0:00:31 > 0:00:36Dwarfed by the vast expanse of the open ocean,
0:00:36 > 0:00:41the biggest animal that has ever lived on our planet.
0:00:46 > 0:00:51A blue whale, 30m long and weighing over 200 tonnes.
0:00:51 > 0:00:56It's far bigger than even the biggest dinosaur.
0:00:56 > 0:01:00Its tongue weighs as much as an elephant,
0:01:00 > 0:01:02its heart is the size of a car
0:01:02 > 0:01:07and some of its blood vessels are so wide you could swim down them.
0:01:08 > 0:01:13Its tail alone is the width of a small aircraft's wings.
0:01:29 > 0:01:34Its streamlining, close to perfection,
0:01:34 > 0:01:37enables it to cruise at 20 knots.
0:01:37 > 0:01:41It's one of the fastest animals in the sea.
0:01:43 > 0:01:46The ocean's largest inhabitant
0:01:46 > 0:01:50feeds almost exclusively on one of the smallest -
0:01:50 > 0:01:55krill, a crustacean just a few centimetres long.
0:01:57 > 0:02:01Gathered in a shoal, krill stain the sea red.
0:02:01 > 0:02:05A single blue whale in a day can consume 40 million of them.
0:02:14 > 0:02:17Despite the enormous size of blue whales
0:02:17 > 0:02:20we know very little about them.
0:02:20 > 0:02:22Their migration routes are still a mystery
0:02:22 > 0:02:25and we have no idea where they go to breed.
0:02:30 > 0:02:34They are a dramatic reminder of how much we still have to learn
0:02:34 > 0:02:39about the ocean and the creatures that live there.
0:02:40 > 0:02:43Our planet is a blue planet.
0:02:43 > 0:02:46Over 70% of it is covered by the sea.
0:02:49 > 0:02:52The Pacific Ocean alone covers half the globe.
0:02:52 > 0:02:55You can fly across it for 12 hours
0:02:55 > 0:02:58and still see nothing more than a speck of land.
0:03:00 > 0:03:05This series will reveal the complete natural history of our ocean planet,
0:03:05 > 0:03:09from its familiar shores to the mysteries of its deepest seas.
0:03:11 > 0:03:14THEY SHRIEK
0:03:20 > 0:03:26By volume, the ocean makes up 97% of the Earth's inhabitable space.
0:03:26 > 0:03:32Its marine life far exceeds that which inhabits the land.
0:03:54 > 0:03:57But life in the ocean is not evenly spread.
0:03:57 > 0:04:02It's regulated by the path of currents carrying nutrients
0:04:02 > 0:04:04and the varying power of the sun.
0:04:04 > 0:04:09In this first programme we'll see how these two forces interact
0:04:09 > 0:04:13to control life from the coral seas...
0:04:14 > 0:04:17..to the polar wastes.
0:04:36 > 0:04:40SEA LION ROARS
0:05:09 > 0:05:14The sheer physical power of the ocean dominates our planet.
0:05:30 > 0:05:35It profoundly influences the weather of all the world.
0:05:35 > 0:05:39Water vapour rising from it forms the clouds
0:05:39 > 0:05:44and generates the storms that ultimately will drench the land.
0:05:59 > 0:06:04The great waves that roar in towards the shores
0:06:04 > 0:06:07are dramatic demonstrations of its power.
0:06:21 > 0:06:24Waves originate far out at sea.
0:06:24 > 0:06:29There, even gentle breezes can cause ripples
0:06:29 > 0:06:32and ripples grow into swells.
0:06:40 > 0:06:46Out in the ocean, unimpeded by land, such swells can become gigantic.
0:07:07 > 0:07:12It's only when an ocean swell eventually reaches shallow water
0:07:12 > 0:07:14that it starts to break.
0:07:19 > 0:07:22As it approaches the coast,
0:07:22 > 0:07:27the bottom of the swell is slowed by contact with the sea bed.
0:07:27 > 0:07:32The top of the swell, still going fast, starts to roll over
0:07:32 > 0:07:35and so the wave breaks.
0:08:03 > 0:08:06The ocean never rests.
0:08:06 > 0:08:10Huge currents such as the Gulf Stream,
0:08:10 > 0:08:14keep its waters on the move all round the globe.
0:08:14 > 0:08:17These currents, more than any other factor,
0:08:17 > 0:08:22control the distribution of nutrients and life in the seas.
0:08:22 > 0:08:27A tiny island lost in the midst of the Pacific.
