Freshwater Animals The Animal's Guide to Britain

Britain.

The history and the culture.

Born of a landscape that we know and love.

But, hang on a minute...

That's just how WE see Britain.

We humans are in a minority. We share our land and our shores here

with hundreds of thousands of other species of animal,

many of which have been here a lot longer than we have.

So what I want to know is what they think of Britain,

what matters to them?

And that's my mission - to see the UK through our animals' eyes.

Where are their favourite places in these crowded islands?

How do their senses affect their view of our country?

And what do they make of us?

Off you go!

I'm starting by diving into the lives

of a hand-picked group of freshwater animals.

I want to understand each one's unique abilities.

And how freshwater animals have adapted to modern Britain.

Together, they're going to reveal our country

as we've never seen it before.

Welcome to The Animal's Guide to Britain.

Rivers, lakes, marshes, ponds, even ditches.

Britain's freshwater habitats form a countrywide life support system

on which all species depend.

But the value of these places is greatest for the animals

that actually live there.

Now you may think

that animals don't have much choice about where they live.

But there's certainly one creature that does, because twice a year,

it literally flies the length of the UK, sampling our lakes and marshes.

So it's the perfect species to give us an insight into the quality of Britain's freshwater habitats.

The osprey, a spectacular bird and a highly specialised fish hunter.

Every spring, several hundred ospreys make the 3000-kilometre

journey from West Africa to Britain.

And most of them come here, to the Scottish highlands.

Beautiful, isn't it?

Very picturesque.

This is Loch Garten and it's only a few hundred metres away

from the ospreys' nest in the woods, there.

So you would think that this would be the most convenient place

for them to come and hunt.

But they rarely, if ever, fish here

because there aren't enough fish in here to warrant the effort.

But luckily, they know somewhere better.

And even luckier, I know it too.

It's 5am and I've come to a much smaller loch where ospreys have

often been seen hunting fish.

Just donning some essential stealth.

A couple of dark green mittens and then, and I can tell you,

you won't have seen this on the catwalk this season. Oh!

Oh, yeah. Just hope it works.

Nothing's going to see me in this.

For me, the prospect of seeing an osprey is always special.

When I was a kid,

these were rare birds, you know, they were super state secrets.

No-one got the look-in as to where they were.

You know, there was a privileged few in the RSPB who got to see them.

The rest of us just saw them on the 9 o'clock news

when they were being harried by egg collectors.

RADIO: 'OK, on top of you now, Chris.'

This is Keith, he's our osprey spotter this morning.

'Female osprey from Loch Garten.'

Look at that!

Wow.

'Three more birds coming up the river, there.'

Unbelievable. Look at this, the sky. It's a flock - a flock of ospreys.

In 40 years of birding, I've never seen this many ospreys,

this close together, anywhere in the world.

Wow...

He missed.

This is a brilliant place

to get your head into that of a hungry osprey.

They're clearly circling around the lake,

looking through the surface for fish.

They identify an area and then they seem to focus on it.

This one's going in, it's going in, it's going. It's gone.

It's got one.

And it's got an antenna on its back,

which means it's one of the study birds from Loch Garten.

And that bird was carrying a fish in the classic osprey manner.

Immediately after catching it, it swings it around

so the fish is head-first

into the direction the bird's flying, keeping it streamlined.

It's tough enough work carrying a heavy fish anyway,

but you don't want it flapping around underneath you.

And to help them with this,

one of their toes is opposable, so they can turn it round.

So there are two toes on one side of the fish and two on the other.

Got it!

And one thing you see is that when they lift themselves out,

they have a jolly good shake, to get that water off of their plumage.

One thing that's immediately apparent here

is how energetically expensive

the process of catching fish is for these birds.

They're using up lots of reserves.

Maybe about one in four, they're going out with a fish.

But the reason that this particular loch is so popular with ospreys

is that this is a fish farm.

It's heaving with prey.

Amazing. Top 10 birding moment.

A whole morning spent watching ospreys fishing.

It's been absolutely remarkable. There's been only one downside.

And that's dealt with that.

My goodness me, the balaclava, never again.

But you know, more than anything, it's given me the opportunity

to get my head inside that of an osprey.

And I think I've come up with a conclusion.

It's not the pure water, it's not the climate up here,

it's not the nesting trees. It's the fish.

It's the fish that are the most important thing to these birds.

But Britain has loads of fish.

So why aren't ospreys everywhere?

