Original subtitles

The Amazon is the mightiest river on earth.

A thousand miles before it reaches the sea

its main channel is already ten miles wide.

By the time it meets the ocean

it carries a fifth of all the river water in the world

spilling an incredible forty five million gallons a second into the Atlantic.

Every year, without fail the Amazon bursts its banks

and floods an area of forest the size of England.

This huge change in water level

over such a vast area

affects everything that lives here in a dramatic way.

Over its four thousand mile course from the Andes to the Atlantic

the Amazon is swollen by some eleven hundred tributaries.

Several are massive rivers in their own right.

Between them they drain forty percent of the continent.

Three thousand kinds of fish live in this waterworld.

And where there are fish there are fish hunters.

Giant otters.

They really are giants nearly two metres from nose to tail.

Local people call them 'Iobos del rio' wolves of the river.

They live almost entirely on fish

though they can kill caiman and anacondas.

They're one of the top predators of the Amazon

the aquatic equivalent of the jaguar.

Most otters are solitary but these are very sociable.

They live in family groups of about half a dozen

a mated pair and their full grown young from previous years.

The otters bridge two worlds.

All their food comes from the river

but they sleep and rear their cubs on dry land.

So their lives are profoundly affected by the changing level of the river

which has its origins far to the west

in the distant peaks of the Andes.

After a first precipitous rush from the mountains

the river's course is flat and winding.

For the next three and a half thousand miles

it drops barely two inches a mile.

As sluggish meanders twist across the floodplain

their course is constantly changing

creating new channels and lakes.

This is more than just a river

it's a whole river system.

It's October the height of the dry season.

The Amazon is at its lowest ebb

and showing its bare bones.

For just a few weeks each year

sandbanks rise clear of the water

giving a brief window of opportunity

for one of the Amazon's most remarkable animals.

Giant river turtles.

They're one of the largest fresh water turtles in the world

over a metre long and weighing more than forty kilos.

They come back to the same place each year to lay their eggs.

But this is a race against time

because the sandbanks will soon be flooded again.

For two weeks after they arrive

the pregnant females bask in the sun.

This hastens the development of the eggs

still inside their bodies.

When she's ready

each huge female digs a hole a metre deep.

In it she'll lay a hundred soft-shelled eggs.

But all this activity doesn't go unnoticed.

Good sandbanks are few and far between

so early clutches of eggs are often accidentally dug up

by turtles nesting later.

For the vultures too this is a once a year opportunity.

But it only lasts a few days.

The turtles will soon be gone.

In forty five days the surviving eggs will hatch

far quicker than other turtles.

Just a week or two later

the sandbanks will be drowned again.

If the rivers rise early all the eggs are lost.

At the peak of the dry season

broad reaches of river can be almost cut off

as sandbars separate them from the main flow.

Fish become concentrated in these lagoons

so for predators the killing is easy.

Caiman and egrets gather to take advantage of the best hunting of the year.

The fish take shelter in the reeds

but in shallow water they're easier to catch.

Striking low and from the side makes it harder

for the egrets' prey to see them until it's too late.

Sometimes the caiman seem to work together

to herd fish into the shallows.

The shoals of prey become even more concentrated.

The water literally heaves with fish.

With so many to choose from

there's no need to single out a target.

A random lunge can be just as effective

and as each caiman strikes

it panics fish into the jaws of another.

But the good times won't last long.

The caiman must make the most of them while they can.

A feast like this could keep them going for months.

For most fish the dry season is not a good time.

They're on everyone's menu

and their own food's so scarce that many will not eat for months.

Some of the birds are having a hard time too.

Tui parakeets are seed eaters

and the seeds of this munguba tree are a rare treat

The munguba uses the wind to scatter its seeds.

Each one is carried on a parachute of fluff

but if it lands in the water

there's not much chance of it germinating.

These are Brycon sometimes called South American trout.

They're so hungry

they jump at the chance of even these meagre mouthfuls.

They can leap up to four times their body length.

Even at low water the larger rivers are still navigable.

On the Amazon's main channel

seagoing ships can reach as far as the city of lquitos

over two thousand miles from the ocean.

The rivers are the main arteries of travel.

