roses in this miniature world.
0:06:25 > 0:06:30A sand bubbler crab in northern Australia. It hunts meiofauna.
0:06:30 > 0:06:35Just a centimetre across, the sand bubbler works at breakneck speed,
0:06:35 > 0:06:42passing sand grains into its mouth, filtering out all the meiofauna and kicking aside the waste.
0:06:47 > 0:06:52The crab will clean every grain of sand within a metre of its burrow.
0:06:54 > 0:06:59Endless practice for the best back-heel in the natural world.
0:07:19 > 0:07:25The crabs work fast because they can only sieve when the sand is damp.
0:07:25 > 0:07:28They work the entire surface of the beach
0:07:28 > 0:07:33within just a couple of hours of the tide retreating.
0:07:38 > 0:07:43Then they simply return to their burrows and await the next tide.
0:07:49 > 0:07:54Underwater, the falling tide is the cue for some bizarre activity.
0:07:59 > 0:08:02These slow-moving clams use their muscular feet
0:08:02 > 0:08:07to bury themselves under the sand.
0:08:07 > 0:08:14If they fail to get under cover, the tide will leave them exposed to the air, and they will perish.
0:08:14 > 0:08:20But once underground, they can wait deep down in the sand,
0:08:20 > 0:08:22safe beneath the beach.
0:08:26 > 0:08:28And not a moment too soon.
0:08:32 > 0:08:35June in south-east Alaska,
0:08:35 > 0:08:40and in just four hours, a vast, sandy beach is exposed by the falling tide.
0:08:47 > 0:08:49The bears are hungry.
0:08:49 > 0:08:55At this time of year, the pickings on land are few and far between.
0:08:55 > 0:09:01But any food here has long since buried itself deep under the sand.
0:09:03 > 0:09:07To a hungry adult bear, that is no barrier.
0:09:07 > 0:09:12They smell the clams through the sand and simply dig them out.
0:09:15 > 0:09:18For such large animals,
0:09:18 > 0:09:23they show extraordinary dexterity at opening the unfortunate shellfish
0:09:41 > 0:09:44Cubs try their luck, too,
0:09:44 > 0:09:47none too successfully.
0:09:50 > 0:09:53For the adults, the shellfish feast lasts
0:09:53 > 0:09:56as long as the tide remains out.
0:10:01 > 0:10:05Table Mountain in South Africa.
0:10:08 > 0:10:14Every day, the retreating waves leave flotsam somewhere on the beach.
0:10:14 > 0:10:17This creature is scenting the currents
0:10:17 > 0:10:20for the odour of rotting fish.
0:10:20 > 0:10:25The tide carries the scent far into the surf zone.
0:10:27 > 0:10:32Responding to the smell, snails emerge from the sand.
0:10:38 > 0:10:43This is a race against the tide. The snails need to find their meal
0:10:43 > 0:10:48before the tide leaves it beyond their reach.
0:10:48 > 0:10:53But snails are slow, and the tides fall rapidly.
0:10:55 > 0:10:58These are no ordinary snails.
0:10:59 > 0:11:01They can surf.
0:11:26 > 0:11:34They ride the waves up the beach. But all too soon, the tide leaves the fish beyond the surf zone.
0:11:40 > 0:11:45Without the sea, there's a danger that the snails will lose the scent,
0:11:45 > 0:11:48but as long as the sand remains damp,
0:11:48 > 0:11:53they can still follow a faint trail to the food.
0:11:53 > 0:11:55Once there, they tuck in
0:11:55 > 0:11:58with macabre relish.
0:12:06 > 0:12:09Good things come to those who wait.
0:12:22 > 0:12:27Before long, the heat of the sun forces them to retreat into the sand
0:12:27 > 0:12:30to await the return of the next tide.
0:12:34 > 0:12:41In the water, incoming tides can create a strong current,
0:12:41 > 0:12:45and flounders are experts at hitching a tidal lift.
0:12:46 > 0:12:49They're shaped rather like a kite,
0:12:49 > 0:12:53a perfect design for gliding on the tide.
0:12:56 > 0:13:04In Newfoundland, on the east coast of Canada, large numbers of flounder ride the currents into the shallows.
0:13:07 > 0:13:13They've come to hunt invertebrates that will emerge now that the water is back.
0:13:30 > 0:13:35The pickings in the shallows can be very good.
