0:00:30 > 0:00:38These seas, thousands of miles from nearest land, are the most sterile on our planet.
0:00:38 > 0:00:41These are marine deserts.
0:00:42 > 0:00:47But here live the swiftest and most powerful of all ocean hunters.
0:00:53 > 0:00:57Simply finding them is an immense challenge.
0:00:58 > 0:01:01But we are about to follow them
0:01:01 > 0:01:06as they search for their food in this little-known part of the seas...
0:01:07 > 0:01:10..the open ocean.
0:01:25 > 0:01:31Striped marlin - voracious predators that can grow to three metres long.
0:01:34 > 0:01:37They hunt mainly in daylight,
0:01:37 > 0:01:41searching the tropical oceans from close to the surface
0:01:41 > 0:01:44down to depths of 100m or so.
0:01:44 > 0:01:49Normally the fish they feed on are widely dispersed.
0:01:49 > 0:01:54But sometimes their prey gathers in dense shoals, like these sardines.
0:03:23 > 0:03:27This feast may last for over an hour.
0:03:33 > 0:03:37Time enough for other hunters to reach the scene.
0:03:38 > 0:03:42Juvenile tuna join in the feeding frenzy.
0:03:59 > 0:04:03The noise attracts a giant - a sei whale.
0:04:10 > 0:04:13It's 14m long and 20 tonnes in weight
0:04:13 > 0:04:16and has an appetite to match.
0:05:19 > 0:05:26Soon the only sign that the sardines ever existed are scales sinking down into the abyss.
0:05:29 > 0:05:31Such feasts don't last long.
0:05:31 > 0:05:37In a few days, waters that swarmed with food will have been cleaned out.
0:05:37 > 0:05:39The hunters must move elsewhere
0:05:39 > 0:05:44and once again start their search of the seemingly featureless open ocean.
0:05:46 > 0:05:49A manta ray - immense -
0:05:49 > 0:05:54five metres across from the tip of one wing-like fin to the other.
0:05:54 > 0:05:59It's travelling economy, wasting as little energy as possible
0:05:59 > 0:06:03as it glides through the waters of the tropics.
0:06:07 > 0:06:11The remora fish that accompany it travel more economically still...
0:06:11 > 0:06:14by hitching a lift.
0:06:14 > 0:06:17Their host is searching for food - plankton,
0:06:17 > 0:06:22the minute fish and invertebrates that float near the surface.
0:06:22 > 0:06:24It needs lots of them
0:06:24 > 0:06:29and may cruise for days before it finds a good feeding ground.
0:06:29 > 0:06:35Dusk on a pacific island, 3,000 miles from the nearest continent.
0:06:35 > 0:06:39Here, surgeon fish have assembled to spawn.
0:06:40 > 0:06:43As they perform their nuptial dances
0:06:43 > 0:06:47they discharge clouds of eggs and sperm into the water.
0:06:47 > 0:06:51The manta must have known this would happen,
0:06:51 > 0:06:54for it arrived at exactly this moment
0:06:54 > 0:06:57and it's not the only one to do so.
0:06:57 > 0:07:00Others are here too.
0:07:01 > 0:07:06Now all they need to do is to sweep the water into their mouths
0:07:06 > 0:07:08and sieve out the eggs.
0:07:34 > 0:07:38Within an hour, the whole event will be over.
0:07:38 > 0:07:43Any eggs left will be so dispersed that they're not worth collecting.
0:07:50 > 0:07:55But other perils await them as they join the eggs, larvae and tiny fish
0:07:55 > 0:08:00that drift through the surface waters of the open ocean.
0:08:01 > 0:08:04These are the eggs of yellowfin tuna.
0:08:04 > 0:08:10If the hatchlings survive, it will take them two years to become adults.
0:08:13 > 0:08:17In three years, they could be nearly two metres long
0:08:17 > 0:08:20and weigh 200 kilograms.
0:08:22 > 0:08:27Perhaps only one in a million will live as long as that.
