NARRATOR: The Arctic is rapidly changing.

And with this change comes a remarkable shift in power at the top of the world, as solid ice turns to open sea.

One iconic Arctic hunter may soon have to give way to the surprising invasion of another.

MAN: This is probably the most terrifying predator since Tyrannosaurus Rex.

NARRATOR: Stories have just started to emerge, from those that live here, of these changing fortunes.

For the first time, Western science and Inuit knowledge are combining to witness and document the arrival in the Arctic of the most successful ocean predator of them all.

Narrator: This is the Arctic as we know it.

An ocean locked in ice.

Everything that survives here is supremely adapted to this frozen world.

Bowhead whales live here year round.

Wrapped in two feet of blubber, they are insulated from water temperatures of 30 degrees.

The narwhal's smooth shape and lack of dorsal fin allow it to swim easily among the ice floes.

Seal pups use the ice as a platform for resting and being fed... ...enticing the largest land carnivore on Earth out across the frozen sea.

The polar bear.

All through the winter, it stalks hunting grounds a thousand miles from land.

Without the ice, the polar bear could not survive here.

The ice is home.

With a solid surface beneath them and a strong sense of smell, polar bears have a great advantage over prey that sooner or later must come up to breathe.

It's the ice that lets them close in on their quarry.

Attacking from above, they smash through to get to seals hiding below.

Good hunting at this time of the year is essential to surviving the lean Arctic summers.

All animals that live here rely, in some way, on ice.

It's not just the polar bear.

But ice is becoming a fragile player on the Arctic stage.

This is one of the fastest-warming regions on Earth, and it's melting at a breathtaking rate.

Large parts of the Arctic that were once frozen all year are now free of ice in the summer.

There are many opinions on how much this place is changing, but it's well-documented that, in the last years, there's dramatically more water and less ice each summer.

And without sea ice throughout the year, polar bears are unable to hunt.

Hunger is forcing some bears to improvise.

In a desperate attempt at a meal, they climb up steep slopes to get to thick-billed murre nests.

It takes a lot of energy to reach these birds, and often there is no reward.

But this isn't just a story about melting sea ice.

On land, the effects of warming are also startling.

Just beneath the Arctic tundra is a layer of permanently frozen soil.

Only mosses, lichens, and grasses can take root here.

But as temperatures increase, parts of this frozen substrate are melting, allowing trees to grow, sometimes with dramatic results.

In the far Western Arctic, the summer of 2008 produced the first forest fire ever recorded here.

This in a region where local Inuit dialects don't even have a word for "forest fire."

In September 2012, Arctic summer ice cover reached a record low.

The polar bear's world is disappearing beneath its feet.

And as it does, stories are emerging of the most powerful marine mammal on the planet taking advantage.

For thousands of years, the sea ice has kept this predator out of the Arctic.

Their immense dorsal fins can't break through the ice.

But increasingly, there is no such barrier.

And this is no ordinary predator.

Armed with the second largest brain of all ocean mammals, and with the ability to learn from one another, killer whales -- also known as Orcas -- are remarkably intelligent and social hunters.

The Inuit were the first to witness their arrival into Arctic waters.

Sam Omik and Gamailie Kilukishsak can testify to the changing scene.

[Speaking native language] INTERPRETER: There are more killer whales now.

In the past, we would see them maybe once in the summer.

Now we see many more than that.

NARRATOR: Inuit spend a lot of time hunting at sea, and their firsthand experience began to shed light on this marine predator.

[Speaking native language] INTERPRETER: We know they eat seals and narwhal.

And as they are predators like us, they can probably eat anything they can get their hands on.

NARRATOR: For Inuit who live here, the summer Arctic has always been home to a natural bounty that may now also be on the killer whale's menu.

The narwhal.

Narwhal spend all their lives in the Arctic, navigating shifting ice packs and channels both winter and summer.

This is one of the most mysterious animals on the planet.

Known as the unicorn of the sea, they have a single tusk that can reach up to 10 feet long.

Until recently, narwhal have faced polar bears and human hunters.

But now a fearsome new presence has entered their waters.

