April

Smolt. Photo: Arnt Mollan

Now the long journey begins

It's almost time! The parr have become smolts and in many rivers have begun to make their way down the river and approach the salty sea. Out of a hundred, only two to five will usually survive the hardships of the sea and return as adult salmon. Most die in the first few weeks after reaching the sea. At the bottom of the river, they gather in small schools and then leave. We have seen that the smolts tend to go near the bottom and where the current is strongest. Perhaps this is a good idea, as it makes it more difficult for both fish and birds to get hold of them. For post-smolts, as we call them once they are in salty water, they move fairly quickly out of the fjord towards the sea. Naturally, post-smolt encounter many dangers on their way to the open sea. Unfortunately, one of these dangers has become unnaturally large: salmon lice.

The salmon louse is a small crustacean that lives part of its life by feeding on the blood and mucus of salmonids such as salmon, sea trout and char. Animals that live by parasitizing others are called parasites. The salmon louse is a natural parasite on our salmonids. They can tolerate some salmon lice, but not too many. Adult salmon can tolerate more than a small post-smolt, which can be so weakened that it dies if it only gets five salmon lice on it.

In the fish farms along the coast, there are many farmed salmon all year round. Of course, farmed salmon are also hosts for salmon lice. In fish farms, salmon lice can multiply and an abnormally large number of infectious larvae can be released. Farmers work hard to ensure that there are as few mature salmon lice as possible in the farms, especially in the spring. This is when the salmon smolts swim past. The efforts are great, but not always equally successful. In several places, wild salmon are hit hard.

Sea trout and sea trout never travel as far as salmon and stay along the coast. They are therefore even more exposed to abnormal and harmful high levels of salmon lice infection than wild salmon.