0:08:27 > 0:08:33It's the tip of a huge mountain that rises from the sea floor below.
0:08:38 > 0:08:42The nearest land is 300 miles away.
0:08:45 > 0:08:49Isolated sea mounts like this one create oases
0:08:49 > 0:08:55where life can flourish in the comparatively empty expanses of the open ocean.
0:09:03 > 0:09:10But the creatures that swim beside it would not be here were it not for one key factor -
0:09:10 > 0:09:13the deep ocean currents.
0:09:17 > 0:09:19Far below the surface,
0:09:19 > 0:09:24they collide with the island's flanks and are deflected upwards,
0:09:24 > 0:09:28bringing from the depths, a rich soup of nutrients.
0:09:30 > 0:09:35Such upwellings attract great concentrations of life.
0:09:42 > 0:09:47Most of the fish here are permanent residents feeding on the plankton,
0:09:47 > 0:09:52tiny plants and animals nourished by richness brought from the depths.
0:09:52 > 0:09:56They in turn attract visitors from the open ocean.
0:09:57 > 0:09:59Tuna.
0:10:23 > 0:10:27The plankton feeders are easy targets.
0:10:37 > 0:10:41All this action attracts even larger predators.
0:10:44 > 0:10:46Sharks.
0:10:50 > 0:10:54Hundreds of sharks.
0:10:54 > 0:10:58These silky sharks are normally ocean-going species,
0:10:58 > 0:11:01but the sea mounts in the eastern Pacific,
0:11:01 > 0:11:04like Cocos, Malpelo and the Galapagos,
0:11:04 > 0:11:08attract silkies in huge groups, up to 500 strong.
0:11:12 > 0:11:15Silkies specialise in taking injured fish
0:11:15 > 0:11:20and constantly circle sea mounts looking for the chance to do so.
0:11:24 > 0:11:28But silkies are not the only visitors.
0:11:31 > 0:11:38Hammerheads gather in some of the largest shark shoals to be found anywhere in the ocean.
0:11:38 > 0:11:43Sometimes thousands will circle over a single sea mount.
0:11:46 > 0:11:49But these sharks are not here for food.
0:11:49 > 0:11:52They have come for another reason.
0:11:55 > 0:12:00Some of the locals provide a cleaning service.
0:12:01 > 0:12:04Following the last El Nino year,
0:12:04 > 0:12:11when a rise in water temperatures caused many sharks to suffer from fungal infections,
0:12:11 > 0:12:16the number of hammerheads at the sea mounts reached record levels.
0:12:23 > 0:12:30Nutrients also well up to the surface along the coasts of the continents.
0:12:30 > 0:12:35This is Natal on South Africa's eastern seaboard.
0:12:35 > 0:12:37It's June, and just offshore
0:12:37 > 0:12:40strange, black patches have appeared.
0:12:44 > 0:12:49They look like immense oil slicks up to a mile long.
0:12:50 > 0:12:54But this is a living slick -
0:12:54 > 0:12:58millions of sardines on a marine migration
0:12:58 > 0:13:04that in sheer biomass rivals that of the wildebeest on the grasslands of Africa.
0:13:06 > 0:13:11Most of the time these fish live in the cold waters south of the Cape.
0:13:11 > 0:13:14But each year, the coastal currents reverse.
0:13:14 > 0:13:19The warm Agulhas current that flows from the north has been displaced
0:13:19 > 0:13:23by cold water from the south and has brought up rich nutrients.
0:13:23 > 0:13:27They in turn have created a bloom of plankton
0:13:27 > 0:13:31and the sardines are now feasting on it.
0:13:40 > 0:13:45As the sardines travel north, a caravan of predators follow them.
0:13:47 > 0:13:51Thousands of Cape gannets track the sardines.
0:13:51 > 0:13:53They nested off the Cape
0:13:53 > 0:14:00and timed their breeding so their chicks can join them in pursuing the shoals.
0:14:06 > 0:14:11Below water, hundreds of sharks have also joined the caravan.
0:14:13 > 0:14:16These are bronze whaler sharks,
0:14:16 > 0:14:19a cold-water species that normally lives much further south.
0:14:24 > 0:14:30These three-metre sharks cut such swathes through the sardine shoals
0:14:30 > 0:14:34that their tracks are clearly visible from the air.
0:14:34 > 0:14:38Harried by packs of predators and swept in by the action of the waves,
0:14:38 > 0:14:42the sardine shoals are penned close to the shore.
0:14:59 > 0:15:04Common dolphin are coming in from the open ocean to join the feast.