INTERPRETER: I was hunting these narwhal, and they were fleeing from some killer whales.

The killer whales were traveling fast and upside down, and they breached the water chasing the narwhal.

When they got close to the shore where the narwhal were, they took bites out of two of the narwhal very quickly.

This is when I realized that they are supreme hunters.

NARRATOR: A report released in 2012 was the first to detail the recent arrival of killer whales, predators that were formerly unknown so far north in Canadian waters.

That study was led by Steve Ferguson, a marine biologist at Fisheries and Oceans Canada, with a wealth of Arctic experience.

FERGUSON: I've worked in the Arctic for over 30 years, and over that time, I've seen a lot of changes.

For most scientists, I think we looked at the Arctic and knew that changes were happening and wanted to understand how they were changing, and we anticipated it to start from the bottom up.

NARRATOR: Warming waters usually affect organisms at the bottom of the food chain first, which in turn impacts the diets of animals higher up.

But here, something else is also happening.

Changes are underway at the very top of the food chain as access to Arctic waters opens up.

Killer whales have begun turning Arctic ecology on its head.

FERGUSON: We really recognized the killer whale as that top predator that, all of a sudden, was making these huge changes in the Canadian Arctic, and it was something we didn't really think about at the start, but there it was, staring us in the face.

NARRATOR: Steve's team became interested in the firsthand reports of killer whale sightings that were starting to reach them back in 2007.

Since then, they've interviewed more than 100 Inuit hunters in over 10 communities and have opened up a gold mine of information.

Charlie Inuarak has been living in this part of the Arctic all his life, and he's one of the region's best hunters.

[Speaking native language] INTERPRETER: Last year, we headed out on a hunting trip to get some narwhal.

We were sitting at camp, and soon, large numbers of narwhal began to arrive close to shore.

Soon after, we saw a pod of killer whales.

there were six or seven or of them.

We started to pack our gear, and we followed the killer whales.

We knew we would catch narwhal if we followed them.

NARRATOR: Narwhal have always been key to the survival of the Inuit.

Their skin contains large amounts of Vitamin C, essential to the human diet.

For the last few summers, Charlie has seen tantalizing evidence that killer whales are also hunting and eating narwhal.

And this was backed up in the summer of 2013, when Charlie's son managed to grab this footage of killer whales in action.

INTERPRETER: By the time we got to where the narwhal were, the killer whales had already killed some.

Over the next few days, the killer whales stayed in the same area, hunting and eating many narwhal.

NARRATOR: This footage is probably the first time killer whale predation on narwhal in this part of the Arctic has ever been captured.

INTERPRETER: What we saw was the bigger killer whales eating the floating blubber.

The rest of the killer whales were eating the narwhal from underwater.

Even after the killer whales were full, they continued to hunt and kill narwhal.

NARRATOR: Charlie isn't the only one to have seen killer whales here.

Sightings have now been recorded throughout the Canadian Arctic.

INTERPRETER: My experience is, in August, when the ice is completely broken up and mostly gone, that they start arriving in force.

NARRATOR: What has become evident from Inuit interviews is that killer whales have started appearing at very specific times of the year.

FERGUSON: The summary here is all sightings of killer whales in the Canadian Arctic, and we have a really obvious pattern here where there's very few killer whale observations during the winter, but they start to arrive in the spring -- June, July, August peaking in the sightings -- and then, after that, they start to decline in sightings, and this pattern that relates to the sea ice.

So we have the loss of the sea ice in the spring brings the killer whales in, and this is the time of year when they're playing around in the Canadian Arctic.

NARRATOR: Several pods of killer whales are travelling into the Canadian Arctic in the summer, from points deep in the North Atlantic.

They're swimming thousands of miles to get here.

It's a journey that appears full of intent.

In August 2013, Steve launches a new expedition to satellite tag the whales and track where they're going.

It's the fifth season they've set out to sea to learn more about killer whale movements in the region.

This year, the science team will be followed by a film crew.

Armed with the latest technology, they'll attempt to document changes in animal behavior happening as a result of a more ice-free Arctic.

