-STEVE BACKSHALL: Whales... -(WHALES CALLING) ...ocean giants, ancient mariners.

Their songs, majesty, and sheer size fill us with joy and awe.

In their vast shadow, a human can feel very small.

(WHALE CALLING) They were here long before us.

Caring for their young, hunting, voyaging.

But now we are changing their world.

So now I want to see through their eyes, meeting them on their terms to find how their future and ours are inseparable.

Whales are the largest mammals on the planet.

Their streamlined bodies, powerful tail flukes, and huge reserves of energy make them perfectly evolved for a life of ocean-crossing voyages... -migrations.

-(WHALE CALLING) Whales undertake the most epic journeys of any animals with some species travelling thousands of miles every year following routes their ancestors have used for generations.

For centuries, their seasonal journeys and the reasons they chose certain destinations, were a mystery.

Now, for the first time, science has mapped their migrations revealing whale superhighways that criss-cross our planet.

I want to know if this offers new insights into why whales travel so far, and what's happening as they try to navigate our changing oceans.

I'm starting my journey in Tahiti, in French Polynesia, an important hotspot on the whale migration map.

Tahiti is an island nation surrounded by miles and miles of open Pacific, perhaps best known for its dramatic and iconic surf.

The swell that creates these waves has travelled for thousands of kilometres to reach here.

But humpback whales, one of our planet's greatest migrants, travel even further, all the way from the chilly waters of Antarctica to come here for the most important moment of their lives.

This may seem like paradise to us, but Antarctica is where the humpback's food is.

So why do females leave that behind and travel thousands of kilometres to give birth here?

This is the most commonly-seen sight of the humpback.

Indeed, it's what gives it its name.

That distinctive humpback shape.

Right now, this female is just hanging at the surface, taking it easy while the small calf moves around her.

This mother has earned her res.

It took three months to swim here, and then she gave birth to her one tonne baby.

(WHALES CALLING) I want to see how she's getting on.

(WHALE TRILLING) Down here, I can feel the reason Tahiti is so appealing.

The water is calm and warm.

Coming here means she can spend less energy keeping herself warm, and more energy nursing her calf.

It looks so calm and tranquil.

For the female though, these are really tough times.

While she's here feeding her calf, she's spending enormous amounts of energy and not feeding at all.

Her fasting can last nine months, so she must conserve every bit of energy.

While she rests, her calf is free to discover the world around it.

(WHALE TRILLING) The female is almost saying, "Go on, now.

"Go and play with your new friends."

Under mum's watchful eye, the calf uses its newfound freedom to play.

Just a few months old, this calf is getting to know its own body.

Soon, it must swim to Antarctia on its first migration.

The calf just wouldn't leave us alone.

It's one of those once-in-a-lifetime moments when an animal sees you, seeks out your company, and wants to interact with you.

It's totally on their terms.

I think that's probably, in 25 years working with animals, the best thing that's ever happened to me.

Whales travel here because these waters offer optimum conditions but we are changing that.

The planet is heating up.

Recently the world's ocean surface temperature was at its highest since records began.

If the waters warm beyond their tolerance, humpbacks could lose these perfect nurseries forever.

Over to the northeast of the Pacific, grey whales face one of the longest journeys of all.

Up to 10,000 kilometres from Alaska all the way to Mexico.

It's winter, and this female has been heading south for two months.

Her goal was to reach warm, safe waters before she gave birth.

But her calf has been born on the way.

It's the last thing she wanted.

Changing sea ice in her Alaskan feeding grounds impacted the food she relies o. Forced to feed further north, her migration is longer than normal and taking more time.

Just days old, her baby is unprepared for the journey.

But they must keep moving.

She tries to support him.

But he is still weak.

Their plight was filmed off the Californian coast, one thousand kilometres short of their safer Mexican birthing lagoons.

It's not known if they ever made it.

Whales need better protection for their habitats and the superhighways that connect them.

I want to see what's being done.

The peoples of Polynesia are some of the world's finest seafarers, able to cross entire oceans in small boats not much bigger than this, va'a, or outrigger.