0:15:16 > 0:15:21There are over a thousand of them in this one school.
0:15:30 > 0:15:35When they catch up with the sardines, the action really begins.
0:15:39 > 0:15:44Working together, they drive the shoal towards the surface.
0:15:51 > 0:15:55It's easier for the dolphins to snatch fish up here.
0:16:09 > 0:16:13Now the sardines have no escape.
0:16:22 > 0:16:29Thanks to the dolphins, the sardines have come within the diving range of the gannets.
0:16:40 > 0:16:44Hundreds of white arrows shoot into the sea
0:16:44 > 0:16:49leaving long trails of bubbles behind each dive.
0:16:59 > 0:17:02Next to join the frenzy are the sharks.
0:17:14 > 0:17:17Sharks get very excited around dolphins,
0:17:17 > 0:17:20maybe because they can feed well,
0:17:20 > 0:17:27once the dolphins drive the sardines into more compact groups near the surface.
0:17:28 > 0:17:33As the frenzy continues, walls of bubbles drift upwards.
0:17:35 > 0:17:40They are being released by the dolphins, working together in teams.
0:17:43 > 0:17:48They use the bubbles to corral the sardines into ever tighter groups.
0:17:51 > 0:17:54The sardines seldom cross the wall of bubbles
0:17:54 > 0:17:57and crowd closer together.
0:18:00 > 0:18:06Bubble netting enables the dolphins to grab every last trapped sardine.
0:18:17 > 0:18:22Just when the feasting seems to be almost over, a Bryde's whale.
0:18:26 > 0:18:29The survivors head on northwards
0:18:29 > 0:18:33and the caravan of predators follows them.
0:18:39 > 0:18:46Nutrients can also be brought up, though less predictably, by rough weather.
0:18:49 > 0:18:54Particularly near the Poles, huge storms stir the depths
0:18:54 > 0:18:56and enrich the surface waters.
0:18:56 > 0:19:01And here in the south Atlantic, the seas are the roughest on the planet.
0:19:03 > 0:19:06And very rich seas they are too.
0:19:06 > 0:19:09The cold Falklands current from the south
0:19:09 > 0:19:12meets the warm Brazil current from the north.
0:19:12 > 0:19:16At their junction, there is food in abundance.
0:19:16 > 0:19:24These black-browed albatross are diving for krill that has been driven up to the surface.
0:19:28 > 0:19:33Like all albatross, black-brows are wanderers across the open ocean.
0:19:46 > 0:19:51A feeding assembly on this scale is a rare sight.
0:19:51 > 0:19:55Usually, the birds of the open sea are widely dispersed.
0:19:55 > 0:20:00But these feeding grounds are close to an albatross breeding colony,
0:20:00 > 0:20:02and a very special one.
0:20:11 > 0:20:16This is Steeple Jason, a remote island in the west of the Falklands.
0:20:16 > 0:20:20It has the largest albatross colony in the world.
0:20:27 > 0:20:31There are almost half a million albatross here,
0:20:31 > 0:20:36an astonishing demonstration of how fertile the ocean can be
0:20:36 > 0:20:41and how much food it can give even to creatures that don't live in it.
0:20:54 > 0:20:58LOUD CAWING OF BIRDS
0:21:07 > 0:21:11Not only nutrients generate these vast assemblies.
0:21:11 > 0:21:14The heat and light that the sun brings
0:21:14 > 0:21:19is essential for the growth of microscopic floating plants,
0:21:19 > 0:21:21the phytoplankton.
0:21:25 > 0:21:30And it's the phytoplankton that is the basis of all life in the ocean.
0:21:35 > 0:21:41Every evening, the disappearance of the sun below the horizon
0:21:41 > 0:21:46triggers the largest migration of life that takes place on our planet.
0:21:52 > 0:21:571,000 million tonnes of sea creatures rise from the deep ocean
0:21:57 > 0:22:00to search for food near the surface.
0:22:05 > 0:22:11They graze on the phytoplankton under cover of darkness.
0:22:11 > 0:22:14Even so, they're far from safe.
0:22:14 > 0:22:16Other marine hunters follow them,
0:22:16 > 0:22:20some travelling up from hundreds of metres below.
0:23:18 > 0:23:24At dawn, the whole procession returns to the safety of the dark depths.
0:23:29 > 0:23:33The moon too has a great influence on life in the oceans.
0:23:35 > 0:23:41It's gravitational pull creates the advances and retreats of the tide.
0:23:52 > 0:23:56But the moon has more than a daily cycle.
