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00:00The power of the Sun drives the seasons, transforming our planet.
00:12Vast movements of ocean and air currents bring dramatic change throughout the year.
00:22And in a few special places, these seasonal changes
00:26create some of the greatest wildlife spectacles on Earth.
00:38Here on the western coast of North America, in the spring of each year,
00:42one of the Earth's greatest travelers comes home.
00:47Over half a billion salmon in the Pacific Ocean start on a 3,000-mile journey,
00:53returning to spawn in the rivers where they were born.
01:03Traveling deep into the continent,
01:05these fish will not only provide food for millions of animals.
01:14They will also bring life to one of the richest habitats on Earth.
01:23The coast of British Columbia and Alaska is rimmed by spectacular mountains.
01:27The coast of British Columbia and Alaska
01:49is rimmed by spectacular mountains.
01:52Although it will be months before the salmon enter the rivers
01:58below these frozen peaks,
02:01one species that has spent the winter sleeping up here
02:05is already anticipating their return.
02:13In January, snug in their dens,
02:16the females have given birth,
02:18and now the family is beginning to stir.
02:41Grizzly bears.
02:42Whether the cubs will live or die
02:49depends largely on one key event, the salmon run.
02:54For the next five months,
02:55the bears will be focused on making their appointment
02:58with the returning salmon.
03:12The first year is hard.
03:19Surviving the first year is hard.
03:23Half of all grizzly cubs don't make it.
03:27Throughout Alaska and British Columbia,
03:43thousands of bear families are emerging from their winter sleep.
03:50There is nothing to eat up here,
03:53but the conditions were ideal for hibernation.
03:57lots of snow in which to dig a den.
04:05To find food, mothers must lead their cubs down to the coast
04:10where the snow will already be melting.
04:17But getting down can be a challenge for small cubs.
04:27These mountains are dangerous places,
04:41but ultimately, the fate of these mountains
04:43bear families, and indeed, that of all bears around the North Pacific,
04:46and indeed, that of all bears around the North Pacific depends on the salmon.
04:47Right now, those salmon are more than 2,000 miles away.
04:52Right now, those salmon are more than 2,000 miles away.
04:56After four years at sea, half a billion Pacific salmon are going home, back to fresh water,
05:09to lay their eggs in the rivers where they themselves where they themselves were hatched.
05:26How the salmon managed to find their way back home across the open ocean is still largely a mystery.
05:45It has only recently been discovered that a salmon's brain contains small particles of iron that,
05:55like a compass, help it steer the magnetic lines of the Earth,
05:59showing them exactly where to go.
06:01For many of these salmon, that destination is here,
06:26along the western coast of North America, in British Columbia.
06:31They're making their way back to their birthplace in one of its many freshwater rivers and streams.
06:47Here, amongst the network of lakes and waterways,
06:51lies the largest expanse of temperate rainforest left in the world.
06:55It stretches from southern British Columbia to Alaska.
07:11It's one of the most fertile landscapes on the planet.
07:15The temperate rainforest supports even more life than its tropical counterpart.
07:33For thousands of years, salmon have returned to this country,
07:42because of the abundance of one element, fresh water.
07:46This is some of the purest water in the world.
08:02Thanks to these forests.
08:04This is some of the purest water in the world, thanks to these forests.
08:18The forest is still undisturbed.
08:31Where the forests are still undisturbed, the soil, held by millions of tree roots,
08:39filters the water, keeping the rivers flowing clean and pure.
08:43In May, grizzly bears come down to the coast to find something to eat
08:59while they await the arrival of the salmon.
09:03This is where spring arrives first.
09:14The cubs, still feeding on nothing but their mother's milk, have grown considerably.
09:25But it has been six months since their mother had anything to eat.
09:29Now they need other food, and the search for it can lead them into danger.
09:49Some males will try to kill cubs.
09:52The breeding season has begun.
10:00And big males are here, looking for females.
10:03But at least there's something to eat here, even if it's only grass and sedges.
10:25These greens, in fact, can keep them going for months, but they will need something more nutritious
10:34if they are to put on enough fat to enable them to survive the next winter.
10:44In some places along the coast, bears find much richer food.
10:49It's buried, but bears have an extremely acute sense of smell,
10:57and can sniff out a meal even if it's beneath the wet sand.
11:07Clams.
11:08It's not only bears that are drawn to the coast in search of food.
11:26There are more than 2,000 gray wolves in the great forest.
11:30They leave their cubs in the tidal areas while they hunt.
11:46This wolf is the pup's eldest brother.
11:49He's babysitting while the adults are away hunting.
11:53He doesn't have any food for the cubs, so they eat whatever they can find,
11:58even chewing the barnacles off the rocks.