0:08:27 > 0:08:31They and the other animals and microscopic plants of the plankton
0:08:31 > 0:08:36constitute the basis of all life out on the open ocean.
0:08:40 > 0:08:44A storm petrel dancing on the water, but this is no amiable waltz -
0:08:44 > 0:08:47it's a hunt.
0:08:49 > 0:08:55They hover, facing into the wind, picking out morsels near the surface,
0:08:55 > 0:08:57including eggs.
0:09:04 > 0:09:10Only a tiny percentage of the eggs will survive long enough to hatch.
0:09:10 > 0:09:14These newly emerged tuna are only three millimetres long.
0:09:14 > 0:09:19Although they can swim, they're still very vulnerable.
0:09:19 > 0:09:26It will be many weeks before they swim strongly enough to make any headway in the ocean.
0:09:35 > 0:09:37After the sun goes down,
0:09:37 > 0:09:42other predators rise from the depths to attack the floating multitudes.
0:09:59 > 0:10:02Darkness shrouds the arrival
0:10:02 > 0:10:06of battalions of dangerous, drifting predators.
0:10:19 > 0:10:23These shimmering comb jellies - sea gooseberries -
0:10:23 > 0:10:26trap their prey with sticky net-like webs.
0:10:26 > 0:10:31One ill-timed fin-stroke could bring certain death to a hatchling fish.
0:10:31 > 0:10:34There are many kinds of these comb jellies -
0:10:34 > 0:10:37all of them very effective hunters.
0:10:49 > 0:10:55By dawn, most nocturnal feeders will have returned to the depths.
0:10:59 > 0:11:02The surviving hatchlings, however,
0:11:02 > 0:11:06have already started on their travels.
0:11:14 > 0:11:18Vast current systems, like immense rivers,
0:11:18 > 0:11:21carry them around the ocean basins.
0:11:22 > 0:11:26The boundaries between these masses of moving water
0:11:26 > 0:11:28form invisible barriers
0:11:28 > 0:11:33that can trap both plankton and nutrients carried up from the depths.
0:11:37 > 0:11:40So, parts of the ocean become rich with food
0:11:40 > 0:11:43for days or even weeks at a time.
0:11:43 > 0:11:48This attracts vast schools of plankton-feeding fish,
0:11:48 > 0:11:51like these sardine.
0:11:55 > 0:12:00They take in water through the mouth and expel it through their gills,
0:12:00 > 0:12:06sieving out the plankton which is then funnelled down their throats.
0:12:10 > 0:12:15The immense schools travel along the boundaries of the currents,
0:12:15 > 0:12:17seeking where plankton is thickest.
0:12:20 > 0:12:25As the position of the current boundaries changes constantly,
0:12:25 > 0:12:30so does both the supply of plankton and the numbers of fish.
0:12:42 > 0:12:46A small pod of Pacific spotted dolphin -
0:12:46 > 0:12:4920 miles from the coast of Panama.
0:12:49 > 0:12:55Like all predators, they seek parts of the ocean where their food is thickest.
0:13:03 > 0:13:06They cover up to 100 miles in a day.
0:13:06 > 0:13:08And while they travel,
0:13:08 > 0:13:10they play.
0:13:25 > 0:13:30They have detected schooling fish from hundreds of metres away,
0:13:30 > 0:13:34and start to track down the shoals using sonar,
0:13:34 > 0:13:37leaving their toys behind them.
0:13:40 > 0:13:43For the hunted, there are few places to hide.
0:13:49 > 0:13:51Schooling mackerel.
0:13:51 > 0:13:56They have already sensed the sonar beams of approaching dolphin.
0:13:59 > 0:14:03Their only defence is to gather into a ball.
0:14:03 > 0:14:07Individuals that stayed out of the shoal would be quickly picked off.
0:14:07 > 0:14:12Within it, there is at least some chance of survival.
0:14:19 > 0:14:22The noise alerts another predator -
0:14:22 > 0:14:26a sailfish, one of the fastest fish in the ocean.