0:23:56 > 0:24:01Each month it waxes and wanes as it travels round the Earth.
0:24:01 > 0:24:06This monthly cycle also triggers events in the ocean.
0:24:08 > 0:24:14The Pacific coast of Costa Rica, on a very special night.
0:24:14 > 0:24:19It's just after midnight and the tide is coming in.
0:24:21 > 0:24:24The moon is in its last quarter,
0:24:24 > 0:24:27exactly halfway between full and new.
0:24:29 > 0:24:34For weeks the beach has been empty, but that is about to change.
0:24:34 > 0:24:39At high tide, turtles start to emerge from the surf.
0:24:43 > 0:24:47At first they come in ones and twos.
0:24:47 > 0:24:52But within an hour, they are appearing all along the beach.
0:24:58 > 0:25:01They are all female Ridleys turtles
0:25:01 > 0:25:06and over the next six days or so, 400,000 will visit this one beach
0:25:06 > 0:25:10to lay their eggs in the sand.
0:25:16 > 0:25:21At the peak time, 5,000 are coming and going every hour.
0:25:21 > 0:25:27The top of the beach gets so crowded that they have to clamber over one another
0:25:27 > 0:25:31to find a bare patch where they can dig a nest hole.
0:25:33 > 0:25:39A quarter of the world's population of Ridleys turtles come to this one beach
0:25:39 > 0:25:42on a few key nights each year.
0:25:42 > 0:25:48The rest of the time, they're widely distributed, searching for food,
0:25:48 > 0:25:51most hundreds of miles from here.
0:25:51 > 0:25:55This mass nesting is called an arribada.
0:25:55 > 0:25:58How it's coordinated is a mystery
0:25:58 > 0:26:01but we do know that arribadas start
0:26:01 > 0:26:05when the moon is in its first or its last quarter.
0:26:10 > 0:26:1440 million eggs are laid in just a few days.
0:26:14 > 0:26:18By synchronising their nesting in this way,
0:26:18 > 0:26:25the females ensure that six weeks later their hatchlings will emerge in such enormous numbers
0:26:25 > 0:26:31that predators on the beach are overwhelmed and a significant number of baby turtles will survive.
0:26:31 > 0:26:38But why do the females use a cue from the moon to help them synchronise their nesting?
0:26:38 > 0:26:42Part of the answer to that becomes clear at dawn,
0:26:42 > 0:26:45on the following morning.
0:27:05 > 0:27:10The day shift of predators are arriving for their first meals.
0:27:14 > 0:27:18Vultures have learnt that the returning tide
0:27:18 > 0:27:23can wash freshly-laid eggs out of the sand.
0:27:23 > 0:27:26The risk of eggs being exposed by the surf
0:27:26 > 0:27:32may be part of the reason why turtle arribadas tend to occur
0:27:32 > 0:27:36around the last or first quarter of the moon.
0:27:36 > 0:27:41It's on days such as this, when the moon is neither full nor new,
0:27:41 > 0:27:46that the tides are weakest and the sea is likely to be calmer.
0:27:59 > 0:28:03So at these times it's easier for the female turtles
0:28:03 > 0:28:05to make their way through the surf
0:28:05 > 0:28:07and there's less chance of their eggs
0:28:07 > 0:28:10being washed out of the sand and being taken by the vultures.
0:28:21 > 0:28:26The moon's monthly cycle, and its influence on the tides,
0:28:26 > 0:28:30triggers many events in the ocean,
0:28:30 > 0:28:35from the spawning of the corals to the breeding cycles of fish.
0:28:35 > 0:28:38But an even longer rhythm has the most profound effect of all -
0:28:38 > 0:28:40the annual cycle of the sun.
0:28:43 > 0:28:48The sun's position relative to the Earth changes through the year
0:28:48 > 0:28:51and this produces the seasons.
0:28:51 > 0:28:56In the north, Spring comes as the sun rises higher in the sky.
0:28:56 > 0:29:01Off the coast of north-west America, the seas are transformed
0:29:01 > 0:29:05by the increasing strength of the sunshine.
0:29:07 > 0:29:12Here in Alaska, the coastal waters turn green
0:29:12 > 0:29:16with a sudden bloom of phytoplankton.
0:29:16 > 0:29:20Herring that have spent the winter far out to sea,
0:29:20 > 0:29:24time their return to the shallow waters
0:29:24 > 0:29:26to coincide with this bloom.
0:29:26 > 0:29:32Their vast numbers start one of the oceans' most productive food chains.
0:29:45 > 0:29:50Humpback whales are at the top of that food chain.