Local people depend on them throughout the year.

Riverside communities rely heavily on fish for their livelihood.

For the people as for the natural predators

low water is also the best time to make a catch.

Laden with silt from the distant mountains

the rivers are often very muddy.

It's hard to see much under water during the day

At night, the problem is even worse.

Infra-red cameras let us follow the action in total darkness.

But how do the fish find their way?

Some, like these Candiru have an acute sense of smell.

They can detect the slightest trace of blood or decay.

Fish trapped in fishermen's nets are easy meat.

Like aquatic vultures

the Candiru home in on the scent of flesh

and strip the carcass from within.

A close relative of these Candiru has the most unsavoury reputation

of any fish in the Amazon.

It's a parasite rather than a scavenger

and it's attracted to blood and urine.

It's been known to swim right up into people's bodies

wedging itself inside with backward-pointing spines.

Other fish deploy much stranger senses

...ones that we can scarcely begin to visualise.

They generate an electric field around their body.

Any object in their path distorts the field.

They detect the changes with receptors in their skin.

This extraordinary sense allows them to find their way

around in the murkiest of water

and to detect and catch their prey.

They also use it to communicate.

Most electric fish generate a very weak field

but not this one.

To local people

it's one of the most feared fish in the Amazon

the electric eel.

Its two metre long body is a gigantic battery.

It can deliver a jolt of six hundred volts a shocking defence

enough to kill a child

and more than enough to stun its prey.

As the rivers shrink to their very lowest

some of the channels are entirely cut off.

Here, predators and prey become even more concentrated.

Territorial male caiman have put aside their differences

to take advantage of the glut of food

but in such cramped conditions disputes are inevitable.

Despite the crowds

there are still plenty of fish to go round.

But that doesn't remove the temptation to steal someone else's.

Even theft is scarcely worth the energy.

The heron keeps its meal.

For fish hunters these are the good times

though not without their annoyances.

Caiman may look armour plated

but there are chinks in their defences.

With only their heads out of water

they're still a target for blood-sucking horseflies.

Even a harmless butterfly can be a nuisance.

The first showers hint at a change of season

a promise of rising water

and better times to come for fish now

trapped at the mercy of predators like caiman and egret

For the cormorants too

the best of the fishing will soon be over.

Released from their dry season prison

the fish will spread out and become harder to catch.

They'll be too scattered to feed a flock like this.

The dry season may be coming to an end but not just yet.

This is only an isolated shower.

It will take much more to set the fish free.

The first rain can even make things worse for them... much worse.

The cold rainwater sinks

displacing warmer and less oxygenated water below.

Decaying vegetation and bubbles of hydrogen sulphide rise from the depths.

This lethal cocktail makes oxygen levels plummet

and fish are soon gasping for breath

The topmost film of water where it touches the air

still has the most oxygen

so desperate fish skim the surface to avoid suffocation.

Most kinds of fish seem to be affected.

For the birds

this may be the last opportunity to make a killing.

The distressed fish are easy prey for the cormorants and egrets.

With so much on the menu they ignore the dead and dying

but not all of these will go to waste.

For the vultures

this is another brief window of opportunity.

Fish kills are sudden and short-lived and the dead fish soon sink.

Black vultures have an acute sense of smell.

The stench of rotting fish draws them to the feast in increasing numbers.

There is too much even for the vultures.

The victims of this natural disaster can number tens of thousands.

Isolated local showers turn to more general rain.

The Amazon basin gets a massive three metres of rain a year

most of it concentrated in just a few months.

The timing of the rainy season varies

from one part of this vast catchment to another

but the result is the same everywhere

a slow but inexorable rise in the river level.

The changing season has a profound effect on everything that lives here.

Swollen by the rain that falls within the basin

the rivers are soon full to the brim.

But there's more to come.

In the cloud forests of the Andes

the annual rainfall can be an incredible six metres.

When that joins with snowmelt from the high peaks

the results are dramatic.

It may be days or even weeks before these torrents

make their full impact on the floodplain forest

hundreds of miles to the east.

But already, animals are on the move.

The rain that has already fallen here waterlogs the soil

and raises the humidity

and that triggers a remarkable migration.