12:09They, like the bears, are awaiting the arrival of the salmon.
12:23The adults return and find an intruder.
12:27A hungry bear has wandered into their patch.
12:36Coastal wolves will often kill and eat small bears.
12:49But this bear is very big.
12:57A hungry bear has wandered into their patch.
13:11Eventually, they decide that this one is just too big.
13:14By July, the bears are all getting very hungry indeed.
13:31And still, the salmon are not here.
13:33And then, after two months of traveling across the open ocean,
13:46the salmon reach the coast.
13:48As they near the shore, they begin to smell fresh water.
14:07There are thousands of rivers flowing into the sea,
14:10and the salmon have to find the particular one that will lead them to their birthplace.
14:19They have a truly extraordinary sense of smell.
14:22They can distinguish a single drop from their home river amongst eight million liters of seawater.
14:32As they detect the waters of home, they converge into the narrow fjords, which act as underwater corridors.
14:47But other creatures also know these corridors.
14:50And so do stellar sea lions.
14:55Killer whales.
14:57They eat a lot of salmon.
15:07And so do stellar sea lions.
15:20Salmon sharks are here too specifically to feed on salmon.
15:36But there is one predator that they can never see coming.
15:44The bald-headed eagle.
15:50Forgive me.
16:18The bald-headed eagle.
16:20Once past these coastal predators, there is little to prevent them from reaching their home river.
16:39It's now late July, and the salmon are poised at the edge of their inland realm.
16:50In the estuaries of the larger rivers, all five species of Pacific salmon mingle together, pink, chum, coho, sockeye, and chinook.
17:09The drive to get into the rivers is strong. Their eggs will only survive in fresh water.
17:20In late July, however, the water level is often too low for the first salmon to enter the smaller rivers.
17:38That doesn't stop them trying.
17:40But the very water that has drawn them back home will eventually kill them.
17:52As their kidneys and other organs adjust to the sudden lack of salt water, they stop eating and even drinking.
18:00So the energy stored in their bodies is all they have to power their swim up river and spawn.
18:12However, the salmon in the smaller streams have a more immediate problem.
18:17The low water has stopped them before their journey upstream can even begin.
18:34But their coast, every year, is swept by great storms.
18:38In the skies above the North Pacific, a huge eddy is forming.
18:49It moves towards the coast and the high coastal mountains.
19:08The clouds are driven up and over this massive barrier, and they drop their load of water.
19:38The great forest gets up to three meters of rainfall a year.
19:55Bears have thick coats, and the heavy rain doesn't seem to bother them at all.
20:17The steep, rocky mountains funnel the rainwater into the rivers, and levels quickly rise.
20:25This is what the salmon have been waiting for.
20:47The first wave of travelers advance upstream.
20:55No sooner do they start, than they are faced with another challenge.
21:06But six million years of evolution have prepared the salmon well.
21:11The salmon well.
21:33Their bodies are solid muscle and perfectly streamlined.
21:37Clearing these falls for a salmon is like a human being jumping over a four-story building.
21:47In many of these falls, however, the salmon face more than just water.
22:01In many of these falls, however, the salmon face more than just water.
22:05The bears know that this is where they can get the first proper meal of the season.
22:18But it's not easy.
22:22There is an art to catching a leaping salmon.
22:25And this young bear hasn't yet acquired it.
22:35This is what salmon were born to do.
22:38They're driven to get up these rivers to their spawning grounds.
22:55Their parents made it up here, and nothing short of death will stop them from repeating that journey.
23:01They're trying to get to the exact stretch of gravel where they hatched.
23:14Some lucky ones may only have to go a few miles inland.
23:18But others are faced with a truly daunting journey.
23:21The farthest that salmon have been known to swim upriver is 2,000 miles.
23:38Some rains can be short.
23:40And when they stop, the water levels in many of the rivers along the coast drop quickly.
23:45The first salmon in the rivers are once again trapped by shallow water.
23:53And worse, they're in bear country now.
23:59In early August, mother bears begin to patrol the rivers looking for fish.
24:04Like this one, they're usually skinny and starving.
24:16She and her cubs have eaten nothing but plants since they emerged from their den.
24:23They're in desperate need of a proper meal.
24:30Bears of all ages and experience come to the rivers to look for salmon.
24:40The first fish of the season, however, are hard to catch.
24:46This young bear is still learning how to do it.
24:50Step number one is spotting a salmon.
24:56A higher perspective usually helps.
25:03In these early days, fish are few and far between.
25:10And when they do appear, they're moving very fast.
25:15The salmon also have lots of places to go.
25:16The salmon also have lots of places to go.
25:17The salmon also have lots of places to hide.