With some Inuit knowledge to draw on, but knowing that killer whale sightings are limited to a few weeks each summer, the team's task is formidable.

MAN: There's sporadic sightings throughout a lot of communities throughout the Arctic, throughout the Eastern side in particular, but it's quite a large area, so, you know, it's kind of like finding a needle in a haystack.

NARRATOR: The team is led by Cory Matthews.

He is accompanied by Natalie Reinhart, who is at the start of her PhD on killer whale predation.

REINHART: Very little is known about killer whales in the Arctic, and there are a few key questions that we hope to answer with our work.

In particular, where killer whales are coming from and what they're feeding upon.

So what I'm really interested in determining is whether killer whales are coming in and focusing exclusively on one prey type, or if they're preying on a wider variety of prey type.

NARRATOR: The team will be helped by Charlie and other Inuit.

They've seen killer whales before in this very area at this time of year.

So there is hope for the film crew and the scientists that this summer will be successful.

They start out from Pond Inlet on the northern coast of Baffin Island.

They're heading west in the hope of being in the right place when the killer whales pass through.

With hundreds of miles to cover and the chance that the killer whales could appear at any time, everyone is on constant watch, helped by regular updates from other Inuit hunters.

[Speaking native language] NARRATOR: The team will use crossbows to dart the whales with satellite tags.

If successful, these tags will enable the team to follow their every movement for about three months.

This year, they get lucky on their very first day.

The killer whales make a dramatic entrance.

REINHART: She's upside down!

MAN: Look at that.

WOMAN: Oh, my God!

Look at her!

MAN: Oh!

WOMAN: Oh, my God!

MAN: Oh!

[Chuckling] MAN #2: What?!

MAN: Whoa!

NARRATOR: In order to tag the fast-moving whales, they must get as close as possible.

MAN: Like that?

-Did you get it?

-I got it.

MAN: She's going behind us.

MAN #2: Good shot!

NARRATOR: This has been an amazing day for the team.

They've not only found and tagged the whales, but the film crew has also captured footage of killer whales here for the first time.

Over the next two days, the team manages to satellite tag five killer whales and biopsy and photograph much of the pod.

REINHART: I was fully expecting to come up here and potentially not even find them.

I was kind of preparing myself for the worst, so that when we did see them, I felt pretty lucky.

It does feel like you've seen something that not many people have witnessed or seen up here.

NARRATOR: It's a remarkable achievement and a great start to the research program.

Now the team must follow the killer whales and discover what they have come to hunt and how they are doing it.

In the rest of the world's oceans, killer whales have been extensively filmed and studied.

It's well known that they develop specific strategies for different kinds of prey.

On the west coast of Canada, they follow the annual salmon run.

In New Zealand, they deliberately sink to the bottom, taking rays with amazing precision to avoid being stung.

In Patagonia, they charge straight at the beach to hunt seals.

They're the dominant predator wherever they go.

And for the first time, these mysterious new Arctic killer whales can be tracked.

Almost immediately, the satellite tags reveal new information about what they're up to.

FERGUSON: So, with the tagging, it looks like they've continued to stay together as a group, these five whales that were tagged.

The satellite locations are all showing them sticking close together, and this is something we weren't sure about as to whether the killer whales are going to group and disperse as small groups, come back together.

But it looks like these guys are sticking really close together and staying as a group of maybe about 20 killer whales in total.

NARRATOR: With the tags pinpointing the whales' location, the film crew sets out to follow the pod and try to film them hunting.

MAN: The radio tags indicating they're in Admiralty Inlet, so it's going to take us probably two and a half days to get there.

NARRATOR: But the whales are moving much faster than their boat can and often travelling through uncharted waters.

FERGUSON: It looks like the killer whales really move fast, and they'll make some very directed movements to a next area.

They seem to know this region of the Arctic, and they know where they're going, and they know probably where they next are likely to get a meal.

NARRATOR: And what the killer whales have in their sights are narwhal.

Right now, the narwhal are on their way to the tranquil bays and inlets that form summer nurseries for more than half the global population of roughly to 25,000 to 45,000 individuals.

But there are now few places left to hide.

