Seasonal Seas The Blue Planet

There are some seas where fish swarm in millions...

..and plankton blossoms in vast clouds.

The sheer quantity of life here is unmatched anywhere in the oceans.

These are the most productive seas on Earth.

They are the seasonal seas.

These seas border the temperate regions where the seasons change.

The seasons also affect the underwater world.

The power of the sun is constantly changing.

In the far north, during the summer, there are long hours of sunlight.

In winter, that dwindles and there can be weeks of darkness.

The summers are warm and gentle.

The winters wracked by savage storms.

In conditions like this, life of any sort has to struggle to survive.

January on Sable Island, off Nova Scotia.

Grey seals have got ashore through the crashing breakers.

Gales here can blow for days on end.

Sable Island has the world's largest colony of grey seals.

100,000 come here to breed each year,

just when the weather's at its worst.

To add insult to injury,

the pups - having suckled for only 18 days - are abandoned.

Their mothers must return to the sea to find food for themselves.

Unable to dive, the pups are marooned and sustained by nothing but their fatty blubber.

It will be five weeks before they're strong enough to swim.

By then, it will be early spring. The ocean will be teeming with food.

By May, spring has reached the coasts of Scotland.

Underwater, it arrived rather earlier.

During March, the seas had warmed enough to trigger a transformation.

These are phytoplankton -

tiny floating algae, each much smaller than a pin head.

They multiply with amazing speed

to produce more annual growth than all the plants on land put together,

six billion tonnes of it.

This immense bloom spreads across the face of the ocean.

Within a couple of months, it turns vast areas of it a dense green.

Animal life reacts to the blooming sea.

These polyps are about to change into something else.

As they separate, they're revealed to be tiny common jellyfish.

They're less than three millimetres across.

Within a few months, they will have assembled into vast swarms.

Minute copepods are part of their staple diet.

These appear every spring in vast numbers

and graze on the phytoplankton bloom.

Their beating legs create currents

that sweep the algae into the filters around their mouths.

On this microscopic scale,

water is so viscous, phytoplankton can't swim against the current.

Whilst feeding, senses on the copepod's antennae

warn of dangers ahead.

Lighting by lasers

reveals that feeding copepods leave wakes behind them, like jet trails.

Slightly larger floating predators

are able to use these trails to find their prey.

A close call,

but some predators are simply too large to avoid...

..jellyfish.

They may appear to be delicate

but they are deadly hunters.

With every pulse of the delicate bells,

plankton-rich water is drawn into their lacey throats

and pushed out again, leaving behind copepods stuck to the membranes.

A single sea nettle jellyfish, only a few centimetres across,

can collect thousands of copepods in a day.

So the killing power of giants, like these, is hard to estimate.

Each of these weighs up to 30 kilos

and has tentacles stretching over eight metres.

But there are even greater dangers awaiting the copepods.

By the late spring, the baby common jellyfish are fully grown.

They gather in millions,

forming immense swarms

which filter out all the small planktonic animals in their path.

Nevertheless, there are such astronomic numbers of copepods

that enough will survive to form swarms of their own.

It's early spring in British Columbia.

Seaweed has started to grow slowly in the cold water.

As the hours of sunlight increase, and the water warms,

these small plants turn into great beds of bull kelp.

The immense 30-metre-long strands have small gas-filled floats,

which keep them within reach of the energy-giving sunlight.

Further south, the sunshine is more powerful.

So here, on the coast of California, the biggest kelp can grow.

This is giant kelp.

By midsummer, each plant grows in length by a metre a day.

Californian sea otters gather in the kelp forest

to rest and snooze in safety.

To prevent themselves being carried away into dangerous open water,

where the big predators cruise, they anchor themselves by winding kelp around their body.

Sooner or later, they have to find food.

That lies on the sea bed, a long way below them.

However, they can stay underwater for up to ten minutes.

That's ample time to find shellfish.

Many of the smaller creatures that live in these forests - such as urchins - graze on the kelp

and can damage it if their numbers are unchecked.

Sea otters feed on these grazers and prevent them getting too numerous.

In effect, sea otters are the forest's guardians.

Garibaldi fish do not, in fact, damage the kelp.

They help it by picking off tiny animals that encrust the leaves.

They graze on bryozoans - tiny colonial animals

which build their colonies like a patchwork of white skins on leaves.

When night falls, there are fewer predatory fish

and the bryozoans emerge from their white shelters.

Now, just like coral polyps, they start filtering out the plankton

under cover of darkness.

They're not the only animals to venture out at this time.

This is an amphipod, just 2cm long.

And it does eat kelp.

In turn, it is excellent food for many predators.

To protect itself, it produces silk, like a spider,

and sews together two sides of a kelp frond and so form a shelter.

This one is in particular need of a secure home.

