Original subtitles

SIAN PHILLIPS (VO): It's early summer

and 20 miles off the coast of Pembrokeshire...

the warming seas of the Celtic Deep are starting to stir...

drawing in long distance travellers.

A fin whale.

One of the biggest animals on the planet.

She's here for a summer of feeding.

She could well have travelled over 6,000 miles to get here.

After centuries of whaling, fin whales have been on the brink of extinction.

But it's believed that numbers are now on the rise.

And these ocean giants are returning to the seas around Wales.

She's not the only one coming to these rich waters

for what they hope will be a time of plenty.

Wales is a coastal nation.

The sea /s in our soul.

Full of hidden treasures...

wonders...

and giants.

A sea of mythical coasts...

with extraordinary creatures...

secretive shallows...

...and hidden depths.

These are the Wonders of the Celtic Deep.

St Anne's Head, the westerly edge of Pembrokeshire.

Reaching out into the St George's Channel, here the Celtic and Irish Sea meet.

Now we /eave /and...

and travel out into the Celtic Deep.

Eight miles offshore stands Grassholm.

BIRDS CAW

Of Wales's 52 islands, it's the most remote.

Making it a perfect breeding ground for the North Atlantic's largest seabird.

The gannet.

With his six-foot wingspan he's well suited to a life out at sea.

He greets his life partner with a knocking of bills to renew their bond.

BIRDS CAW

Ten percent of the world's population of Gannets come here every spring.

Some 40,000 birds crammed onto this tiny island.

Together this pair will raise a single chick.

To keep it fed, they will take it in turns to go far out to sea in search of fish.

BIRDS CAW

A further 12 miles out...

lies the Smalls Lighthouse.

Sitting on the last rocky outcrop Wales has to offer

before losing the fight with the sea.

SEALS BELLOW

It's also the final haul-out of the Atlantic grey seal.

SEALS MOAN

Giving them a resting place from fishing in the open ocean.

From here, the sea floor gives way to the open ocean beyond.

This is the domain of Dylan ail Don

God of the Celtic Deep.

And the territory of the common dolphin.

All who venture here are prime targets for investigation.

These sociable and highly intelligent cetaceans

are one of the few animals to play into adulthood.

The dolphins are drawn to passing boat traffic...

taking it in turns to ride the bow wave.

Game over.

DOLPHINS WHISTLE

Living in pods of up to 50 individuals...

common dolphins have complex social lives.

Which can get even more tangled when families gather to form superpods.

DOLPHINS WHISTLE

It's thought that they come together like this when food is plentiful.

Many fins will make light work when hunting large shoals of fish.

DOLPHINS WHISTLE

But getting this many social creatures together is also a chance for romance.

Females race ahead, pursued by a group of would-be suitors.

Turning onto their backs the males attempt some... upside-down mating.

This frenetic open ocean lovemaking Is so hard to keep up with...

that it has never been filmed here before...

until now.

DOLPHINS WHISTLE

The females will mate with multiple males.

It won't be the fastest swimmer who fathers her offspring...

but the one with the fastest swimming sperm.

Mating over, it's time to go and hunt some fish.

SEABIRDS CRY —- CHILDREN SHOUT

Warmer days draw crowds to the beaches of Pembrokeshire.

But the water /s still cold.

CHILDREN SHOUT

After a long winter it takes time to warm up.

When spring finally comes to the Celtic Deep...

it triggers a massive bloom Of life.

These murky Welsh waters might not be crystal clear

but that's because they are so full of nutrients...

rich with microscopic phytoplankton.

When magnitied around a thousand times

It reveals glassy plant-like cells.

These diatoms contain chlorophyll with which they photosynthesise.

Just like terrestrial plants

they adraw in carbon dioxide and release oxygen.

Phytoplankton accounts for around 50% of all photosynthesis on Earth...

and so, half of the oxygen we breathe.

As they grow, the plant-like phytoplankton are grazed on by microscopic animals

called Zooplankton.