For narwhal, the summer has suddenly become a dangerous time.

Steve's keeping track of the tagged killer whales, and their movements suggest that they know where the narwhal are.

FERGUSON: Killer whales moved all together as a group of five of them at fairly rapid speed again, probably in the range of 200 kilometers a day.

And very quickly, they get down into the next inlet, and from an aerial survey, we found out that most of the narwhal were down near the bottom of this inlet.

NARRATOR: It's in these very inlets that the narwhal have always found security.

These calm waters are their traditional summering grounds where they bring their young, born just a few months ago.

It's the final stop on their summer migration.

It appears to be a time and place for friendly socializing.

This gentle jousting is known as "tusking."

It's thought to be a way of communicating with each other and establishing hierarchy.

Swimming in circles and tusking is a sign of relaxation and well-being.

But how long will this peace last?

Chasing killer whales capable of traveling 125 miles a day has been tough for the film crew.

Since the tagging event, they haven't been able to get anywhere near them.

But 12 days into their trip, they finally manage to track them down.

MAN: Wow!

Look at that!

Got probably, I don't know, 10, 12 Orca going all the way round the boat.

Some that have gone underneath it.

It's quite an amazing sight.

NARRATOR: And as the satellite tags indicated, the killer whales are headed straight towards the narwhal nursery grounds.

We've still got them.

They're still there.

NARRATOR: In the far distance, a large group of narwhal appear, swimming fast and close to the shore.

And not far behind, the tagged killer whales, clearly in hunting mode.

Steve is gripped by what he's seeing.

FERGUSON: The prey has to do something different.

They're not going to outrace killer whales.

NARRATOR: The narwhal are hugging the shoreline, desperately doing all they can to get away from the killer whales.

It's incredible how packed the narwhals' behavior is.

It may not be the best strategy, but I think it's -- when you're scared, you get close to your neighbor.

NARRATOR: It seems the narwhals' only recourse is safety in numbers.

FERGUSON: And I'm sure they're terrified.

This is probably the most terrifying predator since Tyrannosaurus Rex.

NARRATOR: The killer whales' movements are organized and coordinated.

This is a ruthless hunting group, ready to strike.

Working together, the killer whales seem to be herding the narwhal closer to the shore.

FERGUSON: Killer whales, typically, as a predator, will be quiet and not make a lot of communication sounds because the prey can hear them.

But once they've been discovered, they can start using communication to coordinate their activities.

NARRATOR: But it's what the hunting whales do next that comes as the biggest surprise.

Lunging themselves at the beach, the entire team of 20 killer whales pins the narwhal in shallow water.

The crew is stunned.

The killer whales have turned a peaceful retreat into a dead end for an entire pod of narwhal.

FERGUSON: That's amazing.

It's, uh -- it's amazing.

One lone narwhal tries to get away.

[Narwhal squeals] But exhausted from the chase, he won't last long.

Within moments, and without mercy, a killer whale drags the last narwhal survivor underwater.

FERGUSON: I don't recall records of Inuit stories of predation, that kind of lunging behavior.

Having seen it now, it seems to make sense.

Pretty amazing pictures.

It seems like the larger male is staying back a bit.

Because of its size, it might actually create problems in shallow water.

But they'll certainly participate in the feeding.

NARRATOR: Under the water, the killer whales are tearing the narwhal apart.

The kill is shared among the entire pod.

Young killer whales need the protein from the meat so they can grow, while older whales go for the fat to provide stores of energy.

These top predators have efficiently worked out how to trap and take narwhal here.

They've invented a specific tactic for this Arctic prey.

That's the first time I've seen that kind of footage.

That's amazing.

That's exactly the kind of things we wanted to document.

NARRATOR: It's the latest example of the killer whales' amazing ability to adapt everywhere they go.

While these Arctic newcomers are taking advantage of the ice-free waters, the old king of the north is struggling through the summer.

Without an ice platform to hunt from, finding food is tough.

And alternatives are few and far between.

To get through the lean times, some polar bears resort to eating seaweed.

This certainly provides important minerals, but it's not nutrition enough for an animal whose diet is based on large amounts of seal fat.

