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Yesterday, Saturday, October 24, was World Migratory Fish Day, notably scheduled for October 25, but brought forward by one day due to the Covid. The opportunity to understand and protect these endangered species.
We know these fish which come up streams after having crossed thousands of kilometers. What we know less are the reasons for this cyclical phenomenon.
They have to leave a place where they are less well than before, because the environment is too polluted but also for reasons of food and reproduction.Gilles Boeuf, honorary president of the Natural History Museum
And the most fascinating is their mode of navigation in the water. A sort of magnetic compass has been found in the brains of salmon, as well as an « astronomical » navigation system.
Salmon are able to receive light information from the sky at night.
And once the ocean has been crossed, you have to find the river where they were born. « The final approach is chemistry, specifies Gilles Boeuf. The salmon stay in the river they come from; genetic brothers are constantly releasing molecules which act as beacons for its return. If we sterilize the river, he can’t come back. «
Humans also have a lot to learn about the adaptation of migratory fish to switch from freshwater to seawater, and vice versa.
The water of the fish comes out through its gill to dilute the sea water. It drinks constantly. In fresh water, it’s the other way around, he pees all the time!
Lots of intelligence. But their performance is just as amazing, as their propulsion is more efficient than our propellers. Or even those tunas or swordfish that spin at 100 km / h, in an environment 800 times denser than air, with, it is true, a skin so smooth, that it has been copied for submarines and swimmers suits!
For too long, their fishing was out of control:
In 1800, 3,000 tonnes of wild salmon were fished in France. Today, it’s not even 30 tonnes. And especially overfishing in estuaries: we caught them just before their reproduction, which is stupid!
Not to mention the many dams on our waterways, and despite the fish passes, elevators – and even fish vacuum cleaners! – preventing their migration leads them to certain death and, on a large scale, the disappearance of these species. « When we see how we treat the living, I’m ashamed in fact « , adds Gilles Boeuf. Will we have time to unlock all their secrets before they disappear? It’s a real question …