She's a mother.

There are 50 youngsters clustered on her abdomen,

so her home is becoming cramped.

They will soon be old enough to leave

and now, when she can, she kicks them out to get a taste of the world.

Dawn.

Beds of eel grass grow between the kelp forest and the shore.

A harbour seal has found sanctuary here

and is sleeping after a hard night's foraging.

But not for long.

A male seal gives a wake-up call.

More seals are attracted from all directions.

It's June - the time when young male harbour seals

start their strange mating displays.

One listens attentively to the grunting noises made by the other.

These calls are almost certainly a way of establishing which of the two will be dominant.

But if the animals are closely matched in size and experience,

grunts won't settle the issue.

The rivals will have to come to blows.

Seals can be surprisingly violent.

In a month's time, the breeding season will start,

then fights will be in earnest.

But now, in midsummer, these exchanges are harmless.

Many of the creatures in the kelp have to venture out in order to feed.

The bat ray, for one.

Unlikely though it seems, the sandy floor of the open sea is, for the bat ray, a rich feeding ground.

There is food, hidden within the sand.

The bat ray has a special technique for finding it.

It uses jets of water to blow the sand aside,

and expose small invertebrates.

A kelp bass hangs about alongside, waiting for scraps.

Other hunters are also on the prowl.

The fan-tailed sole.

There are mantid shrimps here, living in tunnels.

But, once again, hunger compels them into the open.

That, of course, is a gamble.

They will either eat,

or be eaten.

A sea slug called Janolus.

Its colours suggest that it's poisonous, and so it is,

to everything except another kind of sea slug...

..the predatory Navanax.

Navanax pulls itself along the trail of slime the Janolus leaves behind.

Once caught, Janolus rolls into a ball.

All Navanax gets, is a few yellow tentacles.

And Janolus is swept to safety by the current.

It's now midsummer.

The sun is shining at full strength.

The increasing warmth is the cue for an Atlantic lobster to start on a long journey.

She's spent the winter 250m down, far beyond the reach of the storms.

But it was cold down there, and now she needs to find warmer water,

so she's marching towards the shallows.

They, however, are 150km away.

After a month of walking, she arrives at her favourite sand bank.

But she's not the first here.

Dozens of lobsters have already dug themselves homes in the sand,

and they don't intend to surrender them to newcomers.

Size counts for everything in these battles.

The new arrival is in urgent need of a pit. Since she weighs seven kilos,

she stands a good chance of getting one.

She's won.

These battles continue for the next two months.

They're crucial, for the females need shelter and warm water,

if they're to raise their young.

For the last seven months, each of these females has been carrying around 20,000 fertilised eggs.

But their task is approaching its end.

The warmth of the shallows is speeding the eggs' development.

Two more months, and the eggs are ready to hatch.

At first, they're not very good at swimming.

But within a few minutes, the babies can set off purposefully.

At this time of year, the sea is full of larval animals.

This one is a one-day-old lobster.

And this, a three-week-old crab, ready to start life on the sea floor.

Its feet touch the bottom for the first time.

At this stage, it's a vegetarian with a taste for sea lettuce.

As it grows, it will repeatedly moult and grow into a bigger, thicker skin.

The chances are, it will be eaten.

But if it survives for five years, it'll be a magnificent armoured giant.

Now, it eats meat.

Special adaptations enable it to hunt in the dark.

Its jointed feet are covered in sensors,

which detect the slightest chemical change in its surroundings.

As it walks in the darkness, its feet can, literally, taste the sand.

As soon as it finds suitable food, it passes it to its crushing claws,

which make light work of the soft flesh.

Those claws are also very useful for defence.

A 1.5-metre-long common octopus glides by.

The jet-propelled giant is both powerful and very clever.

With octopus about, it's risky for even a crab to be in the open.

Even in the dark, the octopus' eyes are sensitive to the slightest movement.

Against a hunter like this, the crab's claws are useless.

Late summer in south-east Alaska.

The water is still warming and mysid shrimp swarm near the surface.

It's a final feast for Pacific salmon

returning to the coast from the open Pacific.

They're heading inshore to breed and they arrive in huge numbers.

They have to swim far up the rivers to spawn.

But this river's level is too low.

They'll have to wait until rain causes it to rise.

They are trapped close to the sea shore - the worst place to be.

A three-metre-long salmon shark,

a close relation of the great white.

This one's sensed minute electrical signals from the salmon nearby.

Shark can maintain their blood temperature

at a higher level than the surrounding sea water.

That means they can be quick.

Quicker even than salmon.

Early autumn in Vancouver Island in Canada, 600 miles to the south.

The ocean temperature is slowly dropping.

40 metres below the surface, baby herring feed on the last of the summer's plankton.

Their movements attract attention from the skies above.

