Original subtitles

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

The Seasonal Seas border the Temperate parts of the world,

where conditions change through the year

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,

but in Winter that dwindles and eventually there can be weeks of darkness

The summers are warm and gentle, the winters racked by savage storms

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

January on Sable Island off the coast of Nova Scotia in North America

Grey Seals have managed to get 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 is at its worst

To add insult to injury, the pups,

having suckled for only 18 days are then abandoned

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

As yet unable to dive, the pups are marooned

and sustained by nothing except their fatty blubber

It will be five weeks before they are strong enough to swim,

but by then it will be early spring and 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 magical transformation

These are Phytoplankton, tiny floating algae,

each much smaller than a pinhead

And they multiply with amazing speed to produce more annual growth

than all the plants on land put together - 6 billion tonnes of it

This immense bloom spreads across the face of the ocean

and within a couple of months turns vast areas of it a dense green

Animal life reacts to the blooming sea

Although these polyps look like small anemones,

They're about to change into something else

As they separate, they reveal themselves to be tiny, common jellyfish

They are less than 3 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 little algae

into the basket-like filters around their mouths

On this microscopic scale, water is so viscous that

the Phytoplankton can't swim against the current

Whilst feeding, sensors on the copepod's antennae give warning of dangers ahead

Lighting by lasers reveals that feeding copepods leave wakes behind them,

rather like jet trails in the sky

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 as well as beautiful, but they are deadly hunters

With every pulse of the delicate bells plankton-rich water

is drawn into their lacy 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 jellyfish weighs up to 30 kilos

and has tentacles stretching over 8 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 hundreds of metres long,

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

A basking shark

This huge eight tonne fish feeds on plankton and nothing else

Every hour it filters a thousand tonnes of seawater through its gills

Numbers of these shark appear when the bloom is at its peak

and travel along those water currents where the plankton is thickest

Even so, the supply of plankton is so abundant

that another group of animals' is able to share in the feast

- animals that live on the seafloor

Every centimetre of rock is covered with invertebrates of some kind

- anemones, sea squirts, Gorgonians,

all trapping plankton from the passing water

Not all life in the Seasonal Seas depends upon the plankton

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 high in the water within reach of the energy-giving sunlight

Further south the sunshine is more powerful,

so here, on the coast of California, the biggest kelp of all can grow

This is giant kelp and by midsummer each plant

is growing in length by nearly a metre a day

The main stem is fixed to the bottom 35 metres below

by a small but immensely strong holdfast

Fully grown, these gigantic plants are a hundred metres long

and they form vast marine forests

Great schools of fish find sanctuary here

These are blacksmith fish

They are plankton feeders and whenever it seems safe

they head off into open water to feed where the plankton is thickest

But at the first hint of danger...

...they rush back to the safety of the forest

Fish are not the only creatures that shelter here

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 straps of kelp around their body

Sooner or later, they have to find food

and that lies on the seabed a long way below them

Sea otters, however, can stay underwater for

up to 10 minutes and that's ample time to find the shellfish they need

Many of the smaller creatures that live in these forests, such as urchins,

graze on the kelp and can seriously damage it if their numbers are unchecked

The sea otters, however, feed on some of these grazers

and prevent them getting too numerous

So, in effect, the Sea Otters are the guardians of the forest

Garibaldi fish do not, in fact, damage the kelp

If anything, they help it by picking off tiny animals

that in places encrust the leaves

They graze on bryozoans, tiny colonial animals,

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

When night falls there are fewer predatory fish around

and the bryozoans emerge from their white shelters

Now, just like coral polyps they start filtering out

the plankton under cover of darkness

They are not the only animals to venture out at this time

This is an amphipod, just two centimetres long and it does eat kelp

In turn, it is excellent food for many predators

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

and uses it to sew 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 over fifty youngsters clustered on her abdomen,

so her home is becoming somewhat cramped

They will soon be old enough to leave and now whenever she can,

she kicks them out to get a taste of the outside world

Dawn

Beds of eel grass grow in the shallows 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

And seals can be surprisingly violent

In a month's time the breeding season will start

and then fights will be in earnest

But now in mid-summer these exchanges are harmless

They're just playing

Many of the creatures that shelter in the kelp

have to venture out in order to feed

A 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

and 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 to venture 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 that Janolus leaves behind it

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 mid-summer and the sun is shining at full strength

The increasing warmth is the cue for an Atlantic lobster

to start on a long journey

She has spent the winter 250 metres 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 150 kilometres away

After a month of walking she finally arrives at her favoured sand bank

But she's not the first here

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

and they don't intent to surrender them to newcomers

Size counts for everything in these battles

The new arrival is in urgent need of a pit and since she weighs a hefty 7 kilos,

she stands a good chance of getting one

She's won

These battles continue for the next two months and they're crucial,

for the females must have both shelter

and warm water if they're to raise their young

For the last seven months, each of these females has been carrying

around about twenty thousand 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 are able to set off in a purposeful way

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

- this one is a 1 day old lobster

And this ... a 3 week old crab, just ready to start its life on the sea floor

Its feet touch the bottom for the first time

At this stage in its life it's a vegetarian, with a taste for sea lettuce

As it grows, so it will repeatedly moult and expand into a bigger, thicker skin

The chances are it will be eaten, but if it survives for five years

it will have become a magnificent armoured giant

Now it eats meat and 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

And as it walks through the darkness its feet can, literally taste the sand

As soon as it finds suitable food,

it passes it forward to its powerful crushing claws,

which make light work of the soft flesh

And those claws are also very useful for defence

A one and half 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's 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 are swarming near the surface

It's a final feast for Pacific salmon,

returning to the coast from the open Pacific

They are 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 still too low

- they'll have to wait until rain causes it to rise

So, for now they are trapped in the sea,

close to the shore, the worst place to be

A three metre long salmon shark, a close relation of the great white

This one has sensed minute electrical signals from the salmon nearby

Shark can maintain their blood temperature at a higher level

than the surrounding sea water

And that means they have the energy to be quick

Quicker even than salmon

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

The ocean temperature is slowly dropping

Forty metres below the surface,

this year's baby herring are feeding 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 panic herring a force towards the surface

They gather into a giant defensive ball of swirling fish

The commotion attracts Yellowtail 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

and there even the gulls can get at them

Attacked from all sides, the little 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

Far bigger predators cruise here, too

Pacific white-sided dolphin

But the dolphin are mainly nocturnal hunters

- during the day they concentrate on socialising

They display by releasing streams of bubbles and they play games...

...games like pass the seaweed, for example

Exactly eight months ago in the winter 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 summer 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

is so well developed that it can't 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 through 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 increasingly difficult

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

As the sunlight becomes ever more 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

- and now the plankton is beginning to bloom around Tasmania

Just as in the north, the Southern Seasonal Seas

also have areas of rich green water...

...with their own kelp forests

...and their own swarms of plankton

Some of the inhabitants of these seas 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 and to one another by putting on a ballet,

during which they continually change the colour of their costumes

Eventually they form pairs

A male passes a packet of sperm across to the female

After the eggs have been fertilised, they are 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 quite so good at swimming

There is 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 would be a highly nutritious snack for any predator that found them

But that's not easy because the eggs are attached

to their father's perfectly camouflaged body

They could scarcely be in a safer place

By November, 10,000 miles to the north, winter has arrived

Norway now gets less than 5 hours of daylight in every twenty four

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 entirely on the fat

they accumulated during the summer feasts of plankton

But they are not alone...

Orca's his small pod is part of a population

of some five hundred killer whales

that specialise in hunting North Atlantic herring

Using their echo-location they have detected a shoal

of herring 50 metres below them

With enough air for a 10 minute dive...

they swim deep 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 Seasonal Seas will be full of minerals once more,

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