The small creatures of the leaf litter move upwards, into the trees.

Soon, their world will be transformed.

But up in the treetops predators are waiting for them.

For many of the leaf litter animals

the shift to the treetops proves fatal.

So why do they move?

It can take two months

but finally the rain and snowmelt from the Andes make their mark.

The rivers are already full from rain in the lowlands

and can take no more.

Now they overflow into the forest.

The reason for that mass migration from the leaf litter becomes clear.

The forest floods to a depth of ten metres.

The flood changes the lives of everything

that lives here.

Even the giant otters are on the move.

Their dry season dens are now underwater

so they have to move their young to higher ground.

This year's cubs are still too small to swim far

so the adults carry them in their jaws.

Giant otters are territorial

and the group's range has to include dens for both low and high water.

The whole family share the task of looking after the young.

It will be two or three years before their elder brothers

and sisters leave to start a family of their own.

Few predators will tackle a full grown giant otter

but infants can be killed especially by caiman.

Safely installed in their new home

the otters will stay here until the floodwater recedes.

At the height of the flood

Amazonia is transformed.

An entirely new habitat has been created

a waterworld that can drown the forest

for ten miles either side of the main river channel.

At the peak of this inundation

the rivers engulf an incredible fifty seven thousand square miles of forest

an area the size of England.

The trees can stand in water for six months or more.

But this drowning is something that happens every year

and many will turn the flood to their advantage.

Branches where birds once perched now shelter fish.

Freed from the dry season confines of the riverbed

they spread out into the forest.

This is their season of plenty.

Predators follow the fish into the forest

but now it's much harder for them to hunt.

Their prey are more scattered

and have more places to hide.

The hunters include one of the Amazon's most remarkable animals.

The Boto dolphin.

This river dolphin is almost blind

but the water here is often so muddy that eyes are useless.

It finds its way through the branches by using sonar.

Unlike its marine relatives it has a very flexible neck

so it can sweep its head from side to side to scan the water ahead

and to catch fish.

Because there is so much decaying vegetation

the water in the flooded forest is very low in oxygen.

That's no problem for an air-breathing dolphin

but how do fish survive here?

The Pirarucu is the largest freshwater fish in the world.

It can grow to three metres

and weigh nearly a hundred and fifty kilos.

Like the dolphin this giant has to breathe air.

Every few minutes

it comes to the surface and takes a gulp of oxygen.

All sorts of other fish share this ability.

The electric eel absorbs oxygen through the lining of its mouth.

It acts like a lung.

Many of the Amazon's fish have a high concentration of red blood cells

which makes their gills more effective

but they also take air from the surface.

A swamp eel traps great gulps of air in its inflated throat.

Animals that live in the trees

also need special skills to survive in the flooded forest

like being able to swim.

Sloth and spider may seem unlikely swimmers

but their own form of freestyle is now the best way

for them to move between trees.

If you can't swim, climb or fly you can't stay here

so this waterworld has none of the usual forest animals

like deer or peccaries.

The sloth may not be fast

but it's a good swimmer and climber.

The most unexpected creatures take to the water.

Fire ants even turn the flood to their advantage.

When their forest floor nests are drowned

they set sail on a living raft

formed from their own inter-linked bodies.

Drifting with the current

they can end up far away from their old home.

All they need for a new colony is on the raft

including the next generation.

But the voyage has its hazards.

Fire ants have a painful sting

but that doesn't stop fish picking them off

as they drift through the forest.

Eventually the rafting ants make landfall.

This tree will provide a refuge until the waters fall

and they can return once more to the forest floor.

If their eggs are to survive

they must find shelter from the drying heat of the sun.

A crack in the bark is a perfect nest site

and the tree comes with a built-in food supply.

Others have already taken refuge here.

They arrived even before the flood had started.

The lizard has now got competition.

These insects are all on the fire ant menu

but they seem strangely reluctant to jump.

Those that do take the plunge may find

that they've leapt from the frying pan into the fire.

Fish are quick to take advantage of any insects

that fall into the water.

For some, like the metre-Iong Aruana

they're a large part of their diet.

The fish may not have eaten for months

so now it's time to fatten up.

