"This is a journey made by 300,000,000 animals every single year" "More salmon than that are people in the entire United States are making their way up rivers across Alaska." "The entire journey is fraught with danger." "But they're also heading for a massive confrontation." "At these falls, one of the biggest concentrations of grizzly bears on the planet is preparing an ambush." "And what the bears miss bald eagles will be quick to take advantage of." "The salmon are swimming into the trap and for this salmon, it's about to be sprung." "The chances of the salmon growing up and making it past these bears alive is 2000 to 1." "But is life and death simply a lottery ?" "And can a fish do anything to increase its chances of surviving the looming impact ?" "To find out, we'll look at the chain of events that led to this critical moment." "The story begins 4 months earlier, in the depths of the Alaskan winter." "The salmon, were born in fresh water, but swam to the ocean to feed on protein-packed sea life." "All their lives they've been building their strength for this moment." "It's time to have young of their own." "And for a salmon, only one place fits the bill." "The exact stretch of river where it was born." "But to get home they have to pinpoint a single river mouth in an ocean 16 million miles squared." "How on Earth do they do that ?" "Far inland, most of the welcoming committee isn't even awake." "Snow covers parts of Alaska for 8 months of the year." "Not even grizzly bears tough it out." "They sleep right through it." "This mother has been underground since October." "Her heart beats just 10 times a minute but in January she gave birth." "No restless nights for her" "The cub let her doze through the winter." "When they're not suckling, they're sleeping by her side." "But with spring approaching, bears are forced from their dens, in search of food." "This early in the year, there's almost nothing to eat." "If they've timed it wrong, young families will starve before the salmon arrives." "Older bears have lived through lean times before." "But the cubs, they know nothing but safety." "The longest spring days trigger the eagle's biological clock." "Time to start thinking about a family." "The chicks need to be born the moment there's enough salmon to feed them." "The fish is still months away, but the eagles have to plan ahead." "If they start breeding now the eggs should hatch just as the salmon arrives." "That's the plan, but it's a gamble." "If their timing's off, they'll be hungry chicks and not a salmon in sight." "2 months to impact." "The salmon are far out in the Pacific, but incredibly, they're headed for home in a dead straight line." "So, what's their secret ?" "Every salmon comes with a built-in compass fit as a standard" "Magnetic material in their brain, guides them home, using Earth's magnetic field." "But, an advanced guard is gathering at the beach." "Eager to greet, and eat." "Without salmon, these bears would be in real trouble." "Hundreds will die." "Their fat layers are exhausted after 7 months in hibernation and hunger forces them to dig for clams beneath the sand." "It's that, or chew grass, like cattle." "The eagle's chicks have just hatched and the parents must work hard for every scrap of food." "They're just a few days old and they're surviving on any small fish their parents can catch." "They could use a more substantial meal." "There's no sigh of the salmon." "Generations of fish lure the eagles and bears here." "And now these awesome predators can't live without them." "The sea is where all their hopes lie." "The salmon is swimming hard for home but there's something sinister in the water waiting for the perfect moment to attack." "300 million salmon are about to arrive in this bay and there's no way that that much sushi can slip past unnoticed." "When they arrive, that's when their problems are really going to start." "In an incredible feat of navigation the salmon have pinpointed the right river mouth, in the world's biggest ocean." "But have they come all this way, only to be trapped ?" "The water is so low, that a wall of sand blocks their escape route." "Only rain, or an incoming tide can raise the water and release them." "And through the estuary's murky water their enemy is finally revealed." "Salmon sharks." "These close relatives of the great white now have the perfect ambush opportunity." "The salmon are trapped by the low water, and are at the mercy of the sharks." "At last, rain." "And the tide is turning." "The bears know it's time to make a move." "As the water rises, the salmon will be able to make their escape." "The sharks can't follow them into fresh water" "The sense of relief is almost palpable." "Rain means salmon is back on the menu." "The floodgates are open but the salmon are about