Just as they sometimes had to leave the sanctuary of the lagoons and head out into the terrors of the big blue, the humpbacks, too, can't stay here forever.

They're going to have to set off on one of the most dangerous journeys in nature.

While it's an ideal nursery, Tahiti offers nothing in the way of food.

This female has barely eaten since she left Antarctica six months ago.

She must now return to the freezing waters of the south.

(WHALE CALLING) Her young calf will join her.

It's a huge undertaking swimming over 6,000 kilometres through open ocean.

To survive, the calf will need to be in peak physical condition.

Two here.

Two here.

Mother and calf, I think, breaching in front of us.

Breaching is a really interesting form of behaviour.

Here in these waters, you often see the youngsters, the calves breaching and they'll go over and over and over again.

And the energy they need to expend to get out of the water is immense.

Research suggests this could be a form of training.

Building the strength needed for the vast migration that lies ahead.

Eventually the youngsters are strong enough to make their epic journey to Antarctica, but it's a trip that's fraught with danger.

The discovery of whale superhighways comes at a critical time.

As their long journeys put them right in the path of our increasing boat traffic.

Here in Nanuu Bay on Tahiti's North coast, the problem is clear.

For humpback whale calves, it's incredibly challenging.

They can't dive down as deep as their mothers.

They spend much more time nearer the surface, and ferries like this are heading back and forth between these islands all the time, putting these animals in constant peril.

The risk of collision turns this sanctuary into a danger zone.

A boat this size has hundreds of tonnes of force behind it.

Being struck by that is going to be fatal for any whale no matter how big it is.

Around the world, ship strikes kill dozens of whales each yea.

The number of vessels is risin.

By 2050, shipping traffic is projected to increase up to 1,000%.

For migratory whales, their ocean world has become a minefield.

The search for solutions is underway and one success story can be found in New Zealand.

(SHIP HORN BLARING) The Port of Auckland was a whale-collision hotspot.

The shipping routes plough right through the habitat of resident Bryde's whales.

These sleek hunters consume schooling fish in shallow waters, when they can catch them.

But this means they spend a lot of time near the surface in the path of passing ships.

Professor Rochelle Constantine is an expert in marine mammal conservation.

CONSTANTINE: That's an example of the ships that, you know, transit through the Gulf.

If that ship was going full-speed, the whales would stand no chance if they were in the path of the ship.

About 15 years ago, we had around two to three whales a year washing ashore with injuries consistent with a ship strike.

BACKSHALL: Her solution was simple, to persuade shipping companies to reduce speeds to just 10 knots in specific zones.

CONSTANTINE: Since they've slowed down, no whales have been found dead from ship strike.

Whales have time to get out of the way.

Such a great conservation win.

BACKSHALL: Helping whales doesn't always have to be complicated or costly.

And the more we discover about their superhighways, the better we're able to make positive change.

Whales migrate for many reasons... and one of the strongest motivators, is the need to mate.

This is a Southern right whale.

The white callosities on their head are as unique as fingerprints.

(WHALE TRILLS) This one is a female.

(WHALE CALLS) She's travelled thousands of kilometres from the Southern ocean, to Argentina's Valdes Peninsul.

She's not alone.

Over 1,500 Southern rights arrive each year in late summer.

Calm shallow bays offer ideal conditions to breed in, and to nurture newborn babies.

But not every female is here to give birth.

This one is looking to meet a wide range of male suitors.

She has no intention of mating with the first male to approac.

Instead, she attracts several males who have to compete for her attention.

(WHALE CALLING) Rolling on her back, she keeps her genital region out of reach until she's ready.

She's looking to test their agility and stamina.

When a strong suitor has jostld into the closest position, she rolls onto her front.

(WHALE CALLING) This is his chance to mate with her.

But when another male takes his place, she mates with him too, sometimes repeatedly.

And the competition for paternity continues inside her body.

Their one-tonne testicles produce huge quantities of sperm, to displace that of their rivals.

It's all part of her strategy.

Males that mate the most times and produce the strongest sperm have the best chance of fathering her next calf.

These waters are also vital to Southern right whales for calving.

But infant mortality is on the rise here and I want to find out why.