0:14:30 > 0:14:33It has detected rapid vibrations in the water
0:14:33 > 0:14:36and is searching for the cause.
0:14:48 > 0:14:52Sailfish rely on eyesight for their final approach,
0:14:52 > 0:14:56so they hunt mainly in daylight.
0:14:58 > 0:15:05When sailfish become excited they change colour, lighting up with bright blue stripes.
0:15:05 > 0:15:10Since mackerel eyes are especially sensitive to blue and ultraviolet,
0:15:10 > 0:15:14these colours confuse them, making them easier to catch.
0:15:52 > 0:15:56Far below, a blue shark returns from a squid-hunting trip
0:15:56 > 0:15:59in the cold darkness 300 metres down.
0:15:59 > 0:16:04It's heading for the surface to reheat in the warmer water.
0:16:07 > 0:16:14As it ascends, it detects the smell of oils and proteins shed into the water by the panicked mackerel.
0:16:17 > 0:16:23The trail leads the shark and the pilot fish towards an easy meal.
0:16:28 > 0:16:33Scraps and casualties float in the wake of the passing mackerel school.
0:16:36 > 0:16:39Throughout the ocean,
0:16:39 > 0:16:43predators and prey are locked in a deadly contest of hide and seek,
0:16:43 > 0:16:47played out over immense distances.
0:16:47 > 0:16:50To survive they must travel.
0:16:55 > 0:17:00The huge four metre long blue-fin tuna has special blood vessels
0:17:00 > 0:17:07that enable it to keep its body temperature significantly warmer than the surrounding water.
0:17:09 > 0:17:14They can survive in much colder conditions than any other tuna,
0:17:14 > 0:17:17and they travel thousands of miles
0:17:17 > 0:17:21away from their spawning grounds in the tropics
0:17:21 > 0:17:25to hunt in cold seas where the food supply is richest.
0:17:25 > 0:17:30Ocean travellers come in many guises, and few are stranger than this...
0:17:34 > 0:17:39..A crab that spends much of its life afloat.
0:17:45 > 0:17:51It is a worrying passer-by for booby birds with delicate toes.
0:18:08 > 0:18:13Many floaters are little more than jelly, enclosed in membranes,
0:18:13 > 0:18:17but they may drift for vast distances.
0:18:17 > 0:18:20And turtles, like these olive ridleys,
0:18:20 > 0:18:24migrate thousands of miles every year.
0:18:25 > 0:18:30The ocean is full of such wanderers, riding the currents,
0:18:30 > 0:18:33and doing their best to avoid enemies
0:18:33 > 0:18:38while they search for food and a safe place to breed,
0:18:38 > 0:18:44which is what these rays are doing, forming a two-mile-high club,
0:18:44 > 0:18:47gathering together for courtship on the wing,
0:18:47 > 0:18:49far above the ocean floor.
0:19:13 > 0:19:16More nomads - flying fish.
0:19:16 > 0:19:20They seem to be on every large predator's menu,
0:19:20 > 0:19:25so their whole life is spent on the run in the open ocean.
0:19:25 > 0:19:33They don't scatter their eggs but lay them on pieces of flotsam like this palm frond.
0:19:34 > 0:19:36If the quality of water is right,
0:19:36 > 0:19:40they will attach their eggs to the frond,
0:19:40 > 0:19:45which will then serve as a kind of life-raft for their offspring.
0:19:45 > 0:19:49But it's not only flying fish that seek nurseries.
0:19:53 > 0:20:00Any piece of floating debris can serve as a shelter under which baby fish can hide.
0:20:02 > 0:20:09The only drawback is that predators like this wahoo always check up on who's hanging about in the shadows.
0:20:09 > 0:20:14The wahoo may trail the flotsam for weeks.
0:20:18 > 0:20:22Few bits of flotsam are without their quota of lodgers -
0:20:22 > 0:20:26even man-made junk attracts them.