0:29:50 > 0:29:55They spent the winter breeding in warmer tropical waters off Hawaii.
0:29:55 > 0:29:59But there was little food for them there.
0:29:59 > 0:30:04This herring bonanza provides the majority of their food for the year.
0:30:24 > 0:30:31Stellar and Californian sea lions also return from the open ocean each year to feast off the herring.
0:30:41 > 0:30:45The herring themselves, however, have not come here for food.
0:30:45 > 0:30:48They are about to breed.
0:30:48 > 0:30:53Nothing deters them as they head for even shallower waters.
0:30:53 > 0:30:57Now the waters are so shallow that glaucous-winged gulls
0:30:57 > 0:31:00snatch live fish from the surface.
0:31:10 > 0:31:14In spite of these attacks and losses, the herrings swim on
0:31:14 > 0:31:20until they reach the vegetation the females need if they are to lay.
0:31:24 > 0:31:29Each female produces around 20,000 eggs.
0:31:29 > 0:31:32And they're very sticky.
0:31:34 > 0:31:39The males arrive soon after the females have spawned
0:31:39 > 0:31:42and release their sperm in vast milky clouds.
0:31:46 > 0:31:50Soon the excesses of the herrings' sexual spree
0:31:50 > 0:31:54creates a thick white scum on the surface.
0:31:54 > 0:31:56Through the season,
0:31:56 > 0:32:00curds of sperm clog the shores for hundreds of miles,
0:32:00 > 0:32:05from British Columbia in the south all the way to Alaska in the north.
0:32:11 > 0:32:15After a few days, this gigantic spawning ends
0:32:15 > 0:32:19and the herring head back out to deeper waters,
0:32:19 > 0:32:26leaving behind them fertilised eggs plastered on every rock and strand of vegetation.
0:32:34 > 0:32:37They time their spawning
0:32:37 > 0:32:42so that two weeks later, when these eggs start to hatch,
0:32:42 > 0:32:49the annual plankton bloom will have reached its height and the new-born fish fry will have plenty to eat.
0:32:49 > 0:32:54Meanwhile, all these eggs provide food for armies of animals
0:32:54 > 0:32:57both below and above the surface.
0:33:00 > 0:33:05Millions of birds arrive to collect a share of the herrings' bounty.
0:33:05 > 0:33:11Some of it is easily gathered, for millions of eggs have been washed up onto the shore.
0:33:11 > 0:33:17This encapsulated energy is particularly valuable to migrating birds.
0:33:17 > 0:33:23These surf birds are on the way to breeding grounds in the Arctic
0:33:23 > 0:33:25and they had to come down to refuel.
0:33:25 > 0:33:29Stranded herring eggs are just what they need.
0:33:29 > 0:33:34Bonaparte's gulls collect the eggs just below the surface of the water.
0:33:36 > 0:33:38Further out in the bay,
0:33:38 > 0:33:43huge flocks of ducks gather. They're mostly surf scoters -
0:33:43 > 0:33:48diving ducks that can feed off the bottom several metres down.
0:33:50 > 0:33:54There are such huge quantities of eggs,
0:33:54 > 0:34:00that even such a big animal as a bear finds it worthwhile to collect them.
0:34:00 > 0:34:07The spawning of the herring is a crucial event in the lives of many animals all along the coast.
0:34:07 > 0:34:11The whole event coincides with the plankton bloom
0:34:11 > 0:34:15and within three weeks it's all over.
0:34:19 > 0:34:25The migratory birds leave to continue their journey north.
0:34:31 > 0:34:37They will not come back until the herring also return next year.
0:34:40 > 0:34:42As the herring spawning finishes,
0:34:42 > 0:34:47other migrants are starting to arrive, just off shore.
0:34:49 > 0:34:51Grey whales.
0:34:56 > 0:34:59They have followed the sun north
0:34:59 > 0:35:05and they too are seeking the food generated by the bloom of the phytoplankton.
0:35:05 > 0:35:08Krill are feeding off it
0:35:08 > 0:35:11and these whales are feeding on the krill,
0:35:11 > 0:35:16skimming it from the surface with the filter plates of baleen
0:35:16 > 0:35:19that hang from their upper jaws.
0:35:19 > 0:35:24Grey whales make one of the longest migrations of any marine mammal -
0:35:24 > 0:35:26a round trip of 12,000 miles or so,
0:35:26 > 0:35:34from their breeding grounds off Mexico, along the entire coast of North America to the Arctic Ocean.