Well, once upon a time, they were.

In medieval times, ospreys could be found from the Highlands of Scotland

to the English south coast.

They were believed to have to the mystical ability to hypnotise fish,

which turned belly-up in surrender.

They were held in awe by humans,

featuring on the coat of arms of Swansea, granted 1316.

Ospreys even enjoyed divine protection,

listed in The Old Testament as an animal not to be eaten.

They had it good.

But then, around the 1500s, ospreys began fishing from human-made ponds,

and the relationship soured.

Later, the Victorian humans became obsessed with collecting rare eggs.

The rarer the bird, the higher the demand,

until, in 1916, the last osprey vanished from Britain.

But, thankfully, that wasn't the end of the story.

In 1954, a pioneering pair of ospreys,

migrating from West Africa to Scandinavia, stopped at Loch Garten.

They liked it so much they stayed to raise a family.

As they struggled to survive, a nation watched in awe.

Mr Watson, just how rare are these birds in Scotland?

Well, as far as we know,

they're the only pair nesting in the whole of Great Britain.

Last year, they were robbed, unfortunately,

and the year before this, one of the pair was shot.

Ospreys were giving Britain a second chance and, this time,

one group of humans was determined to protect them from egg collectors.

This big clump of barbed wire is a pretty useful deterrent

to anyone trying to get up to that nest.

The electronic warning devices on the tree are rather interesting.

They are vertical wires all round the trunk

and anyone trying to climb the tree is almost bound to touch the wires,

particularly if they're climbing it in the dark,

because it is during the hours of darkness

that we think an egg collector

might try to raid the nest and collect the very valuable eggs.

In the annals of British ornithology,

this is an historic place.

The hide, where veritable legions of bobble-hatted volunteers

have guarded the oldest ospreys' nest in Britain.

Established 1954.

This is an honour.

And in here, the history continues.

Just look at this.

Legend has it that these binoculars

were requisitioned from a German U-boat.

They're not marked with an osprey,

but with a German eagle clasping a swastika.

I'm rather hoping that I'm not going to spot a battle cruiser

through here, but an osprey grasping a fish.

And there is an osprey.

That is one of this year's young

that has fledged in the last few days.

And it's sat there, having a bit of a preen,

waiting for the adults to bring in a fish.

This chick is an osprey success story,

the fourth generation to be born at Loch Garten

and one of around 100 chicks to have fledged here

since they first came back in 1954.

And many of these have gone on to nest across the Highlands.

In fact, they've become part of the Scottish experience,

along with lochs, monsters, haggis, tartan, you name it.

But it's largely an accident that so many nest in Scotland.

You see, ospreys keep returning here because they're creatures of habit.

They prefer to nest near the place they were born or,

at least, close to other ospreys.

But ospreys have gradually spread out from their Loch Garten home.

And now, some humans are trying to encourage them

to go much further south.

You can't get much further south than this, in the British Isles.

That is Poole Harbour

and I'm standing in Dorset on the RSPB's reserve at Arne.

And the locals here have got a big idea.

They want ospreys to nest here.

After all, Poole Harbour is full of fish

and ospreys regularly pass over Poole on their way back from Africa.

But from an osprey's point of view,

what's missing is other ospreys' nests.

So the RSPB are putting up what you might call show nests,

and I'm here to find out what an osprey needs in a nest.

Do you know what I thought when I saw this nest from the ground?

I know. It wasn't very high! I thought that as well.

For the osprey, is that going to be high enough? It's perfect.

What the osprey are looking for

is a really isolated tree, nice and high, with a good vantage point.

OK. Well, let's get some more twigs up and try and make it look

a little bit more attractive.

The thing is, everyone thinks a big pile of twigs is easy to build.

But birds do a remarkable job, don't they, of weaving them in and out?

They do. They definitely do.

Yes, ospreys know what they're doing.

The males do the heavy construction work,

bringing back up to 100 loads of dead wood every day.

Females, on the other hand,

focus on the comfy bits, like the moss to keep their eggs warm.

So what other desirable features

would a house-hunting osprey be looking for?

Erm, I think we can go for the pooh now.

Pooh? Not literally.

Can we have pooh paint, please?

Well, I think we've, we've really poohed the nest. Yep.

I'm beginning to understand

what an osprey looks for, but this nest needs one final touch.

This is a very fish-filled osprey, if ever I've seen one. Oop.