0:20:26 > 0:20:33And some, like this oceanic trigger fish, defend their squatters' rights with vigour.
0:20:38 > 0:20:43The triggers, in fact, tend to claim all the prime residences.
0:20:48 > 0:20:52Out here, even discarded netting can provide valuable shelter,
0:20:52 > 0:20:55so, in a bizarre twist,
0:20:55 > 0:20:59a wrecked trawl net like this can end up as a sanctuary for fish
0:20:59 > 0:21:02until such time as it finally sinks.
0:21:05 > 0:21:12Indeed, a single large piece of flotsam can be the reason why several square miles of open ocean,
0:21:12 > 0:21:20instead of being empty, will support a fish population of hundreds of tonnes.
0:21:20 > 0:21:24This huge clump of seaweed is doing exactly that.
0:21:24 > 0:21:31It is a giant kelp plant, ripped from the rocks off the coast of California.
0:21:33 > 0:21:38Now, it's floating above thousands of metres of water,
0:21:38 > 0:21:41held up by its gas-filled floats.
0:21:44 > 0:21:49Clouds of young rockfish are growing up in the safety of its shadow.
0:21:53 > 0:21:58Giants also seek out this algal flotsam.
0:21:59 > 0:22:02This is a sunfish.
0:22:02 > 0:22:06It can measure as much as four metres from fin-tip to fin-tip.
0:22:06 > 0:22:12Surprisingly, it has the record as the heaviest bony fish in the sea.
0:22:13 > 0:22:18Sunfish spend much of their time at depth where they feed on jellyfish,
0:22:18 > 0:22:20but it is cold and dark down there,
0:22:20 > 0:22:27so from time to time they seek a little rest and recuperation, and warm up near the surface.
0:22:27 > 0:22:31They too are looking for floating kelp plants.
0:22:31 > 0:22:39Not for shelter, but because here they can find a particular kind of fish that only lives in such places.
0:22:40 > 0:22:43Half-moon fish.
0:22:43 > 0:22:49The sunfish form up in an orderly queue.
0:22:49 > 0:22:54They have a problem. Their skin is covered in parasites.
0:23:03 > 0:23:06The hungry half-moons will help.
0:23:07 > 0:23:14The sunfish turn their heads towards the surface as a clear invitation to their personal hygienists.
0:23:14 > 0:23:19The half-moons nip off - and eat - every parasite they can find.
0:23:34 > 0:23:37If the half-moons don't do the job,
0:23:37 > 0:23:42there is another rather drastic treatment available here.
0:23:42 > 0:23:46Gulls rest on the floating kelp.
0:23:46 > 0:23:51And if the sunfish send the right signals,
0:23:51 > 0:23:54the gulls will investigate.
0:23:58 > 0:24:03Their beaks can dig out the most stubborn parasites.
0:24:23 > 0:24:29Even the very best of health clinics can only trade on a temporary basis.
0:24:29 > 0:24:31The seaweed rafts rot
0:24:31 > 0:24:34and eventually lose their buoyancy.
0:24:42 > 0:24:47Then their lodgers will have to find a new home.
0:24:47 > 0:24:52If they can't, they will be eaten and die and sink down into the abyss.
0:25:01 > 0:25:06But the open ocean is not entirely devoid of permanent shelter.
0:25:12 > 0:25:18A volcano is erupting from the seafloor and it's still growing.
0:25:50 > 0:25:56It has formed an island some 70 miles from the coast of New Zealand.
0:25:58 > 0:26:04Some juvenile reef fish have already arrived, carried by a lucky current.
0:26:04 > 0:26:10Now they're growing up in the reeds growing around the island's fringes.
0:26:16 > 0:26:21More plankton and juvenile fish are being swept towards the island.
0:26:21 > 0:26:25But now, there's a welcoming committee.
0:26:27 > 0:26:32Schools of trevally fish and blue maomao patrol the surface water.
0:26:40 > 0:26:43All are in search of a meal.