0:35:36 > 0:35:43They travel close to the coast, with the males and non-breeding females leading the way.
0:35:43 > 0:35:46The last to start are the cows that have just given birth.
0:35:46 > 0:35:52They have to wait until their new-born calves are sufficiently big and strong.
0:35:54 > 0:35:57Their progress is necessarily slow.
0:35:57 > 0:36:04The mothers must stay alongside their young and even a strong calf can only travel at two knots.
0:36:04 > 0:36:07They stick even closer to the shore,
0:36:07 > 0:36:10often within just 200 metres.
0:36:13 > 0:36:16Killer whales.
0:36:16 > 0:36:20They have learnt that grey whales follow traditional routes.
0:36:20 > 0:36:26The killers have no trouble in overtaking a calf and its mother.
0:36:28 > 0:36:32Normally, they continually call to one another
0:36:32 > 0:36:36but now they have fallen silent.
0:36:36 > 0:36:40The mother grey whale and her calf have no idea
0:36:40 > 0:36:44that they've been targeted.
0:37:00 > 0:37:05Catching up with the grey whales is the easy part for the killers.
0:37:05 > 0:37:10They have to be cautious for they are only half the size
0:37:10 > 0:37:15of the grey whale mother. She can inflict real damage with her tail.
0:37:23 > 0:37:27But the killers are not after her. They're after her calf.
0:37:27 > 0:37:32As long as the mother can keep it on the move, it will be safe.
0:37:32 > 0:37:35She does her best to hurry it along.
0:37:35 > 0:37:42At first, the killers avoid getting too close to the mother, but just keep pace alongside.
0:37:42 > 0:37:47They know that the calf, going at this speed, will eventually tire.
0:37:56 > 0:38:01After three hours of being harried in this way,
0:38:01 > 0:38:06the calf is too exhausted to swim further. The mother has to stop.
0:38:06 > 0:38:10This is the moment the killers have been waiting for.
0:38:10 > 0:38:15They try to force themselves between mother and calf.
0:38:33 > 0:38:38A calf separated from its mother will not be able to defend itself.
0:38:38 > 0:38:44Time and again, the black fins of the killers appear
0:38:44 > 0:38:48between the mottled backs of the grey whales.
0:38:53 > 0:38:56At last, the killers succeed,
0:38:56 > 0:39:01and now that they have the calf on its own, they change their tactics.
0:39:01 > 0:39:06They leap right onto the calf and try to push it under.
0:39:14 > 0:39:17They're trying to drown it.
0:39:26 > 0:39:30The calf snatches a desperate breath.
0:39:43 > 0:39:47The mother becomes increasingly agitated.
0:39:47 > 0:39:53Frantically she tries to push her calf back to the surface so that it can breathe.
0:39:55 > 0:40:02But now it's so exhausted that it has to be supported by its mother's body.
0:40:17 > 0:40:20The killers won't give up.
0:40:20 > 0:40:25Like a pack of wolves, they take turns in harassing the whales.
0:40:43 > 0:40:47Now the whole pod is involved.
0:40:57 > 0:41:00One of them takes a bite.
0:41:11 > 0:41:16Soon the sea is reddened with the calf's blood
0:41:16 > 0:41:19and the killers close in for the final act.
0:41:39 > 0:41:43The calf is dead.
0:41:46 > 0:41:51After a six-hour hunt, the killer whales have finally won their prize.
0:41:57 > 0:42:03The mother, bereft, has to continue her migration north on her own.
0:42:07 > 0:42:11She leaves behind the carcass of a calf that she cherished
0:42:11 > 0:42:17for 13 months in her womb, for which she delayed her own journey to find food.
0:42:20 > 0:42:27The pod of 15 killer whales spent over six hours trying to kill this calf, but now, having succeeded,
0:42:27 > 0:42:33they've eaten nothing more than its lower jaw and its tongue.
0:42:38 > 0:42:43Valuable food like this will not go to waste in the ocean.
0:42:43 > 0:42:49Before long, the carcass will sink to the very bottom of this deep sea.
0:42:49 > 0:42:52But even there, its flesh will not be wasted.
0:42:55 > 0:42:59Over a mile down, in the total darkness of the deep ocean,
0:42:59 > 0:43:04the body of another grey whale, a 30-tonne adult.
0:43:04 > 0:43:07It settled here only a few weeks ago.
0:43:07 > 0:43:12Already it has attracted hundreds of hackfish.
0:43:17 > 0:43:22These scavengers, over half a metre long and as thick as your arm,
0:43:22 > 0:43:25are only found in the deep sea.