Oh, my goodness. Oh, my goodness. That...

He's a beauty.

Right, OK.

I thought everything was going so well with your plans.

Have to hope for a very short-sighted osprey.

From a distance, they actually do look quite convincing.

And, of course, looking out there, we don't have to look far,

do we, to see an amazing source of food for them?

That's got to be full of flat fish and mullet, which they love eating.

Yep, it's perfect.

This sort of habitat, they'll be able to get more young off, hopefully,

be a lot more successful here than they already are in Scotland.

To an osprey checking out Britain,

these artificial nests are a nod and a wink

that this IS a good place to breed.

And it's working. Human-built nests are popping up across Britain,

and ospreys are spreading south.

And if humans continue to make these magnificent birds welcome,

ospreys will once again see the whole of Britain

as a top spot to raise their young.

It's easy to be impressed

by dramatic lochs and lakes and cascading mountain streams.

Not all of us, though, have these sorts of things on our doorstep.

But what we can have

is a very important freshwater wildlife habitat.

It can come in the form of marshes, perhaps village ponds,

or even our garden ponds.

And when you think about it,

if you were to add all of these things together,

they'd become an incredibly important resource for wildlife.

And I wouldn't mind betting that on each and every one of them,

there's a species of one of nature's most miraculous insects.

Dragonflies.

Superbly adapted aerial predators.

Some species can fly at 40 miles an hour! Others can even fly backwards.

They can hover and bother any aerial insect with a diabolical deadliness.

Imagine you're a little blue bottle.

You fly out over the pond, flashing your iridescent blue bottom.

You get spotted by the giant compound eyes of the dragonfly.

It seizes you, snips off your legs and your wings,

and by the time they've fluttered down onto the lily leaves,

the assassin is already looking for its next victim.

I think we can see dragonflies as a triumph of evolution.

I mean, when you think about it,

their body form has been virtually unchanged for millions of years.

And their beginnings can be traced in Britain.

The story of the British dragonfly

goes back to a time long before humans,

and before our islands even existed.

This is the cast of a fossil

that was recovered from the Bolsover coalmine in Derbyshire.

And it shows the wing of a dragonfly.

This is about 320 million years old.

And this insect would have had a wingspan of about 20 centimetres.

Others that lived with it had wingspans of up to 50 centimetres.

And they were able to get this big because

there was more oxygen in the air.

It's incredible to think, isn't it, that this thing would have been

flying around a full 150 million years before birds?

But what about more recent times?

Well, they've had their share of human prejudice.

In The Middle Ages, dragonflies were associated with evil.

Humans believed them to be in cahoots with snakes

and able to wake them from the dead.

A folk tale tells of the Devil turning St George's horse

into a giant evil dragonfly.

Humans also believed that women who scolded their husbands

and men who cursed, might have their mouths sewn shut

by a Devil's darning needle.

Perhaps such a sinister reputation isn't surprising.

But what do dragonflies see in Britain?

Dragonflies are extremely accomplished aerial predators.

And because there's an abundance of good quality aerial insect prey

across much of Britain,

they've managed to colonise just about every corner of it.

But the story is a little bit more interesting than that,

and if we look at the distribution of individual dragonfly species,

because each of Britain's 24 species of dragonfly

has a very different view on where to live.

Some species are common and widespread,

such as the Four-Spotted Chaser and the Emperor.

But others are very picky, such as the Common Clubtail,

which actually isn't common at all, apart from on a few rivers in Wales.

And then there's the Norfolk Hawker,

which is only happy on the Norfolk Broads.

But to understand why different dragonflies prefer

different parts of Britain,

we have to enter the weird world of the dragonfly.

If you take a look around the edge of this pool,

down in amongst the emergent vegetation,

you might find something which is utterly remarkable, truly bizarre

and provides us with a real clue

as to why dragonflies need very particular habitats.

This is a dragonfly larva,

the form in which a dragonfly spends most of its life.

They're terrors of the underwater world,

equipped with one of nature's most vicious weapons.

A double-hinged jaw that flips forward to impale its prey

in just two hundredths of a second.

Despite its ferocity, the larva is very sensitive to its surroundings.

It needs particular varieties of plants and prey,

along with the right levels of oxygen and acidity in the water.

Only if these things are right

will it grow and then one day venture out of its watery nursery

to make one of nature's most incredible transformations.

So, when it comes to the distribution

of the UK's dragonfly species,

it's actually down to the tastes of their larvae.

