10 20 40 60 cm
mittakaava > 60 cm
Gösta Sundman: Suomen Kalat (Kansalliskirjasto, The National Library of Finland), Lauri Urho, Jouko Lehmuskallio
Salmo trutta (L.)
- Family: Salmonids – Salmonidae
- Similar species: atlantic salmon, chum salmon, coho salmon, pink salmon, rainbow trout
- Size: In the sea and large lakes 40–80 cm, 1–10 kg, max. 15 kg. In streams 20–30 cm.
- Appearance: Brown trout vary greatly in shape and particularly colouring, depending on their habitat. Such is the variation that brown trout were once divided into different sub-species or even ecological races. Traditionally three types of brown trout are recognised: sea trout, lake brown trout and the small resident trout of streams. The current thinking is that the brown trout is a single species exhibiting much variation. The anal fin has a maximum of 12 rays, which distinguishes the brown trout from all Pacific salmon. Sea-going brown trout (sea trout) and salmon, and lake brown trout and landlocked salmon are all fairly similar. The caudal peduncle (the “wrist”) of the salmon is longer and slimmer, allowing it to be picked up by the tail, something that cannot be done with trout. The salmon’s caudal fin is usually clearly forked while that of the trout is almost square, though the difference may not be so marked in spawning fish. In salmon, the upper jaw extends only as far as the eye, in trout it extends beyond the eye. The trout’s vomerine teeth point both left and right, while those of the salmon are in a single straight row. Male salmon returning to spawn develop a kype.
The small brown trout that live and reproduce in streams are difficult to distinguish from brown trout preparing to leave their birthplace for a lake or the sea without opening the stomach. Small resident brown trout will have clearly developed gonads, i.e. testes or ovaries.
Brown trout parr and salmon parr are very similar. However, salmon parr have longer, more sharply pointed pectoral fins and appear to have a blunter head because the maxillary bone (upper jaw) extends back only as far as the eye, while in brown trout parr it goes further back.
- Colouring: Brown trout found in the sea (sea trout) are silver in colour with a dark back. There are numerous black spots on the gill cover and sides, both above and below the lateral line. Salmon generally have far fewer spots, especially below the lateral line. The brown trout found in lakes may also be silver in colour, though they are usually brownish with numerous spots surrounded by paler halos. The colour of spawning brown trout darkens like that of salmon, and this can make the two species difficult to tell apart. At this time, salmon tend to have more spots, even below the lateral line, though still not as many as brown trout. The spots on brown trout are more conspicuous and numerous and cover the entire body down to the tail, whereas in salmon they are found more on the forebody. Female landlocked salmon are black when spawning while female lake brown trout are brown with numerous well defined spots.
The dark vertical bars (“parr marks”) along the sides of brown trout parr are usually 10-12 in number and often joined together. The numerous red spots are surrounded by paler halos. The adipose fin is reddish yellow. Young brown trout and sexually mature brown trout spawning in streams also have red spots.
- Reproduction: Brown trout leave the sea or lake and ascend their native river to spawn. Unlike the salmon, which favours large, fast flowing rivers, brown trout will run into smaller rivers and even small streams to spawn. The diminutive brown trout resident in streams spend their entire lives in the stream of their birth. The spawning runs of other brown trout begin during the spring floods and continue throughout the summer. The eggs are deposited on gravel in streamy water in October. Lake brown trout spawn in streamy water between lakes or in rivers feeding into a lake. Leaving aside the small brown trout resident in streams, most of Finland’s brown trout today are stocked fish as man-made obstructions on rivers prevent them from spawning naturally.
- Food: Brown trout parr feed on plankton and small bottom-dwelling invertebrates. In the sea, the main food items are Baltic herrings, sprats and other small fish. Lake brown trout feed on vendace, whitefish, smelts, bleak and other pelagic species. They may occasionally move into rivers to feed off insect larvae and surface insects, a diet that sustains the small brown trout of streams throughout their lives.
- Distribution and habitat: Brown trout are found in freshwater throughout Finland and in all sea areas. By no means all waters contain brown trout, as the species demands the clean, cool, well oxygenated water found only in large lakes and rivers, the streams of Lapland and spring-fed brooks further south. Sea trout now spawn naturally in only a few rivers. However, stocking has established populations close to several river estuaries. Unlike salmon, sea trout do not travel far from their native river in search of food.
In Finland sea-going brown trout (sea trout) stand is critically endangered, inland-waters’ stands towards south from Arctic circle are endangered and towards north threatened.