0:43:25 > 0:43:31They have been attracted by the faint whiff of decay suffusing through the water for miles around.
0:43:34 > 0:43:38With their heads buried in the whale's flesh,
0:43:38 > 0:43:42they breathe through gill openings on their sides.
0:43:42 > 0:43:44They're very primitive creatures,
0:43:44 > 0:43:48not even true fish for they lack jaws.
0:43:48 > 0:43:50They feed not by biting,
0:43:50 > 0:43:55but by rasping off flesh with two rows of horny teeth.
0:43:55 > 0:43:59In just a few hours, a hackfish can eat
0:43:59 > 0:44:04several times its own weight of rotting flesh.
0:44:04 > 0:44:08Next to arrive, a sleeper shark.
0:44:13 > 0:44:18It moves so slowly to conserve energy -
0:44:18 > 0:44:24an important strategy for so large an animal surviving in such a poor habitat.
0:44:29 > 0:44:33Sleeper sharks live over a mile down
0:44:33 > 0:44:36and grow to over seven metres long.
0:44:37 > 0:44:44They can go for months without food, cruising along the bottom, waiting for rare bonanzas,
0:44:44 > 0:44:48such as this one, to arrive from above.
0:44:53 > 0:44:59A whole range of different deep-sea scavengers
0:44:59 > 0:45:06will feast on this carcass for a long time, before all its nutriment has been consumed.
0:45:06 > 0:45:1118 months later, all that is left is a perfect skeleton stripped bare.
0:45:11 > 0:45:17The sun's energy that was captured and turned into living tissue by the phytoplankton
0:45:17 > 0:45:22has been transferred from one link to another in the food chain
0:45:22 > 0:45:27and ended up as far away from the sun as possible on this planet,
0:45:27 > 0:45:30at the bottom of the deep sea.
0:45:30 > 0:45:35But some energy also returns from the deep.
0:45:39 > 0:45:42Millions of opalescent squid
0:45:42 > 0:45:47are on their way to the shallows. They've come up here to mate.
0:45:47 > 0:45:52As the males grab the females, their tentacles flush red.
0:45:53 > 0:45:59For most of the year, these squid live at a depth of about 500 metres.
0:45:59 > 0:46:03They only come together for a few weeks.
0:46:03 > 0:46:10Just one school was estimated to contain animals that weigh around 4,000 tonnes.
0:46:19 > 0:46:23Wave after wave rise from the depths,
0:46:23 > 0:46:30and soon the sea-bed and shallows are strewn with dense patches of egg capsules several metres across.
0:46:34 > 0:46:39As each female adds another capsule to the pile,
0:46:39 > 0:46:43the males fight to fertilise its contents.
0:46:56 > 0:47:01The squid make their huge journey into the shallows
0:47:01 > 0:47:06because their eggs will develop faster in the warmer water here.
0:47:06 > 0:47:13When the young emerge, they will find food more easily than they would in the ocean depths.
0:47:14 > 0:47:17Dawn the next morning,
0:47:17 > 0:47:21and the sea-bed for miles around is covered in egg capsules.
0:47:21 > 0:47:24The squid themselves have all gone.
0:47:24 > 0:47:29Many will have died but some have returned to their home in the deep.
0:47:29 > 0:47:33They will not return to the light of the sun
0:47:33 > 0:47:38until the next time they are driven up by the urge to spawn.
0:47:48 > 0:47:51The enormous size of the oceans,
0:47:51 > 0:47:54and the fact that we know so little about many of the creatures there,
0:47:54 > 0:48:00have presented the film makers behind the Blue Planet series with difficult challenges.
0:48:00 > 0:48:02None more so than the mighty blue whale.
0:48:02 > 0:48:07Despite their massive size, blue whales have hardly ever been filmed.
0:48:07 > 0:48:15And there are good reasons for that. They're among the fastest marine creatures. Faster than many boats.
0:48:15 > 0:48:19Except for a few minutes to breathe, they stay submerged.
0:48:19 > 0:48:24Even when they surface, they're timid and difficult to approach.
0:48:24 > 0:48:28So, to capture blue whales on film is very difficult,
0:48:28 > 0:48:32and it took the team many months and some ingenuity,
0:48:32 > 0:48:35just to get these few rare images.
0:48:41 > 0:48:47Just finding and keeping up with blue whales is a real challenge.
0:48:48 > 0:48:52One of the largest blue whale populations migrates back and forth,
0:48:52 > 0:48:56along the coast of California and Mexico each year.