0:26:47 > 0:26:50These one-kilo fish
0:26:50 > 0:26:54snap up every morsel of plankton they find.
0:27:02 > 0:27:08At times, the currents sweeping in from the open ocean bring with them
0:27:08 > 0:27:14all kinds of small creatures, like these mysid shrimps.
0:27:46 > 0:27:50Very little that is edible is left after such feasts.
0:27:50 > 0:27:55Islands are far from being safe havens for plankton.
0:27:59 > 0:28:04The Pacific Ocean, however, is peppered with over 23,000 islands,
0:28:04 > 0:28:09as well as countless other submerged mountains -
0:28:09 > 0:28:13sea mounts whose summits do not break the surface.
0:28:13 > 0:28:18Juvenile fish, for their first few months,
0:28:18 > 0:28:20would do well to avoid such places.
0:28:20 > 0:28:24These yellowfin tuna are six months old.
0:28:24 > 0:28:28They are 40cm long - big enough to eat fry.
0:28:28 > 0:28:33So sea mounts for them, are promising feeding grounds
0:28:33 > 0:28:36where they may hunt for several months.
0:28:42 > 0:28:44The base of a sea mount.
0:28:44 > 0:28:50As currents sweep towards it, they are deflected up its towering walls.
0:28:55 > 0:28:58The water from the depths
0:28:58 > 0:29:02carries plankton and nutrients to the surface.
0:29:08 > 0:29:14Reef fish take up residence, feeding where the plankton's most dense.
0:29:24 > 0:29:29Where the cold water mixes with warmer water at the surface,
0:29:29 > 0:29:35there is a strange shimmering effect, a sign the currents are strong.
0:29:52 > 0:29:56These currents attract more than just coastal fish.
0:29:58 > 0:30:02Giants come here from the open ocean.
0:30:02 > 0:30:07Hammerhead sharks - and in great numbers.
0:30:13 > 0:30:16During the day, they circle the sea mount,
0:30:16 > 0:30:19looking for small fish,
0:30:19 > 0:30:23but not in order to eat them.
0:30:23 > 0:30:27They, like the sunfish, are looking for cleaners
0:30:27 > 0:30:30to rid them of their parasites.
0:30:34 > 0:30:40White-tipped reef sharks gather here, too. They DO eat reef fish.
0:30:48 > 0:30:54They hunt at night when the reef fish are sleepy and easier to catch.
0:30:54 > 0:30:59Far better to rest by day and allow the cleaners to do their work.
0:31:05 > 0:31:10Even swarms of breeding trigger aren't a serious temptation.
0:31:17 > 0:31:20They spend much of their time
0:31:20 > 0:31:26in open water, but they've come to the sea mount to spawn.
0:31:33 > 0:31:36Trigger eggs are good food.
0:31:36 > 0:31:42The plankton feeders gather what they can before they're swept away.
0:31:42 > 0:31:46This community is only here due to the nutrients
0:31:46 > 0:31:49the sea mount deflected into the water.
0:31:51 > 0:31:53But ocean-going hunters
0:31:53 > 0:31:56are never far away.
0:31:56 > 0:31:59Silky sharks pick off injured fish
0:31:59 > 0:32:04and check over the residents around the sea mount.
0:32:07 > 0:32:10At some times of the year,
0:32:10 > 0:32:15seasonal changes make the currents especially rich in nutrients,
0:32:15 > 0:32:18and the ocean becomes a soup of plankton.
0:32:26 > 0:32:31At such times, hunters gather in astonishing numbers.
0:32:40 > 0:32:43Bonito, smaller relatives of the tuna,
0:32:43 > 0:32:49are searching for smaller plankton feeders attracted by the bloom.
0:32:51 > 0:32:55So are these jacks. Their prey is nearby.
0:33:00 > 0:33:05A school of anchoveta has strayed to the surface
0:33:05 > 0:33:09even though it's broad daylight and hunters are on the prowl.