Human activity is transforming the oceans and whales are facing the consequences.

(WHALE CLICKING) In Argentina, calm, shallow waters used to offer newborn calves safety from predators, but that is changing.

Just metres from shore, are a Southern right whale mum and calf.

The mother travelled up to 3,000 kilometres to find a safe, quiet place to give birth and nurture her baby.

She provides up to 300 litres of milk a day.

But the peace of this tranquil hideaway is being shattered.

Kelp gulls have learned to feed on the skin and blubber of whales.

And, as calves need to surface more frequently to breathe, they suffer the most.

Although it's the gulls that do the damage we've played our part.

Birds, first attracted here by waste from landfill and fish processing, have developed a taste for whale flesh.

Now, the back of almost every calf carries open wounds.

Time for resting and feeding, is lost to avoiding gulls.

And calf mortality is on the rise.

Soon, these infants must embark on the long migration to the feeding grounds... their mothers came from.

The most injured are the least likely to surviv.

We've come a long way in our knowledge of whale superhighways, but we need to know more.

Rapidly advancing technology could help further our understanding.

A good place to see for myself is this vital pit stop for whales in the Atlantic, the Azores.

This is a social group of female sperm whales with a couple of youngsters and juveniles.

Their large heads hold a brain six times the size of a human's, but our knowledge of how they use it is limited.

But one of the ways that science is starting to unravel the mysteries of the sperm whale's secret life is using sound.

I have here a highly-directional hydrophone.

It's essentially a normal hydrophone welded onto a dog's bowl.

I'm gonna listen out and see if I can hear anything.

(TICKING SOFTLY) Ah, there, there, there... (RHYTHMIC CLICKING) (RAPID CLICKING) So what I'm hearing is a ticking sound.

This is coda, which is a kind of dialect, a language, that females use when they're at the surface.

These series of clicks are exchanged in social encounters, much like people talking when they meet.

(CLICKING) Some codas are highly variable.

(WHALES CLICKING) But others are more uniform.

A dialect shared by all whales in the same region.

While we understand the basics, their full meaning has always been beyond our grasp.

But now, scientists are using artificial intelligence to try to decode coda.

The results could transform our understanding of sperm whales.

And one day, may even enable us to talk to them.

Elsewhere, other high-tech toos are shedding light on whales' long-distance journeys.

Until recently, tracking devics that give us data while they're on the move were fairly limited.

Pioneering scientist, Dr Iain Kerr, has made it his mission to change that.

KERR: I've dedicated 32 years of my life to whales.

We developed a system whereby we actually attach a data and a camera tag to a whale using a drone.

BACKSHALL: In Mexico's Sea of Cortez, he and his team are looking for the largest whale of all, the blue whale.

So let's come out here, head up along the inside of the coast, where we can see the blows against the shoreline.

MAN: Here, we can fly the drone, -no problem.

-KERR: Exactly.

(SPEAKING SPANISH) BACKSHALL: (IN ENGLISH) Many blue whales migrate here in winter, from the northern Pacific Ocea.

Oh, hold on.

Over here.

Yeah.

There's a whale over here.

-KERR: Where?

-To the right of the island.

All right.

The game is on.

BACKSHALL: Iain's technology could help show how they are reacting to human influences like noise and shipping.

Tags like this are gonna definitively tell us what affects the whale.

BACKSHALL: The tags record a whale's movement and orientation in the water.

But that's not all.

So we've got a heart rate monitoring tag on here now, which is pretty exciting.

BACKSHALL: They also give information about the whale's health while it's swimming.

It's been done before, but never done with a drone.

So, fingers crossed.

BACKSHALL: Using drones reduces the need to get close to whales with boats.

KERR: Good job, team.

Herat rate monitoring tag is on a whale deployed by a drone.

First time ever.

All right.

BACKSHALL: The tag causes no harm and will automatically detach in several hours.

We collect data in the hope that we could affect change.

The exciting thing for us now is to assemble this data into a picture that makes sens.

Not just for understanding how they live their lives, but how we can help conserve these animals.

BACKSHALL: As more whales are tagged... KERR: Tag on.