0:13:37 > 0:13:41The activity has not gone unnoticed.
0:13:45 > 0:13:48But ospreys can't dive deeply.
0:13:52 > 0:13:58As long as the water is over a metre deep, the flounder will be safe.
0:14:08 > 0:14:13Going too far inshore can be a risky business.
0:14:22 > 0:14:27This fish buries itself completely in the sand at any sign of danger.
0:14:27 > 0:14:32But, when the tide floods in again, as long as the coast is clear,
0:14:32 > 0:14:35these sand lancet will re-emerge.
0:14:41 > 0:14:46After a wait of six hours under the sand, they're desperate for food
0:14:46 > 0:14:50and, unlike flounder, they head out to sea.
0:14:50 > 0:14:58They're looking for shallow, open water where the tidal currents will concentrate their food -
0:14:58 > 0:15:00plankton.
0:15:06 > 0:15:12In their untold thousands, they stream towards the best feeding grounds
0:15:12 > 0:15:17where they simply pick up tiny, planktonic creatures from the water.
0:15:26 > 0:15:30But if they swim too far offshore in search of food,
0:15:30 > 0:15:34they risk running into large predators
0:15:34 > 0:15:37that live out in deeper water.
0:15:38 > 0:15:41Dogfish - small sharks.
0:15:43 > 0:15:49The sand lancet HAVE strayed out of their safe depth.
0:16:48 > 0:16:53The effect of the turning tide can be totally different on a rocky shore.
0:16:56 > 0:17:01Here, on the coast of Vancouver Island in Canada,
0:17:01 > 0:17:04the sun bakes the exposed rock.
0:17:04 > 0:17:09It's virtually impossible to dig underground when the sea retreats,
0:17:09 > 0:17:17so these mussels and barnacles are fully exposed to the sun's heat - literally cooking in their shells.
0:17:20 > 0:17:23And the seaweed simply dry to a crisp.
0:17:29 > 0:17:34It can be a wait of many hours before the water returns.
0:17:36 > 0:17:41Throughout each month, the size and the strength of the tide changes.
0:17:42 > 0:17:45The biggest tides of all happen
0:17:45 > 0:17:50when the gravities of the sun and moon pull in unison.
0:17:50 > 0:17:53That's immediately after the new moon
0:17:53 > 0:17:56and again after the full moon.
0:18:01 > 0:18:08These are the "spring tides". They reveal vast tracts of sea-bed that would normally be covered.
0:18:12 > 0:18:16For these raccoons, it's a chance to look for a seafood feast.
0:18:18 > 0:18:22A mother ventures forth with her kits.
0:18:22 > 0:18:29With the spring tide, they've come further down the beach than smaller tides would normally allow.
0:18:38 > 0:18:42Searching with their extraordinarily sensitive paws,
0:18:42 > 0:18:45they look for suitable prey.
0:18:45 > 0:18:50With the extreme low tide, they could find something special.
0:19:05 > 0:19:08What could be better than a red rock crab?
0:19:08 > 0:19:13That is, if it weren't for the risk of a painful pinch.
0:19:18 > 0:19:22With large crabs, there's no substitute for experience.
0:19:22 > 0:19:25The mother makes an expert's catch.
0:19:33 > 0:19:36But the kits learn fast.
0:19:42 > 0:19:47And for those that don't, begging is always worth a try.
0:19:49 > 0:19:54All too soon, the returning tide will cover the raccoons' table.
0:20:03 > 0:20:10For the invertebrates, it's a welcome relief, but in rough weather they are exposed to the worst of the waves.
0:20:50 > 0:20:53Even when there are no waves,
0:20:53 > 0:20:58the incoming tide can create considerable forces underwater.
0:20:58 > 0:21:04The gaps between these small islands on the east coast of Vancouver Island channel the tidal flow.
0:21:04 > 0:21:10As the tide keeps rising, gradually the water flows faster,
0:21:10 > 0:21:16and soon, these giant, 30m-long bull kelp plants bend to the current.
0:21:27 > 0:21:32They're sufficiently flexible to cope without too much damage,
0:21:32 > 0:21:37but there are some spots where the currents are especially powerful.
0:21:38 > 0:21:41This is the Naquatto Rapids.
0:21:41 > 0:21:46At the turn of the tide, water from almost 700 miles of coastal fjords
0:21:46 > 0:21:51will have to empty through a gap of less than half a mile wide.