25:19And the rivers are often in reach by the sea.
25:20The salmon also have lots of places to hide.
25:23They don't even know it, but they don't know it.
25:25The salmon actually have lots of places to hide.
25:30They can't even move them to there.
25:32The salmon also have lots of places to hide.
25:46The rivers are only shallow in short stretches,
25:49and they can quickly shoot across them and escape into the deep pools.
25:53This mother and her cubs are going to have to wait a little longer
26:05for the conditions to change
26:07before they can get the meals they so badly need.
26:18But for the salmon, these deep-water refuges are becoming prisons.
26:23It may be weeks before it rains again, and they can move on.
26:40Their bodies are now beginning to change.
26:43As their sex hormones stimulate the production of eggs and sperm,
26:48their skin changes color.
26:53Some develop a humped back and a hooked nose.
26:59All these changes use up precious energy.
27:03The longer the fish wait in these pools,
27:05the less likely they will be able to complete the journey
27:08to their spawning grounds.
27:10The mother bear and her cubs, finding little in the shallows,
27:19now try their luck in the deeper salmon-filled pools.
27:22The salmon are easy enough to see.
27:36With so many fish here,
27:37this young bear should surely be able to catch something.
27:40But finding the salmon is only part of the problem.
27:53Bears must pin a salmon to the stream bed in order to catch it.
27:57Not easy in deep water.
27:59Older bears know that it's almost impossible to get a meal this way.
28:10But while the salmon here may be relatively safe from the bears,
28:14they're not out of danger.
28:15The late summer sun is warming the water so that levels are dropping
28:26and the amount of dissolved oxygen is decreasing.
28:29The time spent in these worsening conditions is beginning to show.
28:44The experienced bears show the youngsters what to do.
28:49Catching live salmon in these pools may be difficult,
28:52but there are dead ones for the taking if only the bears can reach them.
28:59The problem is that most bears don't like to get their ears wet.
29:21However, the old bears know a trick or two.
29:29It just needs a little fancy footwork.
29:59It just needs a little fancy footwork.
30:02This year, the water levels are particularly low,
30:05and by September, the salmon are in real trouble.
30:11In the confined, oxygen-poor water,
30:14there is an increased risk of parasites and infections.
30:17In some years, these conditions can get so bad that most of the salmon die
30:27before they even reach the spawning grounds.
30:35What they need is more rain and soon.
30:39Luckily, this year, the autumn rains arrive on time.
30:59The salmon can set off once again.
31:20However, so much rain brings different challenges.
31:24The fish now have to battle against powerful torrents.
31:46But the salmon know how to turn this swift, turbulent water to their own advantage.
31:54The fish now have to go to the sea.
32:05Scarcely beating their tails, they manage to propel themselves forward
32:10by using the energy of the water,
32:12much as a sailboat does when tacking into the wind.
32:24But that doesn't mean there will be no further problem in reaching the spawning grounds.
32:33But that doesn't mean there will be no further problem in reaching the spawning grounds.
32:47This is going to be the end of the road for a lot of salmon.
33:13This bears are really hungry.
33:17They haven't tasted salmon for 10 months,
33:20and the big males battle for the best fishing spots.
33:23The longer the salmon take over their journey upstream, the weaker they become.
33:39And these falls present them with their biggest challenge yet.
33:44Although the falls aren't very tall, the bears hold the high ground.
33:48The salmon make short, exploratory leaps to see where the bears are.
34:11But they don't always get it right.
34:13This mother bear has been waiting months for this moment.
34:25Competition is fierce for these first salmon, even between a mother and her own cubs.
34:31More and more fish arrive at the foot of the falls.
34:51Eventually, they have to go for it, regardless of the danger.
35:01They have to be great.
35:01And the ferry appears at aman.
35:03But finally, one of them is really fun in the mountains,
35:03because they can take left behind and fall,
35:04and they all push us to come in Ut enmity while fishing stops everywhere,
35:05and they get lots of burying with us because of that quiet.
35:06For our enemies have to be oxidation,
35:08and finally they are a ise.
35:09factories have been rich,
35:10and difficult to have water through the last hunger.
35:11The world has kept thoseочร
35:24But this other can catch outilly here,
35:26and the sandali is truly Nintendo thatcorn is able to help us על camera.
35:31But numbers are on their side.
35:36For every salmon that gets caught, hundreds make it past the bears.
36:01By early September, the salmon have almost reached their spawning grounds.
36:20That one particular patch of gravel where they hatched four years ago.
36:25The salmon have now traveled far inland
36:32and can be found from California to the Arctic Ocean
36:36across a fifth of the entire continent of North America.
36:55But the journey has taken a heavy toll.