0:48:56 > 0:49:00For three seasons our camera teams were on standby,
0:49:00 > 0:49:02waiting for passing whales.
0:49:06 > 0:49:11In theory, a microlite is perfect for filming. It's inexpensive,
0:49:11 > 0:49:17it doesn't disturb the whales and it's possible to land and dive in the water.
0:49:18 > 0:49:21But as the migration continued south,
0:49:21 > 0:49:27the whales were soon out of range and another strategy was called for.
0:49:28 > 0:49:32We followed the whales south to Mexico and the Gulf of California.
0:49:32 > 0:49:39On several consecutive years recently, blue whales have been regularly sighted here,
0:49:39 > 0:49:44and the Gulf's calm waters seemed an ideal location for filming.
0:49:44 > 0:49:48However, getting close enough to get film images wasn't going to be easy,
0:49:48 > 0:49:53as experienced cameraman, Rick Rosenthal explains.
0:49:53 > 0:49:58Filming whales around the world, I think that they are like either
0:49:58 > 0:50:02a large aeroplane or... Very streamlined and it's difficult
0:50:02 > 0:50:09to get an image. You're going to have to be very patient, if you're gonna get any behaviour whatsoever.
0:50:09 > 0:50:13'Mary Lee, Mary Lee, Santa Emily, come back?'
0:50:13 > 0:50:16The first problem was to find the whale
0:50:16 > 0:50:19in 30,000 square miles of water.
0:50:19 > 0:50:22OK, yes, you're just turning around the moon for us,
0:50:22 > 0:50:23that's beautiful.
0:50:23 > 0:50:28Using the locations of previous sightings, the team enlisted
0:50:28 > 0:50:35the help of experienced pilot Sandy Lanham, who had a trained eye for spotting blue whales from the air.
0:50:35 > 0:50:40I've been working with scientists for whale research for 10 years.
0:50:40 > 0:50:47We're convinced that the only way to cover a body of water this large, is to use an aeroplane.
0:50:50 > 0:50:52Without directions from the air,
0:50:52 > 0:50:57the team would have found it far harder to catch up with the whales.
0:51:01 > 0:51:06Once the boat had gained visual contact with the blue whale,
0:51:06 > 0:51:10they aimed to get a good shot as it came up for air.
0:51:10 > 0:51:14It was essential not to cause any disturbance.
0:51:14 > 0:51:21The trick was to try and sneak ahead of the whale, predicting where it would surface next.
0:51:24 > 0:51:31You need special permission and a good deal of experience to get this close to whales.
0:51:33 > 0:51:34I dove down
0:51:34 > 0:51:39and could see a white glowing body and turned the camera on
0:51:39 > 0:51:42and the result was, you know,
0:51:42 > 0:51:46a nice image of a huge blue whale
0:51:46 > 0:51:49gliding by me like a giant torpedo.
0:51:49 > 0:51:53Getting close to blue whales in a small boat
0:51:53 > 0:51:55without causing disturbance
0:51:55 > 0:51:57requires a good understanding of their behaviour.
0:51:57 > 0:52:00You NEVER chase whales.
0:52:02 > 0:52:08When they come up for air, their tails leave a smooth patch of water called a footprint.
0:52:08 > 0:52:11These indicate its speed and direction.
0:52:11 > 0:52:16If you're lucky, you can gauge where it will surface in about 15 minutes.
0:52:18 > 0:52:22After countless attempts at this guessing game,
0:52:22 > 0:52:26Rick was finally in the right place at the right time.
0:52:26 > 0:52:31The whale was surfacing near the boat. At one time it came up close,
0:52:31 > 0:52:37we were able to manoeuvre the boat quietly so we were on top of it,
0:52:37 > 0:52:43looking down at it - enormous tail and the entire body stretched out,
0:52:43 > 0:52:45maybe 50-60 feet long.
0:52:50 > 0:52:54It's one of those Captain Ahab, Moby Dick tales
0:52:54 > 0:52:57when you are looking down at the entire whale
0:52:57 > 0:53:01and it was very quiet and not concerned about us.
0:53:01 > 0:53:04It took some breaths and submerged and came up again
0:53:04 > 0:53:07and we manoeuvred again and got in the same position.
0:53:07 > 0:53:11We were looking down at the entire whale underneath us.
0:53:11 > 0:53:13That's an extraordinary feeling for anybody.
0:53:52 > 0:53:55Subtitles by Gabby De Gregorio and Nick Holmes - 2001
0:53:55 > 0:53:58E-mail subtitling@bbc.co.uk.