Atlantic salmon

(Salmo salar) 

Landlocked salmon (Salmo salar m. sebago)

Order: Salmoniformes Family: Salmonidae Subfamily: Salmoninae

Foto: Ville Vähä

Description: It is sometimes difficult to distinguish between the slender, streamlined Atlantic salmon and the brown trout (Salmo trutta). The salmon has, however, a smaller and more sharply pointed head, a more slender caudal peduncle and a more clearly v-shaped tail. Also, the upper jaw of the salmon extends no further than the rear of the eye whereas that of the brown trout extends clearly beyond that. The dark spots on the sides of the salmon rarely occur below the lateral line, but the brown trout is heavily spotted. The salmon has larger scales than the brown trout. Thus, it has from 11 to 15, usually 12–14, scales between the lateral line and adipose fin whereas the brown trout has from 14 to 19, usually 16.

Origin and distribution: The salmon has two ecological forms: an anadromous form, which makes its feeding migration in the sea and spawns in rivers draining into the sea, and a land-locked form, which spends its whole life cycle in fresh water, migrating in lakes and spawning in rivers draining into them. Both forms show strong homing behaviour and return to their natal rivers to spawn, resulting in the development of genetically differentiated stocks. The native range of the salmon embraces northern areas of the Atlantic Ocean and the Baltic Sea. In Finland, salmon currently occurs in two rivers draining into the Barents Sea (Northeast Atlantic), the Tenojoki (Tana) and the Näätämönjoki (Neiden). There were once as many as 20 salmon rivers on Finnish coast of the Baltic, but today only the River Tornionjoki and River Simojoki have indigenous, self-sustaining salmon stocks. The landlocked form of the Atlantic salmon lives in Finland (Lake Saimaa complex in the watershed of the River Vuoksi), in some lakes of northwestern Russia (Onega and Ladoga), in Lake Vänern in Sweden, as well as in some lakes in North America. The landlocked salmon of the Lake Saimaa is nowadays dependent on hatchery rearing and stocking. It has been released also into some other bigger lakes, e.g. Inarijärvi and Päijänne.

Due to the removal of obstacles to the salmon run on the Hiitolanjoki, a river at the Finnish-Russian border, freshwater salmon from Lake Ladoga (in Russia) can now migrate to Finland and have even succeeded in reproducing in the river below the Kangaskoski power plant. The freshwater form of Lake Kuittijärvi also currently has free passage from Russia to Finland along the river Pistojoki. It is thus possible that this stock, too, will start to migrate to Finnish waters. An enhancement project has been set up to improve salmon reproduction in the Tuulomajoki, a river draining into the Barents Sea through northern Russia. Some of the transferred spawners have succeeded in spawning in their old grounds in the upper reaches of the river, in areas of the rivers Luttojoki and Suomujoki, as young salmon juveniles have been found there in recent years.

Reproduction: The spawning run up rivers starts at the time of the spring flood, when snow melt water flows into lakes and the sea. The run usually peaks in June but continues until the end of the summer. Spawning occurs on the gravel bottoms of shallow streams late in the autumn - in September, October or even November - depending on the water temperature. Early maturing salmon return from their feeding migration in the sea after only one sea-year. All one-year-old mature fish are males. A small number of males mature as juveniles while still in the river; known as precocious males, these small fish take part in mating together with the large males. Mature male spawners are usually 5−10 years old, and spend 1−6 years in the river and 1−4 years in the sea. Females mature after 2−5 sea-years. Freshwater Atlantic salmon females mature after 4−7 years in the lake, by which time they weigh about 4 kg (3−8 kg). Mature males are usually older than (5−8 years) and larger (4−9 kg) than females. The majority of catches in the Baltic Sea consist of fish 2−3 sea-years old, with a mean weight of 6−7 kg. Although most spawners die in the river once they have laid their eggs, some survive and may even spawn a second time after a new feeding migration in the sea or lake.