0:33:13 > 0:33:18They can already feel the vibrations of the approaching predators.
0:33:18 > 0:33:23Swimming at speed, they form a ball and wait for whatever comes.
0:33:32 > 0:33:35They've been rumbled.
0:33:49 > 0:33:54At first, the scale of the bait ball seems to daunt the predator.
0:34:08 > 0:34:13But now, the bonito arrive and launch the first attack.
0:34:18 > 0:34:20Still the bait ball holds together.
0:34:28 > 0:34:31The young yellowfin tuna move in.
0:34:31 > 0:34:34The speed of the attack is so great
0:34:34 > 0:34:38that groups of anchoveta are splintered from the main fish ball.
0:35:40 > 0:35:45Before long the currents will shift and the ocean will become once more
0:35:45 > 0:35:48a blue tropical desert - plankton-free -
0:35:48 > 0:35:51and the hunters will have to move on.
0:35:59 > 0:36:03Spinner dolphins - still searching for food.
0:36:10 > 0:36:13Their twisting leaps are social displays.
0:36:13 > 0:36:16Since the hunting has been good,
0:36:16 > 0:36:21many hundred have gathered together in this exuberant super-pod.
0:37:24 > 0:37:29But now the spinners are starting to hunt once more.
0:37:33 > 0:37:36Their skill in tracking food is not a secret.
0:37:36 > 0:37:41Yellowfin tuna must be aware of it for they regularly follow them.
0:37:41 > 0:37:47But only adult tuna in their third year of life have sufficient stamina
0:37:47 > 0:37:51to keep up with the fast-moving spinners.
0:38:05 > 0:38:08This is another kind, common dolphin.
0:38:08 > 0:38:12They too are on the move.
0:38:13 > 0:38:17As they travel, ever inquisitive, they pay a call
0:38:17 > 0:38:21on one of their larger relations - a pilot whale.
0:38:21 > 0:38:24The whale is not hunting.
0:38:24 > 0:38:29It's on its way to its breeding grounds in the Mediterranean.
0:38:29 > 0:38:36Pilot whales hunt in small family groups, but in midsummer they head for traditional socialising grounds,
0:38:36 > 0:38:41where they will assemble in super-herds, several hundred strong.
0:38:41 > 0:38:44Already, two families have joined together.
0:38:44 > 0:38:48Males are starting to compete for females.
0:39:18 > 0:39:25As the weeks pass by, these group rubbing sessions will become more overtly sexual.
0:39:25 > 0:39:29But now, it's just flirting in the sun.
0:39:49 > 0:39:52Timing in the ocean can be crucial.
0:39:52 > 0:39:57In summer, the northern Atlantic waters are beginning to warm.
0:39:57 > 0:40:00The hunting is good here and by June,
0:40:00 > 0:40:05predators from southern waters are heading towards the Azores.
0:40:12 > 0:40:15These are more common dolphin.
0:40:21 > 0:40:24Like most oceanic dolphins,
0:40:24 > 0:40:29they too often travel in huge herds containing many different families.
0:40:34 > 0:40:39There is seldom enough prey in any one place to feed such numbers.
0:40:43 > 0:40:46So, small groups leave the super-pod
0:40:46 > 0:40:49and set off on hunting expeditions.
0:40:51 > 0:40:55This group will be away from the main herd for several hours.
0:41:04 > 0:41:09By midday, they're nearing the Islands of the Azores -
0:41:09 > 0:41:12900 miles west of the Portuguese coast.
0:41:15 > 0:41:20Other hunters are already here - corys shearwaters.
0:41:26 > 0:41:32500,000 of these birds breed on the Azores every year and scour the ocean for food.
0:41:32 > 0:41:40Right now there is insufficient wind to support gliding flight and since flapping is a waste of energy,
0:41:40 > 0:41:44they sit out the calm, clustered in rafts and riding the swells.
0:41:59 > 0:42:04By mid-afternoon the dolphin are starting to hunt in earnest.