It's that that governs where these dragonflies live.

This is a White-faced Darter, a superb dragonfly.

It's a male, and you can see clearly here how it got its name.

Apart from its face, it's dark-coloured

so it can warm up quickly,

making it well-adapted to the cool British climate.

But one thing that might strike you as unusual about this species

is that you can only find it in five sites in England,

the most southerly of which is here, at Chartley Moss. But why?

Well, this corner of Staffordshire

has exactly the conditions that a White-faced Darter larva needs,

and here is what makes this place so special.

Whilst it might look like a piece of old heathland,

I can tell you that, with no divine capabilities,

I'm walking across the surface of a lake.

Only the good news for me is that it's an underground lake.

Look at this, you can see the ground rippling underneath me.

And here, the cover is just about a metre of sphagnum

and then 16 metres of...water.

Yeah, that's quite enough.

It's covered with a thick layer of this stuff,

sphagnum moss, a hugely absorbent species.

Just look at the amount of water

that I can squeeze out of just one handful.

But sphagnum isn't just highly absorbent.

It also makes the water acidic

and this is perfect for the White-faced Darter larva.

What's peculiar about White-faced Darter larvae

is they're not very good with predators.

Rather than freeze when they're approached and avoid detection,

they try to wriggle away, and fish in particular gobble them up.

But the water here is way too acidic for fish,

so there are no predators in it.

And furthermore, the White-faced Darters don't live in the open pool,

they avoid competition with all of the other dragonfly larvae

by living in amongst the sphagnum moss.

They could even be under my feet right now.

So, for White-faced Darter dragonflies,

this is heaven.

Hmm, the only problem is, this type of acid heaven is disappearing.

Over the past 40 years, humans have dug up 95% of acid peat bogs,

mainly to make garden compost.

But whilst bog-loving dragonflies have fewer places to live,

other species find modern Britain more inviting than ever.

Thousands of gravel pits, quarries and opencast mines

have been flooded to create prime real estate for dragonfly larvae.

And here, dragonflies such as the Emperor

and Four-spotted Chaser are thriving.

And global warming is actually helping, too.

Since the 1980s,

almost all of Britain's dragonflies have expanded northwards,

and five new species have arrived from the warmer climates of Southern Europe.

I'm pleased to say that dragonflies are continuing

their 320-million-year reign on these islands.

Imagine you were to wake up one morning and come face-to-face

with the meanest, most terrifying animal in your world staring at you

across the breakfast table.

And you were its breakfast!

Well, this is a fate that's been played out

on rivers all over Britain

to what could be described as our most put-upon mammal.

A creature that's declined by 97% in the last 20 years.

Now, I know what you're thinking.

This now can't possibly be a romantic, happy tale

from the riverbank.

But, honestly, stay with us, because there IS a happy ending.

The water vole.

Surely our cutest British rodent.

Many humans will know them as water rats, but they are definitely voles.

All water voles need to set up home is a river to swim in,

nice, soft banks to burrow in and plenty of grass and roots to eat.

A water vole's home is its castle.

They construct networks of tunnels and chambers

extending up to five metres into a bankside,

with specialised areas for storing food,

sleeping and nesting, where they produce up to 40 offspring a year.

There are two or more entrances, including one by the water,

for emergency exits...

..because a tasty water vole can never afford to let its guard drop.

From the land, the stoat.

The water vole runs for the tunnel

but the stoat's flexible body can follow.

The only escape is a quick dive into the water.

From the water, the voracious pike.

The quick-acting water vole scrambles to safety just in time.

From the air, the barn owl.

But once again, the water vole makes its swift escape.

Now, the reason that water voles

are so good at avoiding their predator compatriots

is that they've been in Britain for a very long time.

Around 10,000 years ago, after the ice sheets retreated,

water voles from Southern Europe made the epic journey

across an ancient land bridge, to colonise Britain.

And these were water voles were black.

Then, several thousand years later,

they were pushed north by the BROWN water vole from the Balkans.

Eventually a truce was reached,

where the black voles hung on to Scotland

and the brown invaders claimed England and Wales.

At the start of the 20th century, there were over

eight million water voles thriving in Britain's waterways.

And in 1908, Ratty the water vole became a superstar.

But sadly, fame didn't do the water voles any good.

First there were two world wars.

Humans, desperate for food, stripped bankside vegetation for crops

and allowed cattle to trample water voles' homes.