Yay!

(LAUGHS) ...Iain's getting new insights into behaviour.

How whales are affected by humans... and where and when we should focus our efforts to protect them.

To find out more about change, I've headed north.

The Arctic Ocean is critically important for the survival of many whale species, with seasonal abundance of everything from plankton to algae to fish, drawing in long-distance migrants from thousands of miles away.

But the Arctic is changing faster due to climate change than anywhere else.

How will this impact migrating whales?

BACKSHALL: My journey following whale superhighways began in the tropics.

Now, I'm in Norway, a seasonal destination for migrating humpbacks of the North Atlantic.

These Arctic seas are completely mirror flat.

But out in the fjord, there is a frenzy of activity taking place.

(WHALES CALLING) Humpbacks feed on krill and small fish, such as herring and capelin.

Darkening the water, shoals can contain hundreds of millions of fish.

A rich reward at the end of their long journey, ten thousand humpbacks come to feast.

While here, they must store enough energy to last the next six months.

Having swum thousands of kilometres to get here, it won't be long before they'll have to return to breeding and birthing grounds in Cape Verde and the Caribbea.

To see what state they're in after the journey, I need to get nearer.

I'm going to take a paddle board and coast in, and hopefully that should give us an intimate glimpse of these animals resting at the surface.

I have to be really careful not to disturb them at this critical time.

So I intend to wait and see if they come near me.

The spouts are going off like cannon fire, one after the other.

Their billowy spray just covers you.

Whoa!

They're right underneath me!

Well, that was much, much closer than I expected or wanted.

Totally overwhelming.

To us, the Arctic Ocean may seem so challenging, so brutal, but to the humpbacks, this is an oasis.

It's truly one of the most impressive spectacles on our planet.

So we're getting towards the last light of the day, even though it's no more than half past 1:00 in the afternoon.

There could be 50, or even 60 humpbacks here now and they've assembled around our boat.

It's a very rare opportunity.

Don't often get this amount of animals packed in close together like this.

MAN 1: Jump, guys.

MAN 2: All good.

MAN 1: All three in the water!

BACKSHALL: This is one of the few places on Earth where I've seen humpbacks in these numbers.

They look healthy and well fed.

But Arctic waters are getting warmer.

And I want to know if these whales are being affected.

(SPEAKING INDISTINCTLY) Professor of marine biology, Audun Rikardsen, is investigating this population.

Now we're off to taking biopsies, skin samples from humpbacks.

BACKSHALL: His research gathers a range of information on individuals.

Then we need to load this.

This is the biopsy where we take the skin sample and also get some blubber.

Okay.

Go, go, go, go.

That direction.

Follow my hand.

Follow, follow.

Yeah, keep that.

Yeah.

There.

BACKSHALL: The biopsies are harmless... RIKARDSEN: We got it?

Yeah.

I have it.

I have it.

...and provide important data.

So here.

That's the blubber.

On the inside, here, is also a piece of skin.

This is a good sample.

BACKSHALL: And his samples also reveal the sex of the humpbacks that migrate here.

RIKARDSEN: It seems to be mostly females that are here, and some of them are pregnant.

The females need that extra energy to feed the calves.

BACKSHALL: For pregnant females, it's vital to fuel up before heading to their birthing grounds.

But according to Audun, things are changing.

What we see when the ocean heats up, we see that several of these fish species, migrate north and they move their spawning areas.

So that means that the whales have to follow and for those that migrate a long distance, they will have a longer migratory distance.

Is this something that we are thinking will happen in the future?

Or is that something that's already happening now?

It's already happening now.

It will have a dramatic effect on the whales.

BACKSHALL: The movement of fish stocks is creating challenging, longer journeys for these migratory humpbacks, but they're not the only ones that rely on the herring.

Orca come here, as do we.

And I witnessed something I've never seen before.

There's a juvenile humpback, which has about ten orca around it.

It's right below us.

Just below us.

It's here, you can see the underside of the fins.

(CALLING AND CLICKING) This slapping with that giant tail.

That's its biggest defensive weapon.

It's hard to know why the orca are pursuing the humpbacks.

