Smolt

A school of smolts on a trip. Photo: Arnt Mollan

After a few years in the river, the little salmon's body undergoes major transformations. It prepares itself to withstand salt water. When the salmon is ready to travel to the sea, we call it a smolt.

When the parr has become about as long as a ballpoint pen, we can call it a youth. Now the transformation is underway. Major changes take place both inside and outside the body. The parr transforms into a smolt, which we call smoltification. On the outside, it becomes longer, leaner and shiny. Inside the body, things happen that allow it to live in salt water without drying out. It excretes the salt through its gills and when it urinates.

The smolt is now ready for the sea. The river current carries them down the river. At the bottom of the river, they gather in larger and smaller groups. They never leave the river alone. It's safer to swim together in schools. Because they are slim, they can swim fast. Because they are shiny, they are less visible when swimming in clear seawater. And because there are more of them, it's harder for birds and other fish to catch them.

The smolt swim out through the fjord quite quickly. Some have to pass through long fjords, while others have a short distance to the sea.

It is spring when the smolts leave, but the smolts in the south migrate to the sea earlier than the smolts in the north. We still don't know exactly what determines when the smolts leave the river.

Freshwater and saltwater

Both humans and salmon need salt in their bodies. There is little salt in freshwater, so salmon must conserve salt. In salt water, there is too much salt.

In the river, fresh water enters the fish through the skin and the fish urinates a lot. In salt water, fresh water is drawn out of the fish and it has to drink a lot. It excretes the salt through its gills and when it urinates.