But worse was to come.

A water vole apocalypse.

So what's happening with the water voles?

You know, when I was a kid, if I'd have wandered up the River Tale

here in Devon, I'd have been listening for this sound...

..the characteristic plop of a water vole diving beneath the surface

and finding shelter in its burrow.

But in 2002, water voles were declared extinct in the Southwest.

That's the whole of Devon and Cornwall, two large counties.

But how could an animal so prolific at procreating,

so security-conscious, completely disappear? Here's a clue.

You may be interested to know

that it's been simply pouring into this country.

Yes, mink coats.

In the 1920s, the North American mink was imported into the UK

to support the British fur trade,

and by the 1970s there were no less than 800 mink ranches in Britain,

some of them with as many as 5,000 animals,

each one of them a hungry predator.

And not only did some manage to escape, but in the '80s and '90s,

thousands were released by animal rights activists.

And ever since, the American mink has been on the rise

in the wild in Britain.

Fast and agile swimmers,

they can attack water voles from water or land.

And they won't stop until they've eaten them all.

Since the 1950s, mink have eaten their way across Britain,

and many black and brown water vole populations have been annihilated.

But the mink kill other animals too, such as water fowl,

and cause so much carnage that an army of humans

has decided to take them on.

Large chunks of Britain have been cordoned off,

creating areas to be defended at all costs,

where mink are trapped and killed.

And the first of these was here in the Southwest.

The surge has been so successful that the mink have been pushed back.

So today's a great day because water voles are being re-introduced.

How's it going, Mervyn?

Yep, pretty good, thanks, Chris.

I've done one hole, and we're just nearly finishing the other.

So if you'd like to finish it off for us.

OK. Perfect. I'll go and get some water voles.

I've not done a lot of augering but I'll do my best.

Pushing and turning at the same time. I think even I can manage that.

The principle here is that we're digging starter burrows

for these animals, somewhere to hide from predators

whilst they settle down to their new life in the wild.

Right. All we need now are the voles.

Look at that. What a beauty.

This is one of this year's young.

It currently weighs just over 100 grams, which is

a small vole, believe it or not.

How are we going to get this animal into those burrows

without risking losing it struggling across the ditch?

It's the trusty crisp tube, Chris.

The trusty crisp tube!

Very Blue Peter, but I presume it works. Now, in you go.

He's gone in there a treat. All right, let's put him in.

Right, do you want to stick yours in, then, Mervyn?

How many have you released here?

Well, this year, about 140.

I think he's gone in, Mervyn, straightaway.

'As long as mink are kept at bay, water voles have a good chance

'of reclaiming this piece of England.'

If it works, in the future there'll be lots of little voles

paddling up and down this pleasant English tributary.

Let's hope that the brown water voles have been rescued

in the nick of time.

But to get a complete water vole's picture of Britain,

we need to see how the black water voles of Scotland

are coping with mink.

This is Loch Muick on the Balmoral estate,

where I've come to investigate a very peculiar triangle of murder.

Researchers here on the Balmoral estate have established that where

there are lots of rabbits - rabbits - there are no water voles.

But where there aren't any rabbits, there are lots of voles.

So, my nature detectives,

this might imply that rabbits are eating the voles. The plot thickens.

But rabbits aren't carnivores, so they can't be responsible.

I suspect that somehow the mink is at the bottom

of this rodent irregularity.

What I need is a sort of Inspector Poirot for water voles.

So I'm meeting one of Britain's leading experts

who, perhaps reassuringly, also happens to be Belgian.

'Xavier Lambin has been investigating

'the black water voles of Balmoral for 12 years.'

Let's check what we have. There she is.

Oh, my goodness, look at that!

What an animal, honestly! I've never seen a black water vole before.

They're beautiful beasts. Lovely charcoal, brilliant black.

I'm giving it points over the brown southern variety, I have to say.

And what's the story between the water voles,

the rabbits and the mink?

Because clearly, the rabbits aren't eating the water voles.

In the presence of American mink,

we don't find water voles within 5km of a rabbit colony,

a rabbit warren.

Ah, so it IS the mink that are making the water voles disappear?

Yes.

It's quite difficult for a mink to make a living in the uplands,

so when they have rabbits,

they are able to spread further than what they would otherwise do.

In doing so, that brings them closer

to the surviving water vole colonies.

So basically, the mink need the rabbits to sustain them in any area

and whilst hunting those rabbits,

they also snack on any voles they find?