Wind and rain dislodge a steady supply of insects

from the crowded canopy above.

Even if it's not actually in the water

an insect still may not be safe.

All sorts of other fish make the most of the windfalls.

Needle fish cruise the surface picking off anything small.

Like some other fish in the Amazon

and the dolphins these little fish have a marine origin.

They spend most of their time near the water surface

hunting by sight.

That slim profile may help to camouflage them.

Their long 'beak' is ideal for seizing prey.

The rising water triggers an event

that will affect the lives of almost everything that lives here.

It's now that the trees in the flooded forest fruit.

Many trees depend on the flood to disperse their seeds.

By the time they germinate

the water will have carried them far away from their parent tree.

But drifting with the flow isn't the only way seeds can travel.

In this strange new world

some trees use fish to scatter their seed.

The fish eat the fleshy part of the fruit

but the hard seeds pass through their gut unharmed.

This is a good time for fish.

They gather wherever fruit falls.

And where there are fish there are fishers.

Most of the predatory birds have gone

but not this one a little green heron.

Most fruit-eaters have an equal partnership with the trees.

They spread their seeds in return for food.

But some fish don't play by the rules.

They eat the seeds

crunching them up and destroying rather than dispersing them.

Tambaqui have a powerful sense of smell to lead them to fruiting trees.

They have powerful jaws, too

well able to crush hard seeds such as this walnut-like supacaio.

Little of what they eat will ever get the chance to germinate.

The flood is also a good time for the Uakari.

These monkeys live in loose groups of thirty to fifty.

They seldom come down to the ground

preferring to travel through the treetops in elegant leaps and bounds.

So when the forest floods

they have no need to swim.

These bizarre monkeys live only in floodplain forests

where they take full advantage of the flush of new fruit.

Local people call this 'the English monkey'.

Its red face reminds them of a gin-swilling white man

who's been out too long in the sun.

In fact the red face is a sexual signal

a sign of potency and good health.

Uakaris are one of the few monkeys

that live in this riverine forest all year round

surviving on leaves and unripe fruit.

With their strong jaws

they can tackle food that would be too tough for other animals.

The seasonal flooding of the forest leaves nothing untouched.

Everything that lives here has to adapt... or move on.

Remarkably people stay here right through the year.

Their houses are built on stilts or rafts

to raise them above the flood

as are their chicken coups.

Watering the plants may be easy

but a floating platform is now the only place to grow vegetables.

This is a challenging time.

Most people depend on fish

but the shoals of the dry season are scattered through the forest

and so harder to catch.

All sorts of predator are having a hard time.

Even fearsome hunters like piranhas are reduced to scavenging for scraps.

Of all the Amazon's fish

these have the most notorious reputation.

They could strip a chicken to the bone in seconds

if it were careless enough to fall in.

Living on the water does have some advantages

at least for children

though most of us wouldn't

want piranhas in our swimming pool.

In fact

their ferocious reputation is undeserved.

Only in the dry season

when they're concentrated in landlocked pools

are they truly dangerous

So for now it's safe to swim.

People can live without dry land

at least for a while but otters can't.

They need a dry den to sleep in and rear their young.

They're now deep in the flooded forest

well away from the open river

hunting fish hidden in the trees.

The easy times of the dry season may have gone

but otters are supreme predators.

Their webbed feet and paddle-like tails

their strength and agility make them a match for even the most elusive fish.

If fish are hard pressed to escape an otter hunting alone

they're even less able to get away when they hunt as a pack.

Often the whole family fishes together

driving their prey into a tight shoal

or trapping them against a bank.

The rewards of communal fishing

are one of the most important benefits of family life.

Over the months to come

the floodwater will slowly drain from the forest.

The rivers will once again be confined to their channels

and the otters will return to their dry season dens.

Giant otters are an indicator of a healthy river.

Only where they have a plentiful supply of fish

and can live undisturbed do they survive.

There are many threats to the world's last wild places

but the sheer scale of the Amazon could be its salvation.

It's far too mighty to control dam, or divert.

The massive yearly flood makes it a hostile home

for all but a few people

and means this watery wilderness may always remain untamed.