0:21:51 > 0:21:56Within a few minutes, the current is picking up speed,
0:21:56 > 0:21:59until water roars by at over 17mph.
0:22:38 > 0:22:41Tidal currents are not always a damaging force.
0:22:41 > 0:22:45Here, in the Poor Knight Islands of New Zealand,
0:22:45 > 0:22:48weak tides run through rock arches.
0:22:48 > 0:22:51It's ideal for resting stingrays.
0:23:01 > 0:23:06These rays congregate here in huge numbers every March.
0:23:06 > 0:23:08They've come to breed.
0:23:15 > 0:23:18The arches funnel the current,
0:23:18 > 0:23:23which the rays can ride with the minimum of effort, so saving energy.
0:23:31 > 0:23:37Nearby, out in open water, a school of two-spotted demoiselle fish feed on plankton,
0:23:37 > 0:23:42and the current is perfect for sweeping their food past them.
0:23:53 > 0:23:56Once the current starts to weaken,
0:23:56 > 0:24:03there is insufficient food to warrant the risks of swimming about out here in the open,
0:24:03 > 0:24:10so the demoiselles head off to find shelter en masse. Safety is in numbers.
0:24:18 > 0:24:26More and more demoiselles pour towards the cave entrances that riddle the Poor Knight Islands.
0:24:54 > 0:25:00Safe. In the cave, they are far less exposed to attack from predators.
0:25:03 > 0:25:11Thousands of demoiselles and blue maomao wait for the return of the current,
0:25:11 > 0:25:15when once again they will head out into the open to feed.
0:25:17 > 0:25:22The moon's gravitational pull is weaker nearer to the earth's equator,
0:25:22 > 0:25:28so the more equatorial the location, the smaller the tides.
0:25:28 > 0:25:34And out here in the Caribbean Sea, the tidal movements are slight.
0:25:34 > 0:25:39Even so, they are sufficient to push free-swimming plankton in their path.
0:25:43 > 0:25:46These are thimble jellyfish.
0:25:49 > 0:25:57They swim towards the sunlight, and invisible boundaries formed by the tidal motion help herd them together
0:25:57 > 0:26:00until they gather in immense swarms.
0:26:00 > 0:26:04They put the tropical sunshine to good use.
0:26:04 > 0:26:08Their brown colour is from algae that live inside the jellyfishes' bodies
0:26:08 > 0:26:12and photosynthesise energy from the sun.
0:26:22 > 0:26:25In open water, they're fairly safe.
0:26:25 > 0:26:30But the tide is sweeping the whole swarm gently towards the Bahamas,
0:26:30 > 0:26:33where hungry mouths are waiting.
0:26:35 > 0:26:38Although the sea level doesn't change much,
0:26:38 > 0:26:44the tides are still pushing an enormous volume of water from the ocean
0:26:44 > 0:26:49through the small gaps between the island cays.
0:26:59 > 0:27:04Underwater, the tidal currents race past soft corals...
0:27:10 > 0:27:14..and on over the sandy banks themselves.
0:27:17 > 0:27:20It's an immense area of coral sand
0:27:20 > 0:27:24that is only just submerged at high tide.
0:27:26 > 0:27:32This incoming tide is bringing in a fresh supply of oceanic plankton,
0:27:32 > 0:27:36and razorfish gather at the best spots
0:27:36 > 0:27:40to catch the pick of the microscopic feast.
0:27:40 > 0:27:45Although there's plenty here for these small fish to eat,
0:27:45 > 0:27:50gathering in one place makes it easy for their predators to find them.
0:27:53 > 0:27:56A nurse shark is little threat...
0:27:58 > 0:28:01SQUEAKS AND CHIRRUPS
0:28:01 > 0:28:05..but this SOUND heralds a different danger.
0:28:05 > 0:28:08WHIPLASH CRACK FAINT CREAK
0:28:08 > 0:28:12CHIRPS AND CLICKS
0:28:12 > 0:28:16CLICKS AND BUZZES
0:28:17 > 0:28:21"ZIP" CALL A bottlenose dolphin.
0:28:21 > 0:28:23ZIP! WHEE!
0:28:23 > 0:28:28It's using its sonar to locate razorfish beneath the sand.
0:28:28 > 0:28:31IT BUZZES, CLICKS AND PURRS
0:28:37 > 0:28:42Once it finds a suitable target, it simply digs out its prey.