Gulls can't dive,

so for now the fish are still safe.

But there are birds which can dive.

Auklets and murres swim effortlessly down beneath the school.

The panicked herring are forced towards the surface.

They gather into a giant defensive ball of swirling fish.

The commotion attracts yellow-tailed rockfish.

They too are hunters.

The marauding fish scatter the herring.

Repeated attacks split the ball into numerous smaller groups.

Now it's easier for the divers

to keep the confused fish penned at the surface.

There, even gulls can get at them.

Attacked from all sides, the fish have virtually no chance.

More and more divers are attracted to the scene.

They harry the shrinking numbers of herring

right down to the very last individual.

Bigger predators cruise here, too.

Pacific white-sided dolphin.

The dolphin are mainly nocturnal hunters.

During the day, they socialise.

They display by releasing streams of bubbles

and they play games.

Games like pass-the-seaweed, for example.

Exactly eight months ago, off the west coast of Scotland,

an egg was laid and securely fixed to a strand of kelp.

Inside, a tiny embryo started to develop.

Protected by the tough egg case,

it endured the worst of the winter storms.

By summmer, it was half-grown.

Now, at last, in the late autumn

it's nearly ready to hatch.

A fully-formed miniature shark swims free.

This year's plankton will soon die, but the young dogfish can hunt immediately for larger prey.

The days are getting shorter now.

In British Columbia, the water begins to chill.

This bizarre-looking creature is searching the kelp for food.

This is melibe - the hooded sea slug.

It catches plankton with its net-like head.

As winter approaches, plankton is becoming scarce.

But melibe is an assiduous searcher.

It can swim.

It flaps away to look for a better feeding spot.

But its search is becoming difficult.

Within the next two weeks, most of the plankton will have disappeared.

As the sunlight becomes feeble, the kelp starts to die,

gradually rotting away to nothing.

Soon it will be winter.

But 9,000 miles to the south,

the sun is rising on a new spring day.

The southern hemisphere too has temperate regions.

The plankton is beginning to bloom around Tasmania.

Just as in the north,

the southern seasonal seas have areas of rich green water,

with their own kelp forests...

..and their own swarms of plankton.

Some of these inhabitants live only in the southern hemisphere.

This is one of them. The handfish, that strolls around on modified fins.

But when needs must, it can resort to tail power.

Every summer, visitors come to the shallows around Tasmania.

These are Australian squid, about half a metre long.

They are here to breed.

The larger males compete for the attentions of a female,

displaying towards her

by putting on a ballet, where they continually change their colour.

Eventually, they form pairs.

The male passes a package of sperm across to the female.

After the eggs have been fertilised,

they're deposited in tough, rubbery egg cases

that other creatures find poisonous.

Within three weeks, the babies are ready to hatch out.

They are already able to change colour,

but they're not such good swimmers.

There's another animal here that is a rather more devoted parent.

This is a male leafy sea dragon,

an exquisitely-decorated relative of the sea horse.

He is carrying his partner's eggs around with him.

They'd be a nutritious snack for any predator that found them.

That's not easy, because they're attached to his perfectly camouflaged body.

They could scarcely be in a safer place.

By November,

10,000 miles to the north, winter has arrived.

Norway gets under 5 hours of daylight in every 24.

The temperature is falling rapidly.

But despite the cold, the sea is far from deserted.

Every winter, 500 million tonnes of adult herring

seek shelter in these deep waters.

They will stay here for four months,

living on fat they accumulated during summer feasts of plankton.

But they're not alone.

Orca.

This pod is part of a population of some 500 killer whales,

that specialize in hunting North Atlantic herring.

THEIR CRIES ECHO

Using their echolocation, they've detected a shoal of herring

50 metres below them.

With enough air for a ten-minute dive, they swim below the herring

and drive the fish upwards.

Bubbles stream from the rising fish

as gas in their swim bladders expands and escapes.

Even an orca finds it difficult to catch a healthy herring.

But they have a devastating weapon all their own.

They use their tail to club the fish with waves of water pressure.

Then, it's simply a matter of collecting the stunned casualties.

The herrings have no chance

and both orca and gulls will eat as much as they can

every day for the next four months.

But there are so many fish wintering here - over 5 billion individuals -

that the losses are almost unnoticeable.

Violent as this winter weather may be,

it's essential for the renewal of the riches of the seasonal seas.

Out in the open oceans, the surging waters stir up nutrients from the depths.

By the end of winter, the seas will be full of minerals once more,

ready for the return of the sun and the next great plankton bloom.

Many creatures in the Blue Planet have never been filmed before and some are hardly known to science.

One such is a relative of the infamous Great White Shark, the Alaskan Salmon Shark.

