Physical characteristics

The general colouring of the fish depends on the colour of the water. If the water is clear, the fish have silvery flanks. In dark water, they turn brownish. If the fish migrate to an area where the water is a different colour, they quickly adapt to the prevailing colour.

As autumn draws near, fish change into a dark spawning colour, complete with patterns on their flanks. The heads of the males grow longer and they develop a clear kype beneath their jaws, by which they are easy to distinguish from the females.

However, distinguishing a landlocked salmon from a trout can be difficult. The bright colours of the fish may look very similar, particularly in lakes. Below, we have listed the best physical characteristics to distinguish the fish at the different stages of their life cycles.

Parrs

Landlocked salmon and brown trout look very similar at the parr phase. Salmon parr can be distinguished from brown trout parr by the longer, sharper pectoral fin. The large pectoral fin helps the parrs cope in the strong currents of the rapids.

The two fish can sometimes be distinguished by the dark spots the parrs bear on their flanks. The parrs lose these markings when they become smolts. Landlocked salmon have 8 to 10 spots while brown trout have 10 to 12. Brown trout have fainter, lighter spots which may have merged together. 

Also, salmon parrs have smaller mouths and their maxilla does not extend beyond the rear of the eye.

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Photo: A 1-year old brown trout

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Photo: A 1-year old landlocked salmon

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Photo: A 2-year‑old landlocked salmon smolt

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Photo: A 2‑year‑old brown trout smolt


Adult fish

The adult fish are easier to tell apart than juveniles. The best distinguishing mark is that brown trout have lots of spots, both above and below the lateral line. Salmon mainly have spots in the front part of the body and mostly above the lateral line.

The tails are another good give‑away. Landlocked salmon have a clearly slimmer tail wrist than brown trout and the caudal fin starts at a steeper angle than that of the brown trout.

Compared with adult salmon, adult brown trout have larger mouths that extend beyond the rear of the eye.

Landlocked salmon and brown trout can usually be distinguished by these characteristics. However, such characteristics may vary considerably according to individual. For example, the shape of the caudal fin or the length of the mouth does not allow reliable species identification.

While the spots are perhaps the most reliable identifying trait, even they make ambiguous grounds for identification. The brightly coloured fish caught in lakes may resemble each other to the extent that they cannot be reliably distinguished based on external traits.

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Photo: A silvery trout with a clipped adipose fin (lake feeding stage)

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Photo: A brown trout in spawning colours, caught in the Pielisjoki River as a brood fish



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Photo: A bright salmon with a clipped adipose fin (lake feeding stage)

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Photo: A male landlocked salmon in spawning colours

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Photo: A female landlocked salmon in spawning colours




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The project is co-financed by the LIFE+ -programme (EC)