0:42:10 > 0:42:13As the sea breeze picks up,
0:42:13 > 0:42:17the shearwaters take to the air once more.
0:42:25 > 0:42:27Out to sea,
0:42:27 > 0:42:30the dolphin have found prey.
0:42:34 > 0:42:39They are driving a shoal of small mackerel up towards the surface.
0:42:46 > 0:42:52The shearwaters crowd the skies, following the dolphins' every turn.
0:42:59 > 0:43:02The mackerel are still some metres down.
0:43:04 > 0:43:08When the baitfish come sufficiently close,
0:43:08 > 0:43:11the airborne division makes its move.
0:43:11 > 0:43:18Far from being mere bystanders, the shearwaters can now become predators themselves.
0:43:21 > 0:43:26Incredibly, they can dive down to depths of several metres.
0:43:26 > 0:43:31The hunting dolphin prevent the mackerel from escaping downwards
0:43:31 > 0:43:34and both predators gorge themselves.
0:43:59 > 0:44:02Soon the diving birds
0:44:02 > 0:44:07outnumber the dolphin and even drive them away from their meal.
0:44:07 > 0:44:12But another squadron of predators arrives to replace the dolphin -
0:44:12 > 0:44:15adult yellowfin tuna.
0:44:16 > 0:44:19These are giants - two metres long.
0:44:25 > 0:44:29They are heading directly for the bait ball.
0:45:25 > 0:45:29Despite the arrival of the giant fish,
0:45:29 > 0:45:34the shearwater continue to press home their attack, unfazed.
0:45:50 > 0:45:56Eventually, the tuna move on. The shearwaters battle among themselves.
0:46:04 > 0:46:08As long as predatory fish or dolphin remain at the scene,
0:46:08 > 0:46:10the mackerel can't escape.
0:46:10 > 0:46:14But when the skipjack tuna start to move away,
0:46:14 > 0:46:19the bait ball begins to sink into the depths towards safety.
0:46:28 > 0:46:31The shearwaters follow it down
0:46:31 > 0:46:36to the limit of their breath-holding ability, maybe as deep as 15 metres.
0:46:43 > 0:46:48At last, even they are forced to leave their quarry.
0:46:56 > 0:47:00However good or bad this summer's feeding may be,
0:47:00 > 0:47:08in three months winter will be on its way and the temperature of these waters will drop by a few degrees.
0:47:08 > 0:47:12Then, the ocean hunters will abandon the Azores once more.
0:47:12 > 0:47:17As ever, they will move on, seeking another feeding opportunity -
0:47:17 > 0:47:23the next pulse of life in the distant reaches of the open ocean.
0:47:26 > 0:47:29The open ocean is so vast
0:47:29 > 0:47:34that finding anything in this big blue arena is difficult.
0:47:34 > 0:47:37The Blue Planet team invested over 400 days
0:47:37 > 0:47:41on often unsuccessful trips far out to sea,
0:47:41 > 0:47:45and many days were spent looking and waiting.
0:47:45 > 0:47:47Somewhere in this marine desert
0:47:47 > 0:47:52there were amazing animal dramas played out every day.
0:47:52 > 0:47:56Their reward for finding them was to gain a unique insight
0:47:56 > 0:48:00into some of the least-known animals on our planet.
0:48:11 > 0:48:17The day is young - a little after four in the afternoon.
0:48:17 > 0:48:21Historically this is the time that the predator hour starts up.
0:48:21 > 0:48:25'The open ocean, which I call liquid space,
0:48:25 > 0:48:28'is the most difficult area to work in the ocean.
0:48:28 > 0:48:34'People wonder how we film out there. Do you throw the camera in?
0:48:34 > 0:48:39'There's hours of boredom. Nothing goes on.
0:48:39 > 0:48:44'A little voice says, "Stay focused."'
0:48:44 > 0:48:47Patience, patience. Looking, looking.
0:48:55 > 0:48:59After weeks of searching,
0:48:59 > 0:49:03the crew finally caught up with a pod of spinner dolphins.