This in turn kick starts the food chain in the Celtic Deep.

The plankton's predators come in all shapes and sizes.

From this thumb-sized comb jelly

to a true ocean giant.

A basking shark.

The second largest fish in the world.

As long as a bus.

This toothless filter feeder is on the hunt for the microscopic plankton.

And the water Is thick with them.

2,000 tonnes of water an hour can pass through her open mouth...

and straight out through the gills.

Long, comb-like structures on the gills then catch the passing plankton.

But to swallow, she needs to shut her mouth

to guip down the catch.

After wintering in warmer, more southerly waters

the basking shark travels north...

following the blooming plankton up through the Celtic Deep.

Heading for a summer of feeding

in the shallow coastal waters around Scotland.

But she isn't alone.

Recent research has revealed

that basking sharks can travel in extended family groups.

Hanging out with relatives

learning the best migration routes and places to feed along the way.

When the autumn comes, this family group will return south.

But for now, it's time to feast in this sea of plenty.

Jellyfish though, are not on the menu

and even the enormous basking sharks are careful to avoid them.

They too are here to feast on the plankton.

Drifting on the currents to follow the blooms.

The long tentacles of the compass /fellyfish are equipped with stinging cells

for capturing Zooplankton and even small fish.

As if from another world, a moon jellyfish

one of the most common to be found around the Welsh coast.

When food and conditions are good

they can be found in swarms of many thousands.

In recent years, the number of jellyfish has increased...

suggesting an imbalance in the health of our seas

linked to pollution, overfishing and climate change.

But one creature able to take advantage of this is following in their wake.

The ocean sunfish, with its bony, beak-like mouth.

Perfect for biting off lumps of jellyfish.

It's the size of a dinner plate, but can grow as big as a Car.

A brief summer visitor to the Celtic Deep.

He loves nothing more than gobbling up gelatinous splodges.

SEABIRDS CRY

The explosion of life just offshore has not gone unnoticed.

Sardines are arriving in their millions.

Fishing boats are drawn out here from far and wide.

These shoaling fish have travelled en masse from deep water off the Isles of Scilly...

here to feed on the plankton.

They also promise rich rewards for those that can catch them.

An old traveller has returned to the Celtic Deep.

This Is a six-foot long Atlantic bluefin tuna.

Extremely partial to sardines.

These are the ultimate open ocean hunters.

Fished to near extinction

they've not been seen regularly in these waters for over 30 years.

But now the tuna are back.

Drawn here by the bounty on offer.

BIRDS CAW

Splashing on the surface announces the arrival of the tuna...

but only hints at the carnage about to be unleashed below.

Using their formidable power to generate massive acceleration

they are able to pick off the fish at will...

preferring to swallow their prey whole.

Tuna are highly migratory...

perfectly adapted to wander the oceans in search of the most productive waters.

For these tuna, the Celtic Deep is the place to be.

Following the tuna and the panicked fish are the gannets

hoping for an easy meal.

GANNETS CAW AND CRY

The gannet must pick its moment carefully.

Dive-bombing on a moving school of fish requires precision.

Hitting the water at 60 miles an hour

allows them to reach fish up to 20 metres down.

Under aerial bombardment, what remains of the bait ball of fish dives deeper.

But there's one bird determined to follow them.

A Manx shearwater.

Preferring to duck dive from the surface...

it can swim down to 55 metres to hunt fish

far beyond the gannets' reach.

GANNETS CRY

The shearwaters are accomplished long distance navigators

spending the Welsh winter off the coast of South America

before returning to the islands of the Celtic Deep to breed.

DOLPHINS WHISTLE

Now the common dolphins arrive.

But a moment too /ate.

A minke whale engulfs the surviving fish in one massive mouthtul.

The minke is the smallest of the baleen whales

found in the Celtic Deep.

But at over ten metres long, she’s still a giant.

With the fish consumed, the feeding frenzy is over.

The smell of the carnage has drawn the attention of another ocean wanderer.