0:28:58 > 0:29:00THEY SQUEAK SOFTLY
0:29:01 > 0:29:03THEY TWITTER
0:29:12 > 0:29:15"ELECTRIC-ARC" FIZZ
0:29:17 > 0:29:23The buried fish have no defence against this attack.
0:29:23 > 0:29:27They will simply have to wait and hope they aren't found out.
0:29:40 > 0:29:44This dolphin appears to have a razorfish craving.
0:29:44 > 0:29:47Well, she IS pregnant.
0:29:56 > 0:29:59Success at last.
0:30:04 > 0:30:07THEY GIVE CREAKY SQUEAKS
0:30:10 > 0:30:15The incoming tide sweeps on towards America,
0:30:15 > 0:30:19flooding across vast, flat plains of seagrass.
0:30:19 > 0:30:25They're so shallow that at low tide, all large fish are forced to retreat into deep-water channels -
0:30:25 > 0:30:28like these nurse sharks...
0:30:31 > 0:30:36..and stingrays. Both predators hunt crustaceans on the seagrass beds.
0:30:36 > 0:30:42But until the flooding tide brings enough water for them to swim in, they will have to wait.
0:30:48 > 0:30:53So now, for this tulip snail, it appears safe
0:30:53 > 0:30:56to patrol the channels in search of a meal.
0:30:56 > 0:30:59But is it?
0:31:06 > 0:31:09This is a rather bigger kind of snail.
0:31:11 > 0:31:15At 5kg, the giant horse conch has little to fear from any shark -
0:31:15 > 0:31:18and it has a taste for tulip snail.
0:31:26 > 0:31:31Sensing the approaching danger, the snail flees.
0:31:34 > 0:31:39But in a world of snail paces, the conch is something of a Ferarri.
0:31:39 > 0:31:41It calls for desperate measures.
0:31:43 > 0:31:47Exhausted by the effort of its last-ditch attempt,
0:31:47 > 0:31:51the tulip snail is slowly gunned down.
0:31:58 > 0:32:01The tide has to rise for another hour
0:32:01 > 0:32:05before the big predators can feed.
0:32:05 > 0:32:10But out on the flats, the scent of dying snail wafts away on the tide.
0:32:12 > 0:32:16It's a scent these hermit crabs are partial to.
0:32:19 > 0:32:24It's vital that the crabs have the best possible protection
0:32:24 > 0:32:28from the heavy teeth of the waiting sharks and rays.
0:32:28 > 0:32:32For that, they need the shell with a perfect fit.
0:32:32 > 0:32:40Today, there is new real estate on offer, and competition in this housing market is fierce.
0:33:04 > 0:33:08The action becomes even more desperate
0:33:08 > 0:33:13when the shell of the devoured snail is ready for release.
0:33:21 > 0:33:25This crab simply can't wait any longer.
0:33:27 > 0:33:31But it's a decidedly risky acquisition.
0:33:34 > 0:33:37The risk paid off handsomely.
0:33:37 > 0:33:43The new shell is both lighter and stronger than the old home - and it's not a moment too soon,
0:33:43 > 0:33:48because the tide is flowing in strongly, flooding the plains.
0:33:50 > 0:33:54At last, the predators are free to start their foraging.
0:34:03 > 0:34:08Both the stingrays and sharks have a highly-developed electrical sense
0:34:08 > 0:34:12which they use to search for buried invertebrates.
0:34:12 > 0:34:16They can sense minute movements beneath the sand.
0:34:17 > 0:34:22Finding a promising signal, this ray digs out its meal.
0:34:22 > 0:34:26An unprotected hermit crab would have no chance.
0:34:32 > 0:34:36Within a few hours, the tide ebbs out once more
0:34:36 > 0:34:39and all the predators are forced to leave.
0:34:45 > 0:34:51They will have to wait. until the next high tide before making another feeding foray.
0:34:55 > 0:35:02At certain times of the year, at the equinox, spring-tides are exceptionally large,
0:35:02 > 0:35:05and rise even higher than normal
0:35:06 > 0:35:12Now, predators can reach the very shallowest fringes of the seagrass flats,
0:35:12 > 0:35:17and these two-metre-long tarpon are going further inshore still.
0:35:17 > 0:35:20They are heading for the mangroves.