These are supposed to be fish-feeding orca, not focused on mammal prey.

But clearly, they've decided there's something about this humpback that interests them.

These species compete for food, so is this just bullying, or could the orca be actively targeting a weak individual?

This has really stepped up a notch.

And they're battering it, biting its fins and tail.

(WHALE CALLING) And you can hear its distress.

We don't know if competition for food will increase or if we'll see more of this extraordinary behaviou.

We do know, though, that our Arctic seas are changing faster than anywhere else on our planet, and the future certainly holds fresh challenges for the great nomads of our seas.

Throughout my journey, I've seen the difficulties migrating whales face on their travels.

Only those completing these voyages, can fulfil all of life's needs, from finding food, to mating, and raising their young.

Just as we start to understand the use of superhighways, they're being rewritten by our changing planet.

But if we can protect both their highways and their destinations, and if we can keep ocean temperatures within safe limits, we'll afford whales a fighting chance.

And secure their future for generations to come.

BACKSHALL: Of all the ocean voyages we set out to film, we knew one would be the hardest to get.

Pregnant humpbacks travel thousands of kilometres to give birth in the tropical waters of Tahiti.

But filming the intimate relationship between a mother and her newborn calf required the utmost sensitivit.

There's lots of whales out there, but the majority of them are not going to be interested in you.

So finding the right one is what it's all about.

Even then, you can't just jump in and swim up to them.

Any splash or strange noise could be startling.

To capture them on camera, you need to win their confidene and wait for them to come to you.

A mother and cub just came up right in between us and our support boat.

Whale expert and camera operatr Denis Lagrange was our guide to choosing the right whales.

What do you think, Denis?

Shall we get in?

LAGRANGE: I think maybe we should try a drop.

-You think?

-LAGRANGE: Yeah.

Let's go.

BACKSHALL: But when we got in the water, they weren't interested in us.

We headed over to the calf who was playing on the surface.

But the mum came up, just as we got there, and took it away.

A tantalising taste of what could be, for camera operator Rob Taylor.

TAYLOR: It's the first time we've been in with a calf.

And they just came up slowly in between us, had a breath and just slowly moved away.

It was amazing.

BACKSHALL: For the next six days, we scoured 100 kilometres of coastline looking for a mum and calf that were comfortable in our company.

We were just too late.

Just too late.

But though we had fleeting encounters... LAGRANGE: They were not stable.

They move as soon as we get into the water.

Yeah.

BACKSHALL: ...the whales never stayed long enough to give us what we needed.

This is getting tricky.

And time was running out.

We're on day six, and we still haven't had a decent encounter.

So we just keep hoping we're gonna get that one individual as interested in us as we are in them.

No one wanted to admit it might not happen.

As we head out on our last morning, suddenly a mother and calf appear just 100 meters from the harbour.

A pair of whales just popped up right alongside the boat.

And I do mean right alongside the boat.

Everything now rides on the decision about when to enter the water.

In this situation, as much as your heart's racing and you're just dying to just jump in and start swimming, it pays to be patient.

It's your call, Denis, you know them better than we do.

I don't know them personally.

(ALL LAUGHING) LAGRANGE: I'm sorry, Steve.

(CHUCKLES) I think we should give one more try.

BACKSHALL: Cautiously, we slip into the water.

(WHALE CALLING) BACKSHALL: The calf is fascinated with us.

It comes right up to the team and begins to twirl playfully.

It's the encounter of a lifetime.

And Denis and Rob finally get the shots wildlife camera teams dream about.

TAYLOR: That was unbelievable.

I was so close.

BACKSHALL: Enabling us to tell the story of these incredible voyagers, and why they travel so far to come here.

TAYLOR: As a human, you're going, "This is amazing" And as a cameraperson, you're going, "Is it recording?

Is it in focus?

"Is it exposed right?

Is Steve in the shot?"

The mother decided the encounter, you know, we didn't decide.

I'm so proud of the team.

You know, we've really stuck at this.

Ten days, we've been doing this, and we've just waited for the right moment and the right whale.

Didn't push it.

And it's all paid off.

(DRAMATIC MUSIC PLAYING)