Tidal Seas The Blue Planet

There is a force sufficiently powerful to move the oceans of this world.

It is a force not of this earth.

The moon is big enough to generate gravity -

and with sufficient force to pull on the earth 230,000 miles away.

As the moon orbits the earth,

its gravity sweeps across the face of our planet.

Its power drags a great bulge of oceanic water in its wake.

The rising tide.

The River Amazon in Brazil.

On some special days,

the gravitational forces of the moon and the sun pull together to extraordinary effect.

A growing tidal wave from the ocean is being forced 200 miles inland.

This is a "tidal bore".

Fortunately, tidal bores are rare.

but the moon does create strong tides

out in the world's oceans on every day of the year.

The Bay of Fundy in Nova Scotia.

The tides here are the largest in the world,

and have a profound effect on marine life,

creating a rich feeding ground...

..a feast that attracts some of the largest diners on the planet.

Humpback whales.

But they are not the biggest threat to the herring.

These are finback whales, at 70 tons, the second-largest animal on earth -

but so beautifully streamlined

that it is the fastest of the great whales.

This combination of speed and immense size

makes the finback a voracious hunter of schooling fish.

The Bay of Fundy can attract so many fish

that during the summer as many as 500 of these magnificent whales hunt here every day.

The feeding is best where the tides run strongly.

So the whales move further into the bay,

following tidal rips and searching for fish.

Their movements are closely watched by flocks of Cory shearwaters.

As the whales dive down towards the fish, more and more birds gather, anxious to pick up scraps.

The flowing tide may provide a feast,

but before long, it will turn.

In just six hours, 100 BILLION tons of water

will flow out of the bay -

the sea level falling by as much as 15 metres

and exposing vast tracts of mud and sand...

..at first sight, a barren place entirely devoid of life.

In fact, the damp sand is packed with microscopic life, the meiofauna,

feeding in a sandy underworld

quite unaffected by the departure of the sea.

But life is not all roses in this miniature world.

A sand bubbler crab in northern Australia. It hunts meiofauna.

Just a centimetre across, the sand bubbler works at breakneck speed,

passing sand grains into its mouth, filtering out all the meiofauna and kicking aside the waste.

The crab will clean every grain of sand within a metre of its burrow.

Endless practice for the best back-heel in the natural world.

The crabs work fast because they can only sieve when the sand is damp.

They work the entire surface of the beach

within just a couple of hours of the tide retreating.

Then they simply return to their burrows and await the next tide.

Underwater, the falling tide is the cue for some bizarre activity.

These slow-moving clams use their muscular feet

to bury themselves under the sand.

If they fail to get under cover, the tide will leave them exposed to the air, and they will perish.

But once underground, they can wait deep down in the sand,

safe beneath the beach.

And not a moment too soon.

June in south-east Alaska,

and in just four hours, a vast, sandy beach is exposed by the falling tide.

The bears are hungry.

At this time of year, the pickings on land are few and far between.

But any food here has long since buried itself deep under the sand.

To a hungry adult bear, that is no barrier.

They smell the clams through the sand and simply dig them out.

For such large animals,

they show extraordinary dexterity at opening the unfortunate shellfish

Cubs try their luck, too,

none too successfully.

For the adults, the shellfish feast lasts

as long as the tide remains out.

Table Mountain in South Africa.

Every day, the retreating waves leave flotsam somewhere on the beach.

This creature is scenting the currents

for the odour of rotting fish.

The tide carries the scent far into the surf zone.

Responding to the smell, snails emerge from the sand.

This is a race against the tide. The snails need to find their meal

before the tide leaves it beyond their reach.

But snails are slow, and the tides fall rapidly.

These are no ordinary snails.

They can surf.

They ride the waves up the beach. But all too soon, the tide leaves the fish beyond the surf zone.

Without the sea, there's a danger that the snails will lose the scent,

but as long as the sand remains damp,

they can still follow a faint trail to the food.