to run straight into another trap." "Far up river, at the falls, the salmon will face one of the largest concentrations of bears on the planet." "But first, the advance guard." "They're past the sharks, but ahead are some very hungry bears." "The water's hardly deep enough to thrash a tail in but the salmon have no choice, it's time to storm the beach." "With the waters so low, the estuary is a maze of shallow channels guarded by hungry bears." "Every salmon has to run the gauntlet." "But a quick tail flick can take a salmon straight through the legs of its enemy." "After eating little but shellfish and grass fresh salmon is something really worth fighting for." "Just one more bear to get past." "And it's away" "At last, the salmon have made it into fresh water." "The rain has brought an added bonus" "It washes the scent of their birthplace downstream, to the sea" "With each individual tributary having their signature smell every salmon can follow its nose to the exact stretch of gravel where it was born." "But to get home, they'll have to fight the current all the way." "The river is a water treadmill, and in places runs so fast they should be swimming upwards." "Not all the salmon have made it this far." "Back at the river mouth, the predators are cleaning up." "For bears and eagles the casualties of the beach assault provide the first substantial meal of the year." "Eagles may stand proud, but they're not about to turn noses up at bear leftovers." "For them, it marks the beginning of a time of plenty." "At last, the chicks will have something they really can get their beaks into." "Buy the time the salmon hit the falls they should be ready to join the fight." "Meanwhile, the salmon are battling their way upstream." "And they're so consumed with the thoughts of salmon sex they've stopped eating." "When they've used up their body fat they'll reabsorb their guts and muscles, turning their flesh into fuel." "They should waste away long before they complete the journey." "How will they survive the fight against the flow ?" "There's just 2 weeks until impact." "At the falls, the trap is almost set." "The bears and eagles are taking their places." "The few million salmon about to pass this bottleneck make the falls a perfect place for an ambush." "Bears have gathered here, at this time of year, for millennia." "Some walk 70 miles to reach the best fishing spots." "A love for fresh fish forces them together." "But they're used to dining alone." "Dominant males fight hard for the prime spots." "They're pumped up, and pose a real threat to another's cubs." "She don't risk the lives of the youngsters and is relegated to the sidelines, where the pickings could be scarce." "And if the cubs don't get enough food, they won't survive the coming winter." "The salmon is still on track, and against the odds, most are beating the water treadmill." "Their secret is to use the underwater terrain to maximum advantage." "Seeking out these areas saves vital energy." "But will they have enough to make it over the falls and pass the eagles and bears ?" "To make matters worse a new generation of predators are almost ready to join the fight." "Thanks to a rich, salmon diet these juveniles will soon be taking their first flight." "At this time of year, the number of bald eagles in Alaska swells to 30000." "There are more here than anywhere else on the planet." "But only successful hunters catch enough salmon to survive." "These birds will soon be fending for themselves and around here, there's nowhere better to head than the falls." "In a pool, downstream the fish are taking a well earned rest." "The slow moving water gives them a chance to recover, before the mayhem ahead." "For a mother trying to avoid the wrath of males at the falls this quieter pool provides a safer feeding ground." "Her family needs food, and they're counting on mom to deliver it." "But the bears are out of their depth." "The salmon have a natural advantage in the deep water, and are tough to catch." "Mom and cubs are going hungry." "Compared to the salmon's journey so far this is relative calm." "But just ahead lies their greatest challenge." "The falls." "That's where the mother and cubs need to be if they're going to have any success." "While the bears sleep, the salmon will be making their final approach." "Under the cover of darkness, they've surged ahead." "And at the falls, their front runners have finally arrived." "It's the day of impact." "Fish mass in the churning water below." "It's like being caught in the washing machine." "On the bank, the predators are preparing for the ambush." "Fish can swim uphill but when the water's this fast, they have to get out of the flow to get ahead." "And incredibly, that means taking to the air." "With each passing minute, even more fish are fighting