Yes.

How did these voles survive,

because presumably this area is pretty good for rabbits?

Where did they go when the mink arrived?

If you go up in the hills there,

there are very extensive populations of water voles there.

So the voles went right to the top of the mountains?

As high as they could go. This was the refuge they could find,

and we've managed to remove the mink

from nearly 10,000 square kilometres

and now we see the first sign of recovery.

Voles are coming down from the hills and are reclaiming the ground,

from which they were excluded previously by American mink.

Look at that! What a top animal.

We should let her go.

Oh, what a beautiful animal!

What an absolute stunner.

Let's hope she's part of this whole recolonisation process.

So, the British water vole story is one that reveals how one small

change to our waterways can have devastating effects.

Thankfully, now mink are being kept under control,

from a water vole's perspective,

Britain is once again starting to look like a great place to live.

So far all of the animals we've been looking at spend most of

their lives in or beside the water,

but to truly get to grips with the state of Britain's waterways,

what we need to do is consider a creature that spends all of its life

beneath the surface of that water.

I think I've got an ideal candidate. It's an effective predator,

it's elusive prey and it tastes good on a plate.

Yes, it's a fish, and not to be confused with the salmon.

This is a trout.

Just look at it - beautiful animal.

Wonderful speckling across the top of its back and down its sides.

So how did the trout

come to be such an important resident of British waterways?

10,000 years ago, as the ice melted,

an inquisitive trout saw an opportunity, and it wasn't long

before almost every river had a healthy population.

8,000 years later, and the Romans also took a liking

to British waterways, building cities along the best trout rivers.

They also brought fly-fishing.

1,400 years after that, the British Empire spread.

Trout and the art of fly-fishing followed until trout from Britain

swam in rivers across five continents.

But with the industrialisation of Britain's waterways,

trout disappeared from many of our once great rivers.

But here is one river,

away from the main industrialised areas, that has survived.

The Itchen in Hampshire.

One of the first to have been fished by the Romans

and still one of the world's top trout habitats.

So where better to come to understand

what is a very unusual fish?

Perhaps the weirdest thing about trout is that this species of fish

can basically lay an egg which can develop into

either one of two completely different types of animal.

Just think about that. And which type is determined by the habitat.

Now, here in the cool, clear waters of the Itchen, where there's masses

of invertebrate life, good food for trout,

paradoxically you find the smaller of the two.

It's a thing called the wild brown trout.

This is one of over a dozen similar streams in the South of England

where there's enough food, such as caddis fly and mayflies,

to sustain brown trout for all of their life cycle.

And the reason it's so fertile is down to the geology.

You see, the rock beneath the crystal-clear waters

of the Itchen here is chalk, and it's very porous,

so when it rains on the surrounding hills,

all the water seeps through this into underground aquifers.

And it's these that feed the sources

of these type of chalk-stream rivers.

And there's a benefit here, because the water

is pretty much the same temperature throughout the course of the year.

It doesn't vary seasonably or due to warm water running off the hills.

And this means that it's a fantastic place for masses of

aquatic plants and, living on these, masses of aquatic insects.

And these in turn are great food for trout.

But even here in brown-trout paradise,

there's a problem on the horizon.

You see, a third of Britain's human population lives in the Southeast

and they all need water.

Much of it comes from the same aquifers

that feed the chalk streams.

As a result, the chalk streams are drying up.

And as anyone boiling a half-filled kettle will know,

less water warms up faster, and no trout wants a warm stream.

But if things get really bad,

this species has a cunning survival strategy,

because trout are capable of surviving

in a very different environment from this.

Just 25 miles south and look,

this New Forest stream is a very different place.

Conditions are different in the water too.

You can see it's much murkier.

There's a lot less aquatic vegetation in there, it's warmer,

and critically, it's more acidic too.

So it's home to a very different type of trout.

They can be very hard to see in this murky water, but we're in luck.

A shoal has just risen to the surface.

These fish are typically twice as big as normal brown trout.

So the question is, then, why and how do they get so big?

Well, these things are sea trout, and the clue is in their name.

Yes, all over Britain, there are rivers like this New Forest stream,

which are poor in food.

But giant trout still live here, and this is how.

Leaving en masse, the young fish head downriver...

..to become sea trout.

And here they can gorge on the plentiful seafood

before heading back to the river of their birth.