Once there, they tuck in

with macabre relish.

Good things come to those who wait.

Before long, the heat of the sun forces them to retreat into the sand

to await the return of the next tide.

In the water, incoming tides can create a strong current,

and flounders are experts at hitching a tidal lift.

They're shaped rather like a kite,

a perfect design for gliding on the tide.

In Newfoundland, on the east coast of Canada, large numbers of flounder ride the currents into the shallows.

They've come to hunt invertebrates that will emerge now that the water is back.

The pickings in the shallows can be very good.

The activity has not gone unnoticed.

But ospreys can't dive deeply.

As long as the water is over a metre deep, the flounder will be safe.

Going too far inshore can be a risky business.

This fish buries itself completely in the sand at any sign of danger.

But, when the tide floods in again, as long as the coast is clear,

these sand lancet will re-emerge.

After a wait of six hours under the sand, they're desperate for food

and, unlike flounder, they head out to sea.

They're looking for shallow, open water where the tidal currents will concentrate their food -

plankton.

In their untold thousands, they stream towards the best feeding grounds

where they simply pick up tiny, planktonic creatures from the water.

But if they swim too far offshore in search of food,

they risk running into large predators

that live out in deeper water.

Dogfish - small sharks.

The sand lancet HAVE strayed out of their safe depth.

The effect of the turning tide can be totally different on a rocky shore.

Here, on the coast of Vancouver Island in Canada,

the sun bakes the exposed rock.

It's virtually impossible to dig underground when the sea retreats,

so these mussels and barnacles are fully exposed to the sun's heat - literally cooking in their shells.

And the seaweed simply dry to a crisp.

It can be a wait of many hours before the water returns.

Throughout each month, the size and the strength of the tide changes.

The biggest tides of all happen

when the gravities of the sun and moon pull in unison.

That's immediately after the new moon

and again after the full moon.

These are the "spring tides". They reveal vast tracts of sea-bed that would normally be covered.

For these raccoons, it's a chance to look for a seafood feast.

A mother ventures forth with her kits.

With the spring tide, they've come further down the beach than smaller tides would normally allow.

Searching with their extraordinarily sensitive paws,

they look for suitable prey.

With the extreme low tide, they could find something special.

What could be better than a red rock crab?

That is, if it weren't for the risk of a painful pinch.

With large crabs, there's no substitute for experience.

The mother makes an expert's catch.

But the kits learn fast.

And for those that don't, begging is always worth a try.

All too soon, the returning tide will cover the raccoons' table.

For the invertebrates, it's a welcome relief, but in rough weather they are exposed to the worst of the waves.

Even when there are no waves,

the incoming tide can create considerable forces underwater.

The gaps between these small islands on the east coast of Vancouver Island channel the tidal flow.

As the tide keeps rising, gradually the water flows faster,

and soon, these giant, 30m-long bull kelp plants bend to the current.

They're sufficiently flexible to cope without too much damage,

but there are some spots where the currents are especially powerful.

This is the Naquatto Rapids.

At the turn of the tide, water from almost 700 miles of coastal fjords

will have to empty through a gap of less than half a mile wide.

Within a few minutes, the current is picking up speed,

until water roars by at over 17mph.

Tidal currents are not always a damaging force.

Here, in the Poor Knight Islands of New Zealand,

weak tides run through rock arches.

It's ideal for resting stingrays.

These rays congregate here in huge numbers every March.

They've come to breed.

The arches funnel the current,

which the rays can ride with the minimum of effort, so saving energy.

Nearby, out in open water, a school of two-spotted demoiselle fish feed on plankton,

and the current is perfect for sweeping their food past them.

Once the current starts to weaken,

there is insufficient food to warrant the risks of swimming about out here in the open,

so the demoiselles head off to find shelter en masse. Safety is in numbers.

More and more demoiselles pour towards the cave entrances that riddle the Poor Knight Islands.

Safe. In the cave, they are far less exposed to attack from predators.

