BBC The Natural World Complete Series

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.