37:02For every thousand that hatched, only four managed to return.
37:11And even for those salmon that have made it back,
37:14there are still more dangers.
37:16They have finally reached the end of their road
37:18and are so tired and battered that they're easy prey.
37:23The advantage is fully to the bears now.
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38:05THE END
38:35The bears are spoiled for choice.
38:41In the best spawning areas, there are thousands of salmon in every mile of river.
38:50The bears here will gorge themselves for the next two months, and the mothers with their cubs can now gain the weight they will need if they are to make it through the coming winter.
39:05The salmon are so abundant that even the little cub is having a go.
39:17He's caught a female pink, the smallest of the salmon species.
39:32He's already learning the skills he will need to survive as an adult.
39:38But he's got a little way to go yet.
39:47Although the salmon are now at the mercy of the bears, they will not leave this place.
39:53Their nature impels them to lay their eggs where they themselves were born.
39:59Even though the bears eat their filth, there are so many salmon that most will survive to spawn.
40:04Even though the bears eat their filth, there are so many salmon that most will survive to spawn.
40:17The sockeye salmon's brilliant color signals that they're ready to breed.
40:28Males battle with each other for position behind the females.
40:33The female digs out a shallow scoop as a nest.
40:46The male nestles up against the female, stimulating her to release her eggs.
41:03When she's ready, she lowers herself over the nest.
41:09She begins to turn out her eggs, and the male releases a cloud of sperm into the water.
41:33These salmon are the lottery winners.
41:36The lucky ones that have succeeded in returning here to spawn.
41:40But there are enough of them to seed the next generation.
41:45The spawning season is a time of extreme abundance.
41:56For in the course of ensuring their own survival, the salmon provide food for a horde of other creatures.
42:02These bone-aparte gulls are collecting one of the season's great delicacies, salmon eggs.
42:17For the bears, the salmon spawning season is the pinnacle of the year.
42:42But for the salmon, it's the pinnacle of their entire lives.
42:54All that have reached it will end their days in the very place where they began them.
43:05The wear and tear of their long journey is now showing.
43:27Their bodies have been deteriorating for weeks, and with this last act of reproduction, they are finally spent.
43:34But even in death, the salmon continue to benefit the animals of the forest.
44:01The mother and her cubs will continue to fatten themselves on the carcasses until they're ready to head back up the mountain to Den in November.
44:16Why Pacific salmon have to die after they reproduce is not clearly understood.
44:27Atlantic salmon don't. They return year after year to spawn.
44:33But the Pacific salmon's decaying bodies nourish the rivers, providing abundant food for their growing eggs.
44:40And that is what it has all been about for the salmon.
44:46All their trials and tribulations have ensured that the baby salmon, when they emerge from these beautiful orange globes, will have everything they need to begin this incredible journey all over again.
45:01But the legacy of the salmon extends far beyond the rivers and streams.
45:19They are at the heart of a massive network of life.
45:25There are more than 200 species in the great forest alone.
45:29Plants and insects, birds and mammals, that depend on the salmon.
45:34It's possible that Pacific salmon, between their time out at sea and their time inland, feed more life than any other animal species on the planet.
45:49And there is one more beneficiary of the salmon's legacy.
46:04The fish are a unique link between the ocean and the forest.
46:08Born in fresh water, they live their life in the sea, and there gather nutrients with which they build their bodies.
46:29Now, scattered by feeding bears and wolves, the last bequest of these salmon is to the forest.
46:37Now, the nitrogen, carbon, and phosphorus that was gathered in the ocean is now released from their decaying bodies.
47:00Providing the nutrients that enable these trees,
47:05sitka spruce,
47:07red cedar,
47:10and western hemlock,
47:13to grow to such prodigious heights.
47:20It's now known that 80% of the nitrogen in these coastal forests, where the salmon spawn,
47:26comes from the sea, carried in the bodies of the returning fish.
47:31The trees may be growing hundreds of miles from the ocean, but they're still nourished by its richness.
47:48The rivers of the great forest, like the veins and arteries of an animal, carry its lifeblood, the Pacific salmon, throughout.
47:56And no animal relies on them more than the grizzly bear.
48:09Thanks in large part to the abundance of the salmon run, these cubs have survived their first and most difficult year.
48:25The bears will sleep easy each winter, as long as the Pacific salmon are able to continue their epic run.
48:32One of nature's great events.
48:33One of nature's great events.
48:34One of nature's great events.
48:39One of nature's great events.
48:42One of nature's great events.
48:44Coming up next this evening here on BBC HD, it's Stuart Lee's comedy vehicle.
48:53Coming up next this evening here on BBC HD,
48:56it's Stuart Lee's Comedy Vehicle.
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