A blue shark, over two metres long.

Her powertul sense of smell has drawn her here.

Highly sensitive to the amino acids found in blood

she can follow a scent on the current for miles.

She's not interested in the departing tuna.

Found throughout the world

this elegant ocean predator is looking for a starter before her night-time banquet.

She will be more active after dark

hunting tish, squid and even other sharks.

But right now

she'll not turn down the opportunity for wholesome scraps.

A nomadic wanderer

the Celtic Deep is her summer hunting ground.

She doesn't appear until the water's warm enough

and disappears as soon as it starts to cool.

With little left to be found in the remains of this bait ball, she cruises on.

The fish remains and dead plankton drop slowly towards the sea floor.

This "marine snow" is full of nutrients.

A bottom dwelling weeverfish, rarely seen swimming freely.

[ts six poison-tilled spines make it the scourge of many a beachgoer.

30 metres down

and the sea floor looms in the near darkness.

Here, another shark patrols what appears to be a desolate landscape.

A metre-long starry smooth-hound.

But on closer inspection

this sea floor is a thriving carpet of Iife.

Thick with horse mussels

some up to 50 years old.

Eerily decorated with a white, soft coral known as dead men's fingers.

Living half-buried in the sea floor

the horse mussel beds provide a reef-like structure

for a whole community of creatures.

Providing shelter...

for some.

It is thought that much of the Celtic Deep floor

once looked like this.

But when damaged by fishing nets dragged across the sea floor

the slow growing horse mussels and the habitat they create

struggle to recover.

But where it remains

it is host to a thriving ocean waste management community.

Life down here relies on what arifts down from above.

Clams, like the mussels, feed on passing plankton.

Transparent sea squirts are also filter feeders.

And as if on guard around them

squat lobster snatching food passing in the current...

eating the sea floor clean of debris.

Hermit crabs take recycling to the next level.

Their mobile homes are empty whelk shells covered in barnacles...

making this a moving, living landscape.

Here, there's a hungry mouth for every scrap of food.

Nothing goes to waste.

A scorpion spider crab.

These curious crabs stick bits of living sponge to themselves.

The sponges then grow to cover the crab.

Adding to the camoutiage, the crabs pulse up and down

waving like plants in the current.

Blending in is key for survival.

With a hungry catshark on the prowl, It pays to lie low.

A curled octopus waits it out between meals.

Others are bolder...

showing no fear of sharks.

A red gurnard.

Where others swim, this fish prefers to walk.

Equipped with sense organs in their finger-like spines

they can feel for prey hiding among the mussels.

There's plenty around.

It takes more than fish fingers to make the catch.

Second time lucky.

Deploying a similar strategy but with deadlier weapons...

the curled octopus is out of hiding.

She is a ruthless hunter

with a keen eye for anything that moves...

and for anything that doesn't.

She has eight taste-sensitive tentacles that feel their way across the reef.

Hidden under her mantle, her tough, bony beak is strong enough

to crack through crab shells.

She can immobilise prey with an injection of toxins

before digesting their dissolved flesh.

But when crossing the reef, she's exposed.

To stop the hunter becoming the hunted, she must always be on her guard.

Other predators continue to /urk.

She is a master of disguise.

With a lack of hiding places

she must rely on her ability to change colour

and blend in with the surrounding shells.

Now you see me...

now you don't.

[t's now mid-summer

and just off the Irish coast

a remarkable and rarely witnessed gathering ls spotted from the air.

Over a dozen adult basking sharks are circling in descending layers.

With mouths shut

these sharks are not here to feed.

This seldom-seen behaviour is thought to be a courtship ritual.

Very little is known about the sex life of basking sharks.

The females give birth to live young

but the mating itself has never been witnessed.

This helter-skelter of basking sharks Aisappears into the depths.

Perhaps, down below this circle, the next generation will begin their lives.

During the summer, new life abounds in the Celtic Deep.

The microscopic plankton is full of tiny, newly hatched animals.

