December

Young salmon and winter sturgeon

Christmas is approaching. In the cold river water, the roe is developing, while young salmon of various year classes are on the run. As a rule of thumb, we can say that when the temperature of the river water falls below 7 degrees Celsius, the growth of the young salmon will stop - or at least slow down considerably. The lower limit for salmon growth can vary from river to river.

Fish are reciprocating heat. This means that salmon in the cold winter river do not need energy to maintain their body temperature. It gets cold, and life processes in the salmon's body slow down.

The salmon is no longer as fast and may have trouble keeping its balance.

Researchers have found that when the water gets cold, juvenile salmon move away from areas of the river where the current is strong. The water temperature also affects the circadian rhythm of the juvenile salmon. In summer, they are most active during the day, while in winter they are most active at night.

They spend a lot of time resting in hiding places, saving their energy. They're tough guys.

There are also large adult salmon in the winter river. These are the toughest of the tough - the ones that have survived the spawning. Now they have hardly eaten since they came from the sea this spring. It's getting on for several months now. Maybe we're talking about six months without food.

We call them winter sturgeons. When spring arrives, the winter sturgeon will head for the sea on another feeding migration. These salmon can grow into really big salmon and are important to have in the river.

Tagging experiments have shown that some salmon have returned from the sea four times. A fish scale from a female salmon from the river Etne in Western Norway revealed that she had made five trips to the sea and spawned five times. This may be a record.