0:35:22 > 0:35:27These flooded forests cover huge areas of the coastal shallows.
0:35:28 > 0:35:34Extraordinarily, the roots of the mangrove trees can live in salt water,
0:35:34 > 0:35:39and they make a perfect nursery for small fish.
0:35:46 > 0:35:51Silversides and snapper find sanctuary in the maze of roots.
0:35:51 > 0:35:55Big predators seldom find a way in here.
0:36:01 > 0:36:04Now the tide is falling once more.
0:36:06 > 0:36:11The water starts losing what little oxygen it contained,
0:36:11 > 0:36:14and quickly becomes stagnant.
0:36:16 > 0:36:20Most predators have abandoned the mangroves,
0:36:20 > 0:36:25but the tarpon are still here, trapped by the falling tide.
0:36:29 > 0:36:32Dissolved oxygen is fast running out.
0:36:32 > 0:36:35They have a vital survival technique.
0:36:39 > 0:36:42They can breathe air.
0:36:52 > 0:36:58Pumped up with fresh oxygen, they easily outmanoeuvre dozy silversides.
0:37:20 > 0:37:23The tide has turned again.
0:37:24 > 0:37:28And this is no ordinary tide.
0:37:28 > 0:37:32Since it is the equinox, the tide is rising fast.
0:37:32 > 0:37:36But now, out to sea, a hurricane is on its way -
0:37:36 > 0:37:39forcing the tide yet higher.
0:38:08 > 0:38:14The passing storm leaves large areas of the coast flooded by the sea.
0:38:16 > 0:38:24And low-lying islands like the Bahamas are particularly prone to the storm flooding.
0:38:25 > 0:38:28The sun's power here is immense.
0:38:30 > 0:38:37As the tide recedes and the remaining floodwater evaporates, a remarkable transformation takes place.
0:38:46 > 0:38:51The mud is coated with a magical world of salt.
0:39:01 > 0:39:04Any remaining water is extremely salty.
0:39:04 > 0:39:10Very few creatures can survive here except brine shrimp...
0:39:16 > 0:39:19..and, on the water's edge,
0:39:19 > 0:39:22brine flies.
0:39:22 > 0:39:27Both are the favourite food of an extraordinary animal.
0:39:29 > 0:39:32The Caribbean flamingo.
0:39:35 > 0:39:40Remarkably, they actually seek out such briny places.
0:39:40 > 0:39:44They're the best spots for them to find their food.
0:39:46 > 0:39:48BIRDS HONK
0:39:57 > 0:40:02They also provide the protection the flamingoes need to raise young.
0:40:05 > 0:40:09nesting sites like this are surrounded by corrosive brine.
0:40:09 > 0:40:12It's a formidable barrier
0:40:12 > 0:40:16to any predators seeking to dine on flamingo chicks.
0:40:18 > 0:40:21The flamingoes take the precaution
0:40:21 > 0:40:24of building raised mud nests
0:40:24 > 0:40:27just in case of further flooding.
0:40:27 > 0:40:30ADULTS HONK AND QUACK
0:40:30 > 0:40:32CHICK PIPES
0:40:34 > 0:40:40Strangely, it's actually the power of the storm tides that gives the flamingoes both their food
0:40:40 > 0:40:44and the perfect habitat in which to breed.
0:40:56 > 0:41:03The breeding of many animals in the ocean is closely co-ordinated with the tidal cycles.
0:41:03 > 0:41:06A half-moon in November.
0:41:06 > 0:41:09It's the time of SMALL tides.
0:41:13 > 0:41:15Christmas Island in the Pacific.
0:41:17 > 0:41:20Strange happenings are afoot.
0:41:27 > 0:41:30It's one of only a few nights each year
0:41:30 > 0:41:33when female Christmas Island crabs
0:41:33 > 0:41:36risk heading down towards the sea.
0:41:38 > 0:41:46Around the island, they number in tens of thousands - and all of them are laden with hundreds of eggs.
0:41:47 > 0:41:50They have to shed them into the ocean
0:41:50 > 0:41:54if the eggs are to develop into baby crabs.
0:41:59 > 0:42:01But these are LAND crabs,
0:42:01 > 0:42:05and they can neither swim nor breathe underwater.
0:42:15 > 0:42:18There's a great risk of drowning,
0:42:18 > 0:42:24so they pick the smallest tides of the month to minimise the danger.