This bug-eyed creature will become a spider crab.

/t carries a rapier spine to deter would-be predators.

Until it becomes an adult crab

and a real hard case.

Other familiar seaside animals are found here.

Crabs, shrimps, urchins and starfish

all spend the first part of life arifting with the plankton.

At the mercy of the open ocean.

The tiny youngsters are here to feed and grow.

These speedy juveniles are the larvae of the static barnacle.

As adults they are firmly glued to a suitable surface

so this is their chance to get far away from the parents.

They spend this time eating and defecating.

lf they survive this juvenile phase

the barnacles and crabs will descend to the sea floor.

Where life continues to be treacherous.

Millions of adult barnacles cover the horse mussel beds

providing a tempting buffet for those with the tools to take them on.

On the march, a chain of onchidoris bilamellata nudibranch.

Don't be fooled by their sluggish appearance.

This is a herd of voracious predators

easily able to outpace their prey.

The onchidoris are rnasters of chemical warfare.

Absorbing the foul-tasting poisons of their prey

they then secrete these as a defence against their own predators.

With this for protection, an onchidoris is a bold hunter...

specialising in devouring barnacles

no matter where they grow.

Despite their armour, barnacles are no match for the onchidoris.

It has a special tooth which chews away at the top plates of the barnacle

before sucking out the soft tissue inside.

Terrorising the reef at their own leisurely pace

the onchidoris still need to be careful.

This common crab has apparently developed a tolerance for toxicity.

In this world of scavengers, even the best defences are not always enough.

Some prefer to come fully armed.

Another plankton arifter has survived to wander the sea floor.

An eagle-eyed, sharp-shooting mantis shrimp...

more commonly found in warmer waters.

This colony in Cardigan Bay was only discovered in 1999.

An ambush hunter, he's equipped with a harpoon-like claw

with teeth-like serrations along its edge.

But to deliver the killer blow, a good aim is required.

And this quicktire crustacean has the most advanced eyesight on the planet.

Each independently moving eye is made up of

tens of thousands of photoreceptor cells...

allowing it to see ultraviolet, infrared and polarised light.

And for accuracy, three pupil-like receptors in each eye

allow it to measure distances perfectly.

With nothing in range, he decides to hide out...

waiting for an unsuspecting victim.

PEOPLE YELL

The long, hot days of summer are drawing to a close.

And the beaches of Abersoch have drawn in the crowds.

Tourists, here to revel in the last of the sunshine...

and the now warm sea.

SEABIRDS CRY

For those gathered out in the Celtic Deep

they too are keen to make the most of the last days of summer.

Common dolphins

seeking fish.

DOLPHINS WHISTLE

Using echolocation, they are homing in on a school of sardines.

DOLPHINS WHISTLE

DOLPHINS CONTINUE WHISTLING

Under attack, the fish form a defensive ball.

The dolphins work together to corral the sardines close to the surface...

until they have nowhere left to go.

The dolphins take turns to plough through the bait ball...

using their tails to create shock waves...

and stun the fish.

The commotion does not go unnoticed...

acting as a dinner gong echoing across the sea.

BIRDS CAW

The gannets and the shearwaters are soon on the scene.

DOLPHINS WHISTLE

And with fish trapped against the surface they throw themselves into the fray.

BIRDS CAW

DOLPHINS WHISTLE

BIRDS CAW

The sound of the splashing travels faster and further underwater.

The bluefin tuna arrive

as big as the dolphins and with an insatiable appetite.

But they are dwarfed by the next arrivals.

Fin whales.

At 25 metres /ong, only the blue whale can rival it for size.

They are here to capitalise on the dolphins’ hard work.

Lunge feeding on its side enables a whale to sieve through

its body weight in water in one mouthtul.

For a brief moment

the visitors and residents of the Celtic Deep feast together.

In a few weeks the fish will be gone.

They must eat while they can.

With nothing left, the party disperses...

to continue travelling the Celtic Deep.