their way upstream." "And things are starting to get really crowded." "Until now, salmon have been able to seek shelter in slow moving water, near the bank but now, many are forced into the faster flow and are quickly exhausted." "They can't keep this pace up forever." "The only way out is up." "So far, this fish has survived everything life has thrown at it." "4 years roam in the world's biggest ocean hungry sharks, bears starved for half a year on the ground energy sucking currents and the talons of hungry eagles." "Now, one final hurdle." "One of the biggest concentrations of grizzly bears on the planet." "They chose to leap at this moment, but wat the right moment ?" "After all this time, all this effort could fate still deal a final blow ?" "In the last few seconds before the leap not one, but two bears have taken their places." "Incredibly, they both miss." "But the salmon is exhausted." "They can take 3 hour to recover from the exertion and this fish barely has enough energy to swim." "For an eagle looking for an easy meal, the falls are a perfect place to hang out." "For this salmon, it's the end of the line." "It's almost too easy, but that's why there are so many birds here." "There is one advantage to the bottleneck." "Strength in numbers." "When these fish go over the top, there isn't a bear alive that could catch them all." "Theoretically, going together means each individual is more likely to survive." "Of course, that's no consolation for the ones that end up as bear food." "Right place, wrong time." "Even after making it this far, fate can still deal a final blow." "Grizzlies here can really make a killing." "These bears are so successful, that can afford to be choosy." "With such easy pickings, they dine on only energy rich morsels." "Fish brains." "And caviar." "But it's only a matter of time before the big boys are bickering over a choice cut." "Moments before, this bear was locked in combat and is still distracted by it." "Through luck, rather than design, the salmon chooses to leap at this moment when the bear is not really on the ball." "And the fish is through." "Before long, the bear is back in the stride." "But in the time it took, many other salmon were able to slip by and survive." "The fish will never face this number of predators again" "They've run the gauntlet, and are on the homeward straight." "Finally, they can get ready to mate." "As they travel, they undergo a startling transformation." "These are the same salmon that made it over the falls, now they're unrecognizable." "They've swapped their camouflage colors for gaudy reds and greens." "The boys beef up to impress the ladies." "With big humps and hook jaws it drives the females wild, and is great for seeing off the rivals." "Males use their humps to intimidate and a tail tackle can leave the competition eating gravel." "The water's thick with the mass of mating salmon space is so short, even the females get feisty." "It's not much of a nest, just a whole in the gravel, dug with a flick of the female's tail" "Only when they're both happy, in a moment of mutual release are eggs and sperm shed." "Their mission is complete." "But it's cost them dear." "Their fat reserves are burnt up." "Their muscles are wasting away." "They are about to pay for their success with the only thing they have left." "Their lives." "Every single one is about to die." "Neither sharks, nor bears, but sex is the ultimate killer." "The females guard their nest until they haven't even the strength to fight the current." "The measure of their success, the future of their species lies in the gravel beneath their fins." "The lake floor is littered with their bodies." "But next spring it will be alive with salmon fry." "And their legacy isn't just to their young." "Across Alaska the bodies of hundreds of millions of salmon nourish the landscape." "In the salmon rivers there are more plants, more birds," "more bears stalking the valleys." "Even when Alaska rushes headlong into winter's cold embrace the salmon is still feeding the nation." "In a few ice-free rivers, some lake spawning salmon still survive." "For eagles driven south by the advancing arctic winter, it's a vital refueling stop." "Up to 3000 eagles gather here, to pull dead and dying fish from the flow." "The salmon sacrifice is keeping eagles one step ahead even now." "The bears are leading a solitary life again, but thanks to the salmon, most have put enough weight on to survive the winter." "Time for them to head for the hills, and settle down for the long winter sleep." "And in the gravel, just beneath their feet the next generation of salmon." "The following spring they'll swim downstream to the sea passing another shoal of adults chancing the luck the other way." "If only they could let them know what they're in for."