0:42:49 > 0:42:52The eggs will develop far offshore,
0:42:52 > 0:42:56and in exactly one month, a swarm of baby crabs will return -
0:42:56 > 0:43:00again, choosing the perfect tide.
0:43:06 > 0:43:10Whether it's the daily or monthly cycle,
0:43:10 > 0:43:14tides are the rhythm of the ocean, its pulsing clock.
0:43:14 > 0:43:19Every tide brings opportunity to marine life somewhere in the world.
0:43:19 > 0:43:22DOLPHINS TWEET
0:43:23 > 0:43:28Now, a spring-tide is flooding the shallows,
0:43:28 > 0:43:31and hunters are on the prowl.
0:43:32 > 0:43:38A small group of bottlenose dolphin are working their way inshore
0:43:38 > 0:43:42to start a quite extraordinary hunting campaign.
0:44:07 > 0:44:12After one successful pass, the dolphin move off to start again.
0:44:13 > 0:44:19One animal peels off from the group and swims rapidly in a circle,
0:44:19 > 0:44:25stirring up the mud and driving the mullet towards the other waiting dolphins.
0:44:25 > 0:44:30It's a remarkable team effort - and it's extremely effective.
0:44:36 > 0:44:43The dolphin will feed like this for as long as the tide grants them access to the shallows.
0:45:31 > 0:45:38Eventually, the falling tide will force the dolphin to leave the flats, and the mullet will be safe again -
0:45:38 > 0:45:41until the next high tide...
0:45:44 > 0:45:47..because, in the ocean,
0:45:47 > 0:45:53every turn of the tide spells the difference between life and death
0:45:53 > 0:45:55somewhere.
0:46:03 > 0:46:10When trying to film the tidal behaviours of marine creatures, timing is of the essence.
0:46:10 > 0:46:14Many events in the sea only take place at certain tidal phases,
0:46:14 > 0:46:19and some, on only one particular tide in the whole year.
0:46:19 > 0:46:25One of the most spectacular displays depends completely on nature's tidal clock.
0:46:25 > 0:46:28It's the spawning of the coral reefs,
0:46:28 > 0:46:33which release their eggs and sperm in synchrony after a spring full moon.
0:46:33 > 0:46:38A crew was there on Australia's Great Barrier Reef to keep the appointment,
0:46:38 > 0:46:42but things don't always go as planned.
0:46:47 > 0:46:52Lizard Island - a backwater behind the centre of the barrier reef.
0:46:52 > 0:46:59A good place to film coral, but the exact details of coral spawning are still not properly understood.
0:46:59 > 0:47:05Only 20 years ago, they discovered that the entire Barrier Reef went off.
0:47:05 > 0:47:11It goes off on three or four nights, the whole reef, all the corals spawn.
0:47:11 > 0:47:17They've been working on it ever since. They still don't know much.
0:47:17 > 0:47:22Because the spawning could happen in several places at once,
0:47:22 > 0:47:27Martha took two cameramen, Peter Scoones, a veteran of coral seas,
0:47:27 > 0:47:35and Mike Pitts who, although familiar with these waters, had never seen coral spawning before.
0:47:36 > 0:47:41- Mike?- It's working.- Can you light balance away from the reef?
0:47:41 > 0:47:44So we're not shining on the corals.
0:47:44 > 0:47:49The spawning activity starts normally starts two days after a full moon,
0:47:49 > 0:47:54and the crew were ready with a complex system of underwater lights.
0:47:54 > 0:47:59What I wanted to achieve with the lights was that it was natural,
0:47:59 > 0:48:04certainly not front lit with a light stuck on the front of the camera,
0:48:04 > 0:48:07it had to have more atmosphere.
0:48:07 > 0:48:14As well as directing, Martha is in charge of dive safety and has to wait on the surface.
0:48:14 > 0:48:19Just so want to get the best images. When they are all underwater,
0:48:19 > 0:48:26I'm praying that someone's seen a fantastic coral going off and they've lit it beautifully.
0:48:26 > 0:48:30That first night, the corals showed no signs of spawning.
0:48:30 > 0:48:35Meanwhile, the team was also trying another strategy.
0:48:35 > 0:48:38On shore in a laboratory on nearby Lizard Island,
0:48:38 > 0:48:45cameraman, Steve Downer and director, Hugh Pearson were hoping to film close-ups of coral spawning
0:48:45 > 0:48:47in more controlled conditions.