Food, growth and migrations: During their first summer, juveniles living in rivers feed on zooplankton, small insects and bottom fauna. Later they eat large aquatic insect larvae and adults, which are often transported by flowing water. At the beginning of their feeding migration in the sea or lake, they still feed on water surface insects and water invertebrates for some time, but after reaching a length of about 25 cm they switch mainly to fish. In the Baltic Sea, the most important food species are herring (Clupea harengus) and sprat (Sprattus sprattus) and in lakes usually vendace (Coregonus albula). The adult Atlantic salmon is the fastest growing fish species in Finland, though there is great variation between years and individuals. After the first sea-winter, the average weight is 1−3 kg, after the second 3−7 kg, and after the third 7−13 kg. The landlocked salmon grows more slowly than the anadromous form. In Lake Saimaa, landlocked salmon weigh about 0.5−0.9 kg after one lake-winter, 1.5−3 kg after the second, and 2−6 kg after the third winter. The anadromous salmon of the Tenojoki (Tana) migrates for long distances in the North Atlantic; the majority of large salmon grow in the sea north of the Faroe Islands and some migrate even as far as the eastern coastal waters of Greenland. The most important feeding grounds for the Baltic salmon stocks of the Gulf of Bothnia are in the south of the Baltic Main Basin, implying a migration route of about 2000 kilometres. In years of abundant Bothnian Sea herring stocks, some of the northern salmon stocks may stay in that sea to feed. Salmon stocks originating from the Gulf of Finland tend to remain there to feed, though some also go to the Baltic Main Basin. In the Saimaa water system, the most important feeding areas are the two lakes Paasivesi and Pihlajavesi. Feeding migrating salmon may, however, go to southern parts of Lake Saimaa, too.

Fishing and catches: In 2004, the total salmon catch in the Baltic Sea was 2010 tonnes, which was the smallest since the late 1970s and early 1980s. The catch by Finnish fishermen was 579 tonnes, of which commercial fishery accounted for 505 tonnes. The main types of gear used by professional fishermen were trap nets and drift nets; recreational fishermen favoured trolling in the sea and spinning or fly fishing in rivers. In 2004, the total salmon catch on the Finnish side of the river Tornionjoki was about 20 tonnes. In 2003, river catches were the smallest since 1995. In the Simojoki, the salmon catch was only about 560 kg, which was less than would have been expected from the smolt production level. In 2004, the annual catch from the Tenojoki (Tana) was 70 tonnes, of which 37 tonnes was taken on the Finnish side of the river. This was the smallest catch in 30 years and only about half of the long-term average, which is about 140 tonnes. On the Finnish side of the river, 58% of the catch was taken with rod and line and lure. The catch from the other river draining into the Barents Sea, the Näätämönjoki (Neiden), was 6.1 tonnes, of which 1.8 tonnes was taken in Finnish territory.

Vulnerability, threats and management: Environmental changes in Baltic Sea rivers have had a serious impact on the survival of salmon. Rivers has been dammed for hydro-electric power generation and dredged for log floating; ditching has silted up the gravel in spawning grounds; and effluents have impaired water quality. As a result, only two of the original 18 Finnish Baltic salmon rivers remain. Recovery stocking has taken place in many rivers. In marine feeding areas a threat is posed by intensified fishing, which has depleted the numbers able to return to their home rivers to spawn. An additional threat is posed by the M-74 syndrome, which causes mortality among yolk sac larvae and has occurred with varying intensity since 1974. In the 1990s, M-74 mortality increased heavily, but varied among larvae from different females. In recent years, the mortality level has been somewhat lower, and was less than 10% in 2003. The cause of the syndrome is not known for sure, but it is thought to be related to the food of the fish. The greatest threat to Atlantic salmon stocks in rivers draining into the Barents Sea is posed by the parasite Gyrodactylus salaris, which has destroyed stocks in a dozen or so rivers in Norway. Efforts have been made to prevent the parasite spreading from rivers entering the Baltic into more northerly waters by prohibiting fish translocation, by disinfecting gear and by public education.

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