Thousands of demoiselles and blue maomao wait for the return of the current,

when once again they will head out into the open to feed.

The moon's gravitational pull is weaker nearer to the earth's equator,

so the more equatorial the location, the smaller the tides.

And out here in the Caribbean Sea, the tidal movements are slight.

Even so, they are sufficient to push free-swimming plankton in their path.

These are thimble jellyfish.

They swim towards the sunlight, and invisible boundaries formed by the tidal motion help herd them together

until they gather in immense swarms.

They put the tropical sunshine to good use.

Their brown colour is from algae that live inside the jellyfishes' bodies

and photosynthesise energy from the sun.

In open water, they're fairly safe.

But the tide is sweeping the whole swarm gently towards the Bahamas,

where hungry mouths are waiting.

Although the sea level doesn't change much,

the tides are still pushing an enormous volume of water from the ocean

through the small gaps between the island cays.

Underwater, the tidal currents race past soft corals...

..and on over the sandy banks themselves.

It's an immense area of coral sand

that is only just submerged at high tide.

This incoming tide is bringing in a fresh supply of oceanic plankton,

and razorfish gather at the best spots

to catch the pick of the microscopic feast.

Although there's plenty here for these small fish to eat,

gathering in one place makes it easy for their predators to find them.

A nurse shark is little threat...

SQUEAKS AND CHIRRUPS

..but this SOUND heralds a different danger.

WHIPLASH CRACK FAINT CREAK

CHIRPS AND CLICKS

CLICKS AND BUZZES

"ZIP" CALL A bottlenose dolphin.

ZIP! WHEE!

It's using its sonar to locate razorfish beneath the sand.

IT BUZZES, CLICKS AND PURRS

Once it finds a suitable target, it simply digs out its prey.

THEY SQUEAK SOFTLY

THEY TWITTER

"ELECTRIC-ARC" FIZZ

The buried fish have no defence against this attack.

They will simply have to wait and hope they aren't found out.

This dolphin appears to have a razorfish craving.

Well, she IS pregnant.

Success at last.

THEY GIVE CREAKY SQUEAKS

The incoming tide sweeps on towards America,

flooding across vast, flat plains of seagrass.

They're so shallow that at low tide, all large fish are forced to retreat into deep-water channels -

like these nurse sharks...

..and stingrays. Both predators hunt crustaceans on the seagrass beds.

But until the flooding tide brings enough water for them to swim in, they will have to wait.

So now, for this tulip snail, it appears safe

to patrol the channels in search of a meal.

But is it?

This is a rather bigger kind of snail.

At 5kg, the giant horse conch has little to fear from any shark -

and it has a taste for tulip snail.

Sensing the approaching danger, the snail flees.

But in a world of snail paces, the conch is something of a Ferarri.

It calls for desperate measures.

Exhausted by the effort of its last-ditch attempt,

the tulip snail is slowly gunned down.

The tide has to rise for another hour

before the big predators can feed.

But out on the flats, the scent of dying snail wafts away on the tide.

It's a scent these hermit crabs are partial to.

It's vital that the crabs have the best possible protection

from the heavy teeth of the waiting sharks and rays.

For that, they need the shell with a perfect fit.

Today, there is new real estate on offer, and competition in this housing market is fierce.

The action becomes even more desperate

when the shell of the devoured snail is ready for release.

This crab simply can't wait any longer.

But it's a decidedly risky acquisition.

The risk paid off handsomely.

The new shell is both lighter and stronger than the old home - and it's not a moment too soon,

because the tide is flowing in strongly, flooding the plains.

At last, the predators are free to start their foraging.

Both the stingrays and sharks have a highly-developed electrical sense

which they use to search for buried invertebrates.

They can sense minute movements beneath the sand.

Finding a promising signal, this ray digs out its meal.

An unprotected hermit crab would have no chance.

Within a few hours, the tide ebbs out once more

and all the predators are forced to leave.