FERGUSON: It depends on the ice.

It has to have sea ice.

So the more sea ice that's lost, the worse it is for polar bears, and in contrast, the more open water, the better for killer whales.

So it's, um... As far as the top predators go, we think there's a switch going on that a lot of the Arctic, especially in southern areas, is switching over to be open water kind of habitat, where killer whales are the top predator, and polar bears are going to have trouble.

NARRATOR: Is it possible for polar bears to adapt to such huge changes in their habitat?

For killer whales, the open water simply provides more opportunity.

Routes once inaccessible into the Arctic, from both the Atlantic and Pacific, are now opening up, allowing killer whales to freely access these waters.

FERGUSON: When these killer whales come into the Arctic, they are coming from a long ways away.

They've probably made a big migration, and so they're hungry, and they come into their prey, and they start feeding, and we think they feed a lot during summer and maybe put on a lot of fat and store it up so that they can go through the next winter, when there's less food available for them.

But where they go really depends on the sea ice, and in the past, it's probably been restricted to places that typically have open water in the summer, and this is changing.

Now we're getting open water far into the Canadian Archipelago, places that, in the summer, usually had ice, and so killer whales have been observed further west than we ever had any records of them.

It's almost as if the Eastern Canadian Arctic killer whales are expanding right on through, and this huge playground is now available for them to move around in during the summer months, and they're taking advantage of that.

NARRATOR: And with enormous areas to hunt in, killer whales are not just taking narwhal.

They also have the fortune to encounter much larger prey.

Bowhead whales, true Arctic giants, twice the length of killer whales and outweighing them by 50 tons.

[Whales calling] But even these titans are a target.

As part of their research, Steve Ferguson's team has also been working with these whales, photographing and taking samples to find out more about their existence.

Biopsies provide information about where a whale that spends its entire life in the Arctic goes to find food.

And Bowheads are also revealing for the very first time what happens when they encounter killer whales.

FERGUSON: We had bowhead whales that were satellite tagged this past spring, the same year, and as it turned out, our killer whales ended up going down into an area that had almost all of the tagged bowhead whales, and so we were pretty interested in seeing their response.

NARRATOR: This map shows the movements of killer whales in red and bowhead whales in yellow.

FERGUSON: And what we found is that the bowhead whales kind of separated away from the killer whales.

Somehow they knew where they were -- whether they could hear them or whether actually some of them encountered them.

But their hearing, we know can cover large distances.

And, typically, the killer whales try to be quiet, but sometimes they also celebrate a bit when they kill an animal, and so that might have given them away.

But we certainly see a dramatic movement of the bowhead whales towards the coastline, towards the shoreline, and not surprisingly, the bowhead whales went to those pockets of ice.

NARRATOR: We know that killer whales avoid ice, and it seems the bowheads know it, too.

They hide in the sea ice, close to the shoreline.

It's a good way to reduce the risk that killer whales will find them.

But there is some compelling evidence showing how successful killer whales are at hunting these giants.

When Natalie started her work, she was handed nearly 6,000 bowhead whale photos.

Intriguingly, some had gruesome scars on their tails, which Natalie immediately recognized.

REINHART: The photos show the fluke of a bowhead whale, and the fluke has rake marks on it, which are teeth marks caused by killer whales.

And we know them to be caused by killer whales because these marks have been observed on different prey species in different geographical areas.

We've looked at museum specimens of killer whales and measured the space between their teeth and matched these to the rake marks on the bowhead whale flukes.

NARRATOR: The rake marks were extremely obvious, but these wounded bowheads are the ones who've survived killer whale attacks.

We found very few calves and juveniles to be scarred with rake marks compared to older whales, so we think this might relate to killer whales might be more successful when they target calves.

NARRATOR: And killer whales use this technique the world over.

Off California, they are expert hunters of gray whales.

Working as a pack, they isolate the mother and close in on her calf.

INTERPRETER: Killer whales are great hunters.

I've heard stories of them hunting bowhead whales in packs.

A couple of them hang on to the fins and the tail.