So the question is,

why on Earth do the sea trout bother to come back to it?

Well, as with so many puzzling aspects of life,

it comes down to...sex.

They gather in the estuaries

before making the epic journey back upriver to breed.

They need a clean gravel bed where their eggs can be safely anchored

under the cover of a bit of vegetation.

Because of their large size, they can lay up to 38,000 eggs,

so their time of feeding at sea has paid off.

It strikes me as absolutely incredible

that these two types of trout are, in fact, the same species.

They can be genetically identical.

It's sort of a case of brothers and sisters making lifestyle choices.

One decides to be the stay-at-home brown trout,

the other, the more adventurous sea trout.

And these two strategies make British trout more resilient.

Even though wild brown trout are declining in some chalk streams,

elsewhere the sea trout are recolonising industrial rivers

as they're cleaned up.

Already they've returned to the Thames, the Taff and the Tyne.

It's easy to forget, but each of the freshwater creatures

that we've looked at so far does, in fact, exert a positive effect

on the world it shares with humans.

Think about the water voles. They naturally plough and fertilise

the banks of the streams where they live.

Then there are dragonflies. Their larvae are great food for fish.

Fish like trout, which we like eating, and so do ospreys.

And when ospreys eat them,

they spread those nutrients into the wider environment.

Now, I know what you're thinking. All this is terribly subtle

and not at all obvious.

But you know, there is one classic freshwater animal whose effects on

the natural environment are far from subtle.

In fact, they've always been hugely controversial.

The beaver.

An animal with the ability to build its own habitat,

a bit like us humans.

Its view of Britain is going to be especially fascinating because,

having been extinct here for some 400 years,

it's now on the verge of being reintroduced.

So let's take a look at what makes a beaver a beaver.

This one, Peter, is a babe, in both senses.

It's a youngster, of course, and it's beautiful.

It's about 14 weeks old, this fella,

so he's still very much a youngster and that's why he's

still chilled and happy to be held like he is.

He's got all the beaver features already. He has.

He's just a miniature version of the adult.

Look at these back feet. I mean, look at that spread of webbing.

He can swim fantastically fast, manoeuvre really well.

Look at this tail. I mean, this is an amazing adaptation, isn't it?

Mmm. It's a little bag of muscle in there

and that is what gives them a huge thrust.

What about their teeth? They're huge and incredibly tough, aren't they?

That's the business end of a beaver. Their teeth does all their real work.

They're fantastically adapted.

They are like two sets of chisels,

constantly self-sharpening to a razor edge. What about the coat, though?

I mean, this is something else.

These long guard hairs keep it waterproof, but beneath it

there's this really thick downy layer,

which is going to keep them warm, isn't it?

This is the softest, most lovely fur you could ever feel.

And that's unfortunately one of the reasons they went extinct.

The extinction of beavers in Britain was the unhappy last chapter

of a story which had started so well.

Thousands of years ago,

beavers may have paved the way for humans to settle Ice Age Britain,

providing thick fur coats and energy-rich meat.

But gradually, human demand for beaver parts escalated.

Beaver teeth made excellent tools.

And from Roman times, they were hunted both for castoreum -

the anal secretion used to mark their territory -

and for their testicles.

It was believed that these had painkilling properties.

And medieval humans were convinced that a desperate beaver would even

bite off its own testicles, leaving them for the hunter,

in return, saving its own life.

But the thing that wiped out the beaver was its fur.

One of the last ended up as a hat for Henry VIII.

In the 1600s, the beaver finally went extinct in Britain.

All that was left were a few archaeological remains

and place names such as Beverley, which means Beaver Stream.

So beavers disappeared 400 years ago.

But does it really matter to anything other than a beaver?

After all, we humans now have aspirin.

Well, it could be that the beaver still has a lot to offer Britain.

In recent times, some humans of the scientist variety have

concluded that the way beavers modify the environment

is good for many other species of wildlife.

And to get an idea of what we might expect from a British beaver,

here are some of its American cousins in action.

The first thing the beavers want to do is to flood the area

by building a dam.

Each day, a beaver can shift ten times its own body weight.

That's 200kg.

As the dam grows, the water rises.

But you know, they're not just dam builders.

The beavers then excavate a whole network of channels

leading to a supply of trees, whose bark and leaves they like to eat.

Using their strong teeth, they can cut down up to 200 trees a year.

They float them back to where they can build a lodge to live in,

and raise a family safe from predators.