They will have to wait. until the next high tide before making another feeding foray.

At certain times of the year, at the equinox, spring-tides are exceptionally large,

and rise even higher than normal

Now, predators can reach the very shallowest fringes of the seagrass flats,

and these two-metre-long tarpon are going further inshore still.

They are heading for the mangroves.

These flooded forests cover huge areas of the coastal shallows.

Extraordinarily, the roots of the mangrove trees can live in salt water,

and they make a perfect nursery for small fish.

Silversides and snapper find sanctuary in the maze of roots.

Big predators seldom find a way in here.

Now the tide is falling once more.

The water starts losing what little oxygen it contained,

and quickly becomes stagnant.

Most predators have abandoned the mangroves,

but the tarpon are still here, trapped by the falling tide.

Dissolved oxygen is fast running out.

They have a vital survival technique.

They can breathe air.

Pumped up with fresh oxygen, they easily outmanoeuvre dozy silversides.

The tide has turned again.

And this is no ordinary tide.

Since it is the equinox, the tide is rising fast.

But now, out to sea, a hurricane is on its way -

forcing the tide yet higher.

The passing storm leaves large areas of the coast flooded by the sea.

And low-lying islands like the Bahamas are particularly prone to the storm flooding.

The sun's power here is immense.

As the tide recedes and the remaining floodwater evaporates, a remarkable transformation takes place.

The mud is coated with a magical world of salt.

Any remaining water is extremely salty.

Very few creatures can survive here except brine shrimp...

..and, on the water's edge,

brine flies.

Both are the favourite food of an extraordinary animal.

The Caribbean flamingo.

Remarkably, they actually seek out such briny places.

They're the best spots for them to find their food.

BIRDS HONK

They also provide the protection the flamingoes need to raise young.

nesting sites like this are surrounded by corrosive brine.

It's a formidable barrier

to any predators seeking to dine on flamingo chicks.

The flamingoes take the precaution

of building raised mud nests

just in case of further flooding.

ADULTS HONK AND QUACK

CHICK PIPES

Strangely, it's actually the power of the storm tides that gives the flamingoes both their food

and the perfect habitat in which to breed.

The breeding of many animals in the ocean is closely co-ordinated with the tidal cycles.

A half-moon in November.

It's the time of SMALL tides.

Christmas Island in the Pacific.

Strange happenings are afoot.

It's one of only a few nights each year

when female Christmas Island crabs

risk heading down towards the sea.

Around the island, they number in tens of thousands - and all of them are laden with hundreds of eggs.

They have to shed them into the ocean

if the eggs are to develop into baby crabs.

But these are LAND crabs,

and they can neither swim nor breathe underwater.

There's a great risk of drowning,

so they pick the smallest tides of the month to minimise the danger.

The eggs will develop far offshore,

and in exactly one month, a swarm of baby crabs will return -

again, choosing the perfect tide.

Whether it's the daily or monthly cycle,

tides are the rhythm of the ocean, its pulsing clock.

Every tide brings opportunity to marine life somewhere in the world.

DOLPHINS TWEET

Now, a spring-tide is flooding the shallows,

and hunters are on the prowl.

A small group of bottlenose dolphin are working their way inshore

to start a quite extraordinary hunting campaign.

After one successful pass, the dolphin move off to start again.

One animal peels off from the group and swims rapidly in a circle,

stirring up the mud and driving the mullet towards the other waiting dolphins.

It's a remarkable team effort - and it's extremely effective.

The dolphin will feed like this for as long as the tide grants them access to the shallows.

Eventually, the falling tide will force the dolphin to leave the flats, and the mullet will be safe again -

until the next high tide...

..because, in the ocean,

every turn of the tide spells the difference between life and death

somewhere.

When trying to film the tidal behaviours of marine creatures, timing is of the essence.

Many events in the sea only take place at certain tidal phases,

and some, on only one particular tide in the whole year.

One of the most spectacular displays depends completely on nature's tidal clock.

