Brown trout

(Salmo trutta)

Order: Salmoniformes Family: Salmonidae

Brown trout. Foto: Lauri Urho

Description: In some lakes and on the coast the brown trout is still often mistakenly referred to as salmon (Salmo salar). These two do resemble each other in many ways, and even an experienced fisherman may sometimes find it difficult to distinguish between them, especially because there is also great variation in the appearance of the two species; identification is particularly difficult at spawning time. The brown trout usually has more spots below the lateral line, and its caudal peduncle is deep and thick. In the salmon the caudal peduncle is slender and tapered towards the tail, so one can get a good grasp of it. The tail of a large trout is usually straight and almost notchless, in contrast to that of a salmon, which tends to be clearly v-shaped. The salmon usually has brighter colours and larger scales than the brown trout, which often displays numerous red and black spots. The mouth of a trout is also slightly larger than that of a salmon, and its upper jaws extend behind the eye.

Different forms or ecotypes of brown trout are distinguished on the basis of their habitat and migrating behaviour. These forms even look somewhat different and are known as sea, lake or brook dwelling trouts. Ecological and genetic research has proved, however, that the division into forms is not based on long-term evolutionary differentiation. Brown trout ecotypes of one water system often interbreed, and the proportion of migrating and local ecotypes in one environment is determined by both genetic and environmental factors.

Origin and distribution: The original range of the brown trout covers almost the whole of Europe. In addition, the brown trout occurs in the Caucasus, Western Asia and locally in North Africa. In the 1880s, the species was transported from Europe to North America. The trout, favoured by sport fishermen, has also been released in water systems of Chile, Argentina, South Africa, Australia, Tasmania and New Zealand. It occurs throughout Finland, in the sea, in rivers draining into the sea, and in other freshwater environments. Migratory and local brown trout types may occupy the same water system and reproduce with each other as long as there are no obstacles to migration.

Reproduction: All brown trout types spawn in running waters, but the sea trout migrates into the sea for feeding, and the lake dwelling trout into a lake. In reality, however, differences are not so clear-cut. Probably some brook dwelling trout spend their whole life as dwarfs in small brooks, but some may feed on the still waters of the main stream, or even migrate downstream into a lake or the sea. It is difficult to distinguish the larvae and juveniles of migratory trout from those of local dwarf trout. Keeping stocks separate is not easy, especially in areas where local and migratory trout spawn in the same area.

Anadromous brown trout run upstream to spawn for the first time after spending 2−5 years in the sea, by which time they weigh about 2 kg. In lakes, too, females reach maturity at about 2 kg. In northern Finland, females reach maturity at a weight of 1−2 kg and males at 1 kg. In brooks, trout usually reach maturity at 3−5 years of age and at a length of 25−30 cm. Trout spawn into pits they dig into the bottom gravel of running waters between September and November. In the fertilized eggs embryos develop in the gravel during the winter, and the larvae hatch in early spring at a length of 20 mm.

Diet, growth and migrations: Brown trout juveniles commonly live in rivers for 2−5 years. At a length of 18−25 cm they turn into smolts, form schools and migrate with the spring flood into the sea or a lake. Some of the larvae spend their entire life in the river. In the sea, brown trout remain near the coast. Most tagged fishes are caught within 100 km of the release area. In large freshwater catchments, brown trout may also move over large areas. From the lakes the spawning trout migrate into rivers and brooks late in the summer or in early autumn. From the sea they already start migrating to large rivers in summer, but to smaller rivers not until shortly before spawning time.

Trout larvae feed on insects and their larvae drifting in streams or living on the bottom. During the feeding migration in the sea or in a lake, young trout feed first on insects and later on fish. In the sea, they prey on herring and sticklebacks, and in lakes on vendace, smelt, perch and nine-spined stickleback. In brooks, they feed mainly on insects. Trout larvae usually reach migration size in 2−5 years. In the Gulf of Finland, trout grow relatively fast, attaining an average weight of 2 kg in their second year in the sea, and a weight of 5 kg in their fourth year in the sea. In their fourth year in the sea, trout in the northern Gulf of Bothnia weigh about 1 kg less than those in the Gulf of Finland. Growth tends to be slower in lakes than in the sea, and to vary with the abundance of prey species. Growth is slowest in brooks, where trout seldom weigh more than 1 kg.

Fishing and catches: The brown trout is a prized catch for many anglers. Huge sums have been paid to make trout fishing possible, and fish have been transported to new waters around the world to enrich fishing opportunities. In Finland, the annual trout catch was from 650 to 2300 tonnes during the 1990s and early 2000s. The yield from sea trout releases has been decreasing since the early 1990s even though the numbers of fish released have not substantially declined. Due to the increase in net fishing for pike perch (Zander lucioperca) in the Gulf of Finland and the Archipelago Sea, a larger proportion of trout are caught by gill-nets. Further, too intensive fishing with small mesh-size gill-nets has impaired the results of brown trout releases in inland waters. In the recreational fishery, trout are most commonly caught with drift nets, rods, spinning gear and hook-and-line.

Vulnerability, threats and management: Originally there were about 60 sea trout stocks in Finland, but most of them have been destroyed or heavily depleted due to the harnessing of rivers for hydro-electric power, deteriorating water quality and overfishing. Stocks classified as original are currently to be found only in nine rivers, and all of them are endangered (EN). Trout stocks, with feeding migrations into lakes, were originally to be found in most of Finland's large lakes. Today, only about 30 original and self-supporting stocks remain, and 24 of them are classified as near threatened (NT). More than half of them are supported by releases. Furthermore, the original local stocks living in small running waters have decreased drastically in recent decades owing to obstacles to the spawning run, forest drainage, dredging of brooks and rivers, contamination of waters and fishing. Since the main reason for the decrease in migratory stocks is the reduction in larvae and juvenile production, the situation could be improved either by restoring the environment or by releasing larvae into waters where they would have a better chance of survival. Both methods have been in common use. For the restoration of the natural lifecycle it is also important that enough fish can return to spawn, and this often requires the imposition of fishing restrictions during the feeding period. Under Finland's fishing legislation, the minimum catch size is 40 cm, and today for sea trout 50 cm. Fishing with more sparse gill-nets would have a favourable effect on protecting the natural brown trout stocks and also on the yield from releases. On the other hand, net-fishing restrictions based on the abundance of trout stock alone would interfere with the fishing of other species.

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