The by-product of all this beaver land management

is a fantastic new habitat for fish, insects and birds.

So beavers hugely improve the health of the whole ecosystem.

As a consequence of that, they've been reintroduced

into many Western European countries.

There are only a handful of exceptions - Albania and...

Britain. Yes, Britain!

Or, to be precise, Britain, with one tiny exception.

An exciting but highly controversial trial

tucked away in a remote part of Scotland.

This is Knapdale in Argyll, Western Scotland.

And it was here, in May of 2009, that beavers were translocated

from Norway into an unfenced area of Scotland -

that is, into the wild -

for the first time in Britain in 400 years. So, why this part of Britain?

Well, from a beaver's point of view, it's fantastic.

Fertile lochs surrounded by acres of its favourite food -

alder, rowan and willow trees.

And from the human point of view,

it's miles away from any large populations.

And one thing's clear - the beavers are already rebuilding the place.

This may look like a pile of brash

you might find at the bottom of your grandfather's garden, but it isn't.

It's the structural strength of this beaver dam.

And all of this flooded woodland is brilliant for biodiversity.

And look at this too. The beavers have toppled this over

but already there's lots of natural regrowth, productivity.

Deer are coming in and nibbling away and enjoying this.

And when you think about it, we did this for years.

This is just natural coppicing.

And by coppicing, these beavers are creating more space, light

and new growth in the surrounding woodland.

But you can understand, I suppose, why some humans aren't keen.

Imagine if this flooded woodland was in your backyard.

Well, here in Scotland, tempers have flared.

So I'm anxious to see how well these beavers are settling in

to this corner of Britain.

I've joined Roisin Campbell-Palmer of the Scottish Wildlife Trust.

That's it. That's the lodge, isn't it?

Yes, that's right. It's quite well camouflaged.

It's very well camouflaged.

We've waited until dusk.

And we need to be extremely quiet.

Wow. Look at that.

If that one's feeling relaxed enough, it might even

come out onto the bank and start feeding.

That's fantastic.

It's my first British beaver. Is it?

My first British beaver. I'm so glad you've seen it.

I never thought I'd see it. Really?

Well, I've lived through years of delay, hesitancy, bickering.

But now that it's happening, I can't tell you, I'm so excited.

It's going over to the bank.

If we're quiet, we might see it come out.

(Just look at that.

(It really doesn't come any better

(when it comes to British wildlife watching.

(You can hear and see this beaver who's just

(25 metres in front of the canoe, gnawing on a piece of rowan tree.

(I didn't come with any expectations of sitting so close

(to one of these animals and being able to observe it like this.

(In fact, here's another beaver. Another beaver's coming in.

(Oh, they've joined up on the shore.

(There seems to be a bit of bickering going on about the branch.

(In fact...

(..the beaver that's come in has stolen the branch.

(It's dragging that piece of rowan across the top of the loch.

(It's trailing out behind it.

(Look at that!

(To get so close to one of these reintroduced animals,

(to be able to actually hear it gnawing away

(and then see it on the surface.

(Honestly, the project gets my thumbs up, big-time.)

But maybe the strongest sign that the beavers are happy here

is that they've already bred.

This is a rare glimpse of one of Britain's first wild baby beavers,

born in 2010.

As long as they have a supply of water and trees, beavers can create

their own habitat and live virtually anywhere.

So for them, Britain is a land of opportunity, free from

competition and predators, just waiting to be settled.

But whether beavers make a comeback or not

will depend on whether humans are prepared to share the UK

with another species that likes to redesign the landscape.

What really strikes me about our freshwater animals is

that their lives are woven together with ours,

through our mutual need for healthy freshwater habitats.

It's not just that they are dependent on us.

We need them too.

But if they could speak, I wonder what they'd say to us?

I wouldn't mind betting they'd say, "Keep the water pure and clean,"

and, "No more introductions of dangerous non-native animals."

But surely the main thing, the thing they'd be

crying out for, begging for, is more good-quality freshwater habitats.

Perhaps bigger, better wetland nature reserves.

But then, hey, it's also down to me and you, and if we were to

put anything, from a puddle to a pond in our back garden,

I'm sure they'd appreciate that.

Next time on The Animal's Guide To Britain, our grassland animals...

Off you go!

..their astonishing capabilities...

Look at that.

..and their surprising view of our landscape...

What a fantastic thing.

..in Britain's most dynamic habitat.

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