It's the spawning of the coral reefs,

which release their eggs and sperm in synchrony after a spring full moon.

A crew was there on Australia's Great Barrier Reef to keep the appointment,

but things don't always go as planned.

Lizard Island - a backwater behind the centre of the barrier reef.

A good place to film coral, but the exact details of coral spawning are still not properly understood.

Only 20 years ago, they discovered that the entire Barrier Reef went off.

It goes off on three or four nights, the whole reef, all the corals spawn.

They've been working on it ever since. They still don't know much.

Because the spawning could happen in several places at once,

Martha took two cameramen, Peter Scoones, a veteran of coral seas,

and Mike Pitts who, although familiar with these waters, had never seen coral spawning before.

-Mike?

-It's working.

-Can you light balance away from the reef?

So we're not shining on the corals.

The spawning activity starts normally starts two days after a full moon,

and the crew were ready with a complex system of underwater lights.

What I wanted to achieve with the lights was that it was natural,

certainly not front lit with a light stuck on the front of the camera,

it had to have more atmosphere.

As well as directing, Martha is in charge of dive safety and has to wait on the surface.

Just so want to get the best images. When they are all underwater,

I'm praying that someone's seen a fantastic coral going off and they've lit it beautifully.

That first night, the corals showed no signs of spawning.

Meanwhile, the team was also trying another strategy.

On shore in a laboratory on nearby Lizard Island,

cameraman, Steve Downer and director, Hugh Pearson were hoping to film close-ups of coral spawning

in more controlled conditions.

We spent four or five nights in the laboratory getting these shots.

In the end, the coral did spawn and we got some shots of a spawn leaving the corals in a big close-up.

Although that was successful, out at sea, everyone were still hoping that the big event would finally happen.

Today is night three of a potential spawning.

Although Craig has said tonight is going to be the quietest,

they also know up until an hour before spawning,

so they can give warning before it happens.

The third night seemed a little more promising.

Some of the corals began to release sperm in clouds into the water.

But this wasn't the spectacle the team wanted.

Just looks like a foggy cloud coming out of it.

Lots and lots of tiny eggs wafting around.

It takes a long time. Once they start it doesn't go in one bang.

It just goes one, two... four, five...one.

-You wait five minutes and you might get two.

-Watching a stone getting ready to spawn is...

Some pleasing images, but only a start,

and not enough to complete the sequence.

-We're not balanced for that... That's mine!

-I know.

Tonight is the big one. Trouble is, the wind's got up.

When the conditions get rough, the corals don't spawn

because the sperm and the eggs would mix so much.

There's a possibility things won't happen tonight.

But, hey. Positive thinking.

Martha's fears proved right, and despite the hopeful start

spawning was sparse on the fourth and fifth night,

making it difficult to catch the spectacle on camera.

One little coral, and I thought, "I can get an angle on that."

I was getting into position and it went poof!

Come back! I didn't get that.

She had expected the spawning to be more vigorous.

There was only one day left to complete the filming.

The scientists were convinced of spawning that night.

Elation was dampened by another setback.

Ten minutes ago we were sitting out on the back deck,

Peter said his camera was dead.

We've got an hour or so until the spawning starts.

I just had a letter from the coral expert

who said tonight's the night.

I'm hoping it isn't because we've only got one camera.

I'm sure Mike will do a great job.

At the last minute, Peter succeeded in repairing his camera.

The team's patience was rewarded.

Large areas of the reef spawn together in a spectacular simultaneous display.

There's mystery in seeing six branching corals go off at the same time.

They're not connected to each other,

yet they know they had to spawn at the same time.

Nobody has worked that one out yet.

Timing had been everything. Within a few hours

the team returned to the boat to review the results.

It didn't look anything like that.

There!

When they're coming through...

Wow! Well done, everybody.

It's amazing. Thank you very much. I know it's been a struggle.

ALL: Whoa!

Can you rewind that?

-Spikey bits.

-Whoa!