The rest try to suffocate it by covering their blowhole, and they take turns doing that.

NARRATOR: These photos show that killer whales are clearly targeting bowhead mothers and their young.

REINHART: Female bowhead whales are known to protect their young.

Inuit have observed this occurring, and in other whale species such as gray whales or humpback whales.

We do know that females protect their calves during killer whale attacks.

NARRATOR: The Arctic summer has always been a stark contrast to its winter.

But it's the speed of change, now and over the next few decades, that's going to shift the balance between winners and losers.

And it's not just killer whales who are benefiting.

Many species are moving north towards a warming Arctic.

The invading red fox is now showing up in the territory of the Arctic fox.

The more aggressive red fox is already out-competing its Arctic cousin for food.

On the face of it, the Arctic appears to be home to more species.

Capelin and Atlantic cod, fish of famous abundance much farther south, are moving north.

Ultimately, the ice-adapted species may simply lose out and be replaced by ones that are used to a warmer world.

If current trends continue, research suggests that, in just a few decades, there will be no summer ice on the Arctic Ocean at all.

And, so, what of polar bears?

How can they survive this fundamental change?

At the very time when polar bears most need food, there is an event that might help some of them.

This is the George River in Northern Quebec, home to one of the largest migrations in the Arctic of salmon known as Arctic char.

Char cannot survive in freezing saltwater, so having fed all summer at sea, they race inland on an epic journey to breed before winter returns.

But, first, they have some huge hurdles to get over.

Once at the spawning grounds, warm upwellings provide the necessary conditions to prevent their eggs from freezing.

And all this is happening just at the time that polar bears are looking for food here.

Their ability to find food as summer becomes longer is crucial to their survival.

Polar bears must try their luck at a skill perfected by their grizzly bear relatives not far to the south.

This is very different from their usual ice hunting strategies, and they're going to need some practice.

Some are more successful than others.

Polar bears feeding on char is rarely seen and has never been filmed before.

This behavior shows that some polar bears are capable of adapting to a changing Arctic.

But how many of them can survive an ever increasing ice-free season is an urgent question.

For the team studying the killer whales, it's been an extraordinary summer.

It turned out to be their most successful effort since the start of the tagging project five years ago.

The killer whales travelled more than 2,500 miles in just eight weeks, eating a huge amount as they went.

This is not entirely what we expected to see, these changes occurring from the top down, that killer whales could be this dominant a predator that could actually result in the death of a lot of whales from southern populations.

You know, this may be how the distribution will change, how narwhal and beluga will end up even further towards the poles.

They're not necessarily going to move there.

It's just that the ones in the south might be suffering predation by killer whales at the rates that they can't maintain as a population.

NARRATOR: As the ice retreats even further, the kingdom of narwhals and polar bears will have to retreat with it and slowly disappear.

FERGUSON: This is our fear into the future, that as we lose more sea ice, the prey are losing their means of avoiding killer whale predation.

NARRATOR: A marine mammal dominant in the rest of the world's oceans is now clearly dominant in them all.

This predator is a symbol of the changing Arctic, change that is not just affecting its wildlife.

FERGUSON: This is a big problem for the Inuit.

I mean, they rely on ice-adapted whales and seals as their food, their cultural subsistence hunts, and it's important to them.

They're not necessarily going to be able to learn to use the new species.

It could be a real change of life for them.

But it's something to watch into the future.

As the changes unfold, we can get an idea of what might be happening, but we're probably going to be surprised.

There's going to be some more things that we didn't anticipate that'll happen.

Yet some Inuit actually foresee a benefit in having killer whales here.

[Speaking native language] INTERPRETER: We like hunting narwhal, and killer whales herd them closer to the shore for us, which makes hunting easier.

We appreciate having the killer whales around to help us hunt.

NARRATOR: How long these Arctic invaders will be considered helpful to the Inuit remains to be seen.

As a realm once locked in ice emerges from its long embrace, whatever happens next is bound to be dramatic.

The Arctic will continue to surprise us, and like many stories in a changing world, nobody knows how this one will end.

To learn more about what you've seen on this "Nature" program, visit pbs.org.