The habitats of Alaska are both complex and fragile,
the result of the extreme conditions and thousands of years of evolution.
And any change, however small it may appear, can cause irreparable damage.
This delay in the arrival of the salmon can, itself, bring severe consequences.
It's now the end of June, the salmon have still not arrived, and many animals are starting to get impatient.
Day after day they enter the estuaries of the rivers in the hope of seeing the tireless travellers.
And day after day they return to their rocks empty-handed.
Alongside them, on a tiny nearby island, the colony of puffins seems much calmer.
They too have specialised in fishing,
but their prey is much smaller than the salmon.
For them, the herring are their own particular feast
and this year fishing will be plentiful.
While the puffins are hard at work fishing,
on the neighbouring island the seals and sea lions continue their wait.
Both species arrived at the start of the summer in order to reproduce.
They came with plenty of fat reserves, because the males don't eat during the breeding season
and the females have to breastfeed their young, which is an additional drain on their resources.
For them, the arrival of the salmon means an incredible source of energy just at the time they most need it.
The low temperature of the water means they need a layer of fat to protect them from the cold.
But, if food is scarce, the sea lions, like the seals,
will be forced to stop breastfeeding their young and return to their winter habitats.
The pups acquire this layer of fat during lactation,
so if their mothers have to return early they will not be ready to face the journey and will die.
This problem has already arisen in Alaska, and has forced the law courts to place a ban on trawl net fishing in the area
in order to save these colonies of marine mammals.
At last, there is a sign from the sea: orcas.
For once, their worst enemies are a reason to rejoice, as they come in pursuit of the salmon.
Still, it would be a good idea to wait a little longer before diving into the banquet.
The orcas are still in the vicinity
and if they spot the sea lions they might just decide it's time to change the menu and try a little meat instead of fish.
After several years out in the ocean, the five species of salmon that live in the rivers of Alaska return home.
Millions of them approach the coast, heading for the same watercourse in which they were born,
in order to carry out their final mission to perpetuate the species.
The seals watch the orcas move off while the salmon begin their journey upriver.
The first ones are the most fortunate
they have a little time while the seals make sure there is no danger before diving into the water.
The salmon hunting season has begun.
The passage is very narrow, so it's very easy for the seals.
All they have to do is wait near the entrance for lunch to arrive.
For the salmon their own personal Calvary has just begun..
The water is teeming with fish, and the predators hardly need make an effort.
But it is precisely their sheer numbers that enable the salmon to survive
for every one caught, hundreds of others manage to slip past and into the river.
Finally, after the terror, the first rest.
Protected by the narrowness of the entrance, the salmon remain at the river mouth while they adapt to this new medium.
In a few days, they will make the transition from salt water to fresh water,
and in order to be able to tolerate this change, which would kill any other fish,
their bodies have to undergo profound physiological transformations.
Their nutritious bodies are also eagerly awaited inland,
and when they arrive thousands of birds gather at the river mouths.
The salmon have finally reached the land they originally came from,
but their odyssey has just begun.
Now, they have to face seagulls, eagles, otters, and above all the great predator of Alaska,
the largest land carnivore in the world
the grizzly
Hundreds of bears will take up their places along the river banks,
eagerly awaiting the nutritious salmon.
For weeks, they have been feeding on grasses and berries, waiting for them to arrive.
The time of abundance has finally arrived and they will be sure to make the most of it.
The first days are the most intense.
The fishing territories are not yet established and everyone wants the best place.
Hunger and strength will decide who gets first choice.
In just a few days the rivers are full of salmon and hungry bears in search of food.
The large males have taken up the best positions, but the young ones are not so fortunate,
and a group of them decides to try its luck in a nearby stream.
It would seem to be the ideal place, and there's no competition.
But there is one problem
there are no salmon in the water.
They appear to have chosen the wrong stream.
Sure enough, there are no salmon.
Each individual salmon returns to the same river in which it was born,
and the proximity of two streams doesn't mean there will be salmon in both of them.
Until they arrive, the young bears will have to feed on crustaceans,
larvae or other small animals hiding among the rocks on the bed of the stream.