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 tons 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 cope pods 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, senses on the cope pods' antennae
give warning of dangers ahead.
Lighting by lasers
reveals that feeding cope pods 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 cope pods stuck to the membranes.
A single sea-nettle jellyfish only a few centimetres across
can collect thousands of cope pods 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 meters.
But there are even greater dangers awaiting the cope pods.
By the late spring,
the baby common jellyfish are fully grown.
They gather in millions,
forming immense swarms hundreds of meters long,
which filter out
all the small planktonic animals in their path.
Nevertheless, there are such astronomic numbers of cope pods
that enough will survive to form swarms of their own.
A basking shark.
This huge 8-ton fish feeds on plankton and nothing else.
Every hour,
it filters a thousand tons 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 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 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-meter-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 isn't as 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 meter a day.
The main stem is fixed to the bottom 35 meters below
by a small but immensely strong hold fast.
Fully grown, these gigantic plants are 100 meters 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 for 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're not the only animals to venture out at this time.
This is an amphipod just 2 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 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 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 fantail 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 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 has spent the winter 250 meters 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 intend 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 20,000 fertilized 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 one-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 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-a-half-meter-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 3-meter-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.
40 meters 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 panicked herring are forced towards the surface.
They gather into a giant defensive ball of swirling fish.
The commotion attracts yellow-tailed rock fish.
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 socializing.
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 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 hand fish 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 meter 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 fertilized,
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'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 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 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 tons 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 small pod is part of a population
of some 500 killer whales
that specialize in hunting North Atlantic herring.
Using their echolocation,
they have detected a shoal of herring 50 meters 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.