0:49:03 > 0:49:08I keep my eye on areas of the herd where there's an open spot,
0:49:08 > 0:49:12an area that allows spinning. It happens in a minute or two.
0:49:12 > 0:49:15An adrenaline rush and it's over.
0:49:15 > 0:49:20The action is over almost as quickly as it started.
0:49:20 > 0:49:25Then there is nothing to do but keep scouring mile after mile of empty ocean.
0:49:27 > 0:49:33Our crew spent six weeks off Panama searching for tuna and the smaller fish they attack.
0:49:33 > 0:49:39They look for signs of feeding activity at the surface, and when it finally happens,
0:49:39 > 0:49:43the crew must work hard to make the best of the action.
0:49:45 > 0:49:51The prey fish crowd together in a bait ball - there is some protection for an individual in numbers.
0:49:53 > 0:49:58Diving into this frenzy requires a great deal of care.
0:49:58 > 0:50:04One thing when filming bait balls, particularly sardines, that you must be careful of is that
0:50:04 > 0:50:12when you're in the water, sardines will often swarm you to try to seek shelter, to hide from predators.
0:50:12 > 0:50:19And through the camera viewfinder, I could see a piece of bait ball break off and come directly at me.
0:50:19 > 0:50:25Fish were on me; the tuna knew it and were rocketing out of the deep.
0:50:25 > 0:50:30Just for a second, I felt like I'd become part of this feeding frenzy,
0:50:30 > 0:50:33like a big sardine in the middle of a bait ball.
0:50:33 > 0:50:37Finally, when it was all over, all that remained
0:50:37 > 0:50:42was just a shower of fish scales slowly sinking into the deep...
0:50:42 > 0:50:44and me.
0:50:44 > 0:50:49Clues to what's happening underwater are circling birds or moving dolphins
0:50:49 > 0:50:52and Taco the sea-dog had spotted some action.
0:50:52 > 0:50:56I should probably try to get in the water here, huh?
0:50:56 > 0:51:02Why don't you just go on by 'em here and we'll slow up, mellow out...
0:51:02 > 0:51:04Get my grey!
0:51:08 > 0:51:11Hundred yards!
0:51:11 > 0:51:15'You're pumped up, you're hunting, and you're hunting with a camera.
0:51:15 > 0:51:21'You have to filter out an awful lot of what's going on around you.
0:51:21 > 0:51:25'Get specific, get in there where the action is.'
0:51:28 > 0:51:36'A lot of our success is based on intuition, so it's called sixth sense - that we follow down trails
0:51:36 > 0:51:43'and sometimes they lead us nowhere, and other times you just have that feeling something's going to happen.
0:51:43 > 0:51:50'I can't put a handle on when you get that feeling, but you know it's special and you better be ready.'
0:51:52 > 0:51:54Good stuff, lots of marlin.
0:51:54 > 0:51:57OK, Gary, let's do it again please.
0:51:59 > 0:52:06Getting these shots of marlin was the culmination of over four years of persistence and many failed trips.
0:52:06 > 0:52:13It was a thrilling but somewhat dangerous moment - these underwater javelins can swim at 70mph.
0:52:18 > 0:52:22All the marlin activity attracted other predators...
0:52:22 > 0:52:23tuna...
0:52:25 > 0:52:29..and then an extraordinary bonus - a sei whale.
0:52:34 > 0:52:41'Doug and Dave went into the water. I stayed on deck to try to link the shot
0:52:41 > 0:52:45'between the underwater world and the surface world.
0:52:49 > 0:52:54'Here we were at the right time, we followed groups of whales feeding
0:52:54 > 0:52:57'and we put cameramen in the water
0:52:57 > 0:53:03'at the same time we were filming from the surface, so we could get proper coverage.'
0:53:36 > 0:53:39Subtitles by BBC Subtitling, 2001
0:53:39 > 0:53:41E-mail us at subtitling@bbc.co.uk