0:48:47 > 0:48:53We spent four or five nights in the laboratory getting these shots.
0:48:53 > 0:48:59In the end, the coral did spawn and we got some shots of a spawn leaving the corals in a big close-up.
0:49:05 > 0:49:13Although that was successful, out at sea, everyone were still hoping that the big event would finally happen.
0:49:13 > 0:49:18Today is night three of a potential spawning.
0:49:18 > 0:49:23Although Craig has said tonight is going to be the quietest,
0:49:23 > 0:49:27they also know up until an hour before spawning,
0:49:27 > 0:49:30so they can give warning before it happens.
0:49:30 > 0:49:34The third night seemed a little more promising.
0:49:37 > 0:49:42Some of the corals began to release sperm in clouds into the water.
0:49:42 > 0:49:46But this wasn't the spectacle the team wanted.
0:49:46 > 0:49:51Just looks like a foggy cloud coming out of it.
0:49:51 > 0:49:54Lots and lots of tiny eggs wafting around.
0:49:54 > 0:49:59It takes a long time. Once they start it doesn't go in one bang.
0:49:59 > 0:50:03It just goes one, two... four, five...one.
0:50:03 > 0:50:11- You wait five minutes and you might get two.- Watching a stone getting ready to spawn is...
0:50:11 > 0:50:15Some pleasing images, but only a start,
0:50:15 > 0:50:18and not enough to complete the sequence.
0:50:18 > 0:50:22- We're not balanced for that... That's mine!- I know.
0:50:22 > 0:50:26Tonight is the big one. Trouble is, the wind's got up.
0:50:26 > 0:50:30When the conditions get rough, the corals don't spawn
0:50:30 > 0:50:35because the sperm and the eggs would mix so much.
0:50:35 > 0:50:38There's a possibility things won't happen tonight.
0:50:38 > 0:50:41But, hey. Positive thinking.
0:50:41 > 0:50:46Martha's fears proved right, and despite the hopeful start
0:50:46 > 0:50:50spawning was sparse on the fourth and fifth night,
0:50:50 > 0:50:53making it difficult to catch the spectacle on camera.
0:50:53 > 0:50:58One little coral, and I thought, "I can get an angle on that."
0:50:58 > 0:51:02I was getting into position and it went poof!
0:51:02 > 0:51:05Come back! I didn't get that.
0:51:05 > 0:51:09She had expected the spawning to be more vigorous.
0:51:09 > 0:51:13There was only one day left to complete the filming.
0:51:13 > 0:51:18The scientists were convinced of spawning that night.
0:51:18 > 0:51:21Elation was dampened by another setback.
0:51:21 > 0:51:25Ten minutes ago we were sitting out on the back deck,
0:51:25 > 0:51:28Peter said his camera was dead.
0:51:28 > 0:51:33We've got an hour or so until the spawning starts.
0:51:33 > 0:51:37I just had a letter from the coral expert
0:51:37 > 0:51:39who said tonight's the night.
0:51:39 > 0:51:44I'm hoping it isn't because we've only got one camera.
0:51:44 > 0:51:47I'm sure Mike will do a great job.
0:51:47 > 0:51:53At the last minute, Peter succeeded in repairing his camera.
0:51:53 > 0:51:55The team's patience was rewarded.
0:51:55 > 0:52:02Large areas of the reef spawn together in a spectacular simultaneous display.
0:52:02 > 0:52:08There's mystery in seeing six branching corals go off at the same time.
0:52:08 > 0:52:11They're not connected to each other,
0:52:11 > 0:52:15yet they know they had to spawn at the same time.
0:52:15 > 0:52:17Nobody has worked that one out yet.
0:52:24 > 0:52:27Timing had been everything. Within a few hours
0:52:27 > 0:52:32the team returned to the boat to review the results.
0:52:32 > 0:52:36It didn't look anything like that.
0:52:40 > 0:52:41There!
0:52:43 > 0:52:46When they're coming through...
0:52:49 > 0:52:52Wow! Well done, everybody.
0:52:52 > 0:52:57It's amazing. Thank you very much. I know it's been a struggle.
0:52:57 > 0:52:59ALL: Whoa!
0:52:59 > 0:53:02Can you rewind that?
0:53:03 > 0:53:06- Spikey bits.- Whoa!