(Coregonus lavaretus (L.))
Order: Salmoniformes Family: Salmonidae Subfamily: Coregoninae
Foto: Ari Saura
Description: The whitefish is characterised by its silvery sides, adipose fin, relatively small head, slender body and protruding upper jaw, which is longer than the lower jaw. It is one of Finland's most diverse species. Today, only one indigenous whitefish species is known in Finland and Scandinavia. It is usually divided into six forms or ecotypes on the basis of its morphological characteristics, notably the number of gill-rakers, and its diet and spawning behaviour. The diversity of the whitefish has been an advantage in helping it adapt to different types of waters and environmental conditions.
Distribution: The whitefish is to be found throughout northern parts of the Northern hemisphere. In Europe the most southerly indigenous whitefish stocks are in the Alps and Carpathian Mountains. In Finland, the whitefish occurs countrywide, especially in relatively oligotrophic inland waters and in the sea, where cold and oxygen-rich waters exist. The whitefish has been one of most important species for stocking in this country. Since the 1900s, releases have been conducted so widely that it is almost impossible to define the distribution of different forms or ecotypes. Moreover, original and released stocks have probably been mixed in many places.
Reproduction: The most common spawning time of whitefish is October-November, but there is variation among the stocks. Even in the same lake, the first fish may begin spawning in September and the last ones not finish until early winter. The anadromous whitefish males make their first spawning run when they are 3−5 years old, and the females one year later. The bottom whitefish in northern Lake Inarijärvi reach maturity at 5−7 years of age. Some of the whitefish stocks in the sea spawn in coastal waters and some run upriver to spawn. Breeding grounds in rivers, lakes and the sea are in shallow water, often at a depth of a few metres, on gravel and sand bottoms. Larvae hatch after the ice cover has melted in spring.
Diet, growth and migrations: The growth of whitefish varies, depending on the form, stock density and environment. After their first summer, juveniles are about 10 cm long. Later, differences in growth rate arise between stocks. Slow-growing whitefish may live long, and still remain less than 25 cm in length and 150 g in weight. The fast-growing anadromous whitefish in the Gulf of Finland, on the other hand, may attain a weight of 1 kg at 4 years of age, and 2 kg at 6 years. The density of a vendace stock in the same water may also affect whitefish growth. Especially whitefish stocks specialized in plankton feeding have to compete with vendace, which are more effective plankton feeders. The high density of a whitefish stock itself together with dense cyprinid stocks may also slow down the growth-rate of the whitefish stocks. The whitefish diet depends on the number of gill rakers on the inner side of the gill arches. When small, all whitefish feed on zooplankton. The forms with dense gill-rakers remain on a plankton diet, