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 zones,
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 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.
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,
sustained only by their fatty blubber.
It will be five weeks before they are strong enough to swim,
but then it will be spring and the ocean will be teeming with food.
By May, spring has reached the coasts of Scotland.
Underwater, it arrived rather earlier.
In March, the seas had warmed enough
to trigger a magical transformation.
These are phytoplankton —
tiny floating algae, each much smaller than a pinhead.
They multiply with amazing speed
to produce more annual growth than all the plants on land together —
six 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.
These look like small anemones, but they're about to change.
They separate, revealing themselves to be tiny common jellyfish.
They are 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 little algae
into the filters round 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,
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 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 eight metres.
But there are even greater dangers awaiting the copepods.
By late spring, the baby common jellyfish are fully grown.
They gather in millions,
forming 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 1,000 tonnes of seawater through its gills.
Numbers of these shark appear when the bloom is at its peak
and travel along currents where the plankton is thickest.
Even so, the supply of plankton is so abundant
that another group of animals can feast —
animals that live on the sea floor.
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 plankton.
It is 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 100 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 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 ten minutes,
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 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 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're 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 50 youngsters clustered on her abdomen,
so her home is becoming somewhat cramped.
They will soon be old enough to leave,
and, 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 midsummer, 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.
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,
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 is 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 midsummer, 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's 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 arrives at her favoured sand bank.
But she is not the first here.
Dozens of other lobsters have already dug 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 need of a pit,
and since she weighs a hefty seven kilos, she'll probably get one.
She has won.
These battles continue for the next two months,
and they are crucial,
for the females must have shelter and warm water to raise young.
For the last seven months,
each female has been carrying about 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 are able to set off in a purposeful way.
At this time of the year, the sea is full of larval animals.
This one is a day old lobster.
And this — a three—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, it's a vegetarian with a taste for sea lettuce.
As it grows, 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 become an 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.
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 a 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 swarm 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 till rain causes it to rise.
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 water.
And that means they have the energy to be quick.
Quicker 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,
this year's baby herring feed on the last of the summer 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 with ease 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 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.
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 Western 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 late autumn, it's nearly ready to hatch.
A fully—formed miniature shark swims free.
This year's plankton will soon die,
but the dogfish is so well developed
that it 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 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 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, continually changing costume colour.
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 aren't 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 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 five 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 entirely on the fat they accumulated
during the summer feasts of plankton.
But they are not alone.
Orca.
This pod is part of a population of some 500 killer whales
that specialise in hunting North Atlantic herring.
(WHALE SONG)
Using their echo location, they have detected a shoal of herring
50 metres below them.
With enough air for a ten—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 tails 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.
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 five 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.