Biology

At the end of the last ice age, more than 10,000 years ago, Namdalen was about 180 meters lower than today, and Namsfjorden almost reached Brekkvasselv. The land rose relatively quickly when the ice masses melted after the ice age. Already then, salmon were probably present in the Namsen. As the land rose, high waterfalls gradually appeared in the river, and the small fjord was separated from the sea above these waterfalls (Trongfoss, Aunfoss and finally Fiskumfoss).

In most salmon populations, there are dwarf males that reach sexual maturity without migrating to the sea (spawning pairs). Sexually mature females without a sea stay are, however, very rare. A salmon population that is separated from the sea in this way will therefore usually die out, because all the females migrate to the sea and cannot later return to the river above the waterfall (they do not come up the waterfall on their own). However, it seems that females that mature in freshwater occur more often when the summer temperature in the water is consistently low and the growth of the fry is poor. Such water temperatures were probably present in Namsen shortly after the last ice age, and the temperature conditions may have been favorable for the formation of sexually mature females. The sexually mature females were able to survive and reproduce above the newly formed waterfalls in Namsen, and thus became the origin of the small salmon. It was probably isolated from ordinary, sea-migrating salmon 9,500 years ago.

The small brown salmon is the only population of freshwater stationary salmon in Europe that lives its entire life in rivers without migrating to lakes. They prefer areas of the river with coarse substrate and high availability of cavities. They only occur in low numbers in completely stagnant areas, such as above thresholds and ponds. In threshold basins with stagnant water, there are dense concentrations of trout. These therefore appear to be areas that significantly favor trout over small brown salmon. The density of small brown salmon in catches is generally low compared to the density of trout throughout the distribution area.

During monitoring of the small whitefish population, the dominant age groups in the catches are 2-5 year olds. The oldest individuals were 7-9 years old. Food availability in the river is limited compared to salmon that migrate to the sea, which explains their small size. At the same time, they appear to have a later growth rate and a lower growth potential than sea-migrating populations. Males can reach sexual maturity as early as 12 cm long, while females are 14 cm long. They have large eggs in relation to their body size, with an egg diameter of 5 mm. Each female usually has 100-150 eggs. It is not known whether the small whitefish spawn once or more. It is also not known whether they make spawning migrations, and the spawning grounds have not been mapped. However, suitable spawning grounds are likely to be found both in the main river and in tributaries where there is flowing water.

Genetically, the small salmon is very special, both on a Norwegian and international scale. The small salmon is genetically very different from migratory salmon both in the Namsen and other waterways. The small salmon consists of at least three genetically different populations, perhaps more. The genetic difference between the different small salmon populations is large compared to what is common between migratory salmon populations in nearby waterways.

Characteristics

Small salmon and trout can also be difficult to distinguish, but small salmon have all the same characteristics that distinguish common salmon fry from trout. Salmon have a slimmer body shape than trout, a slimmer tail root, a more forked and less rounded tail fin, and larger and rounder pectoral fins. The jawbone reaches to the middle below the eye in salmon, while in trout it only reaches the back edge of the eye. The small salmon's adipose fin is usually completely gray, while it is red or has a red edge in the trout in the Namsen. Small salmon often have one to three large black spots on the gill cover, while trout often have several smaller spots.

Below is a picture of both Namsblank and trout and the most important differences are commented on the page for the pictures.