Until recently, these sharks had only been encountered by local fishermen,

but often it has been an alliance between film-makers and scientists that's brought new insights.

To get these pictures, a Blue Planet crew joined with a research group,

making one of the first ever studies of this extraordinary shark.

Each summer, hundreds of Salmon Sharks visit Alaska's Prince William Sound

in search of salmon returning to breed in freshwater rivers. Fishermen want to start harvesting the sharks

and there is an urgent need for research.

The shark's lifestyle is a mystery and no-one knows how big the population is.

Biologist Ken Goldman wants to learn more, but first he has to catch his study animal.

Lower, now!

-Down!

-OK.

OK.

They work as quickly as possible, to mimimise stress to the shark.

Down, tag, 9-8-1-6-3. You got pre-caudal and feet, flip it over...

Marking sharks with special tags will help reveal how many there are and where they go.

-You want to cut?

-No, pre-caudal, do it now! Sixty pre-caudal, fork!

-Ninety.

-Total?

-A hundred.

-Let's get this fish out of here.

-Go ahead, start down.

Catching and quickly releasing sharks four metres long is not easy work,

but it's essential to the long-term survival of the species.

All right.

-One down.

-One down, five hundred and...

-As many more of those as we can get.

Nearby our crew were trying to film the sharks' natural behaviour.

They have a mini-camera on a pole and a viewing monitor on the surface,

but to get reasonable pictures, the sharks have to come close.

I think one of the things that people don't realise is just how long these sequences take to film.

I've worked on this two years, setting it up, and Peter and I will be here four weeks.

So, all we've seen is a few fins around and so we've been charging after these sharks, individual ones.

All we've managed is to keep up behind them and just get some shots of the tail.

We can film without getting in the water, using the pole cam system,

which can produce very good results. But generally you get better results in the water.

Just a slight safety issue!

You don't get in the water unless you're bloody confident these things will be benign.

Getting in with these wouldn't worry me.

But we obviously must be very safety-conscious

cos the last thing we'll do is get in if there's a risk of being eaten. Peter says they're fish-eaters

and therefore don't pose a threat.

My concern would be that we wouldn't get pictures.

I certainly want to get in just to see them - they're superb creatures. But I'm slightly more nervous,

cos after all, they're sharks!

Curiosity did get the better of our team and they cautiously entered the water.

With these sharks there is no prior art. It's the first time anybody's filmed them or even swum with them.

It's certainly pretty scary and underwater visibility was ten or twelve feet.

All you saw was a big curtain of green. A shark could come at you from underneath, behind, in front...

and you'd have very little notice of it.

-Lots of green.

-Lots of green!

As soon as we dropped in the water, they sank out of sight.

It's only in the last ten years that Salmon Sharks have been sighted here in such large numbers.

At times the crew saw 100 swimming near the surface -

behaviour which might help these sharks regulate body temperature -

but also means they're regularly caught in salmon fishermen's nets.

OK, we got problems.

INDISTINCT VOICES

On this occasion it was for their own good, or at least for the conservation of the species,

as Ken works with local fishermen to increase the number of tags on his study group.

-NOISE OF WINCH

-Okay, big girl. Everybody back, everybody just back.

Only for a month each summer do sharks appear in Prince William Sound

and otherwise nobody sees them or knows where they go.

It's OK, sweetie, it's OK. It's OK.

Over 90% of the animals Ken catches are females

and he hopes that his tagging will help reveal the breeding grounds so that at least these can be protected.

This is almost the only close up view that Ken gets of his sharks,

so he was fascinated to see what our crew came up with.

See you later, bye, girl. Good job, gentlemen.

Way to go captain, beautiful.

The crew were into their third week of filming. Time to compare notes with Ken.

-That's nice.

-Dink!

-That's nice.

-Right on the lens!

This kind of activity is what I DON'T get to see - like that.

That's splendid.

Our vision under water is terrible - they're seeing that camera at 30 feet.

-Well...

-Their vision is adapted to that environment.

When you look at them, they have a really pointy face.

-Their eyes are very forward.

-That explains why we can sneak up behind them.

But once the camera sees their eye, bang - they're off.

Ken hadn't seen his sharks so close before.

To get those shots of the sharks coming at the lens, we had to put dead salmon

that we'd got from the fishermen, close to the camera.

The sharks would come and we'd get those close-up shots of the teeth and eyes.

Just under the boat and he's gonna come up right under the bait.

He's right at the bait, oh, he's dragging it. Oh-ho-ho-ho!

LAUGHTER

-Was that a shot?

-Yes, it was a very good shot!

-It was great, there were sharks everywhere.

-We got some really nice shots today.

It was amazing at one point, because wherever you looked, you saw shark fins.

One or two of them actually charged straight at the lens.

Subtitles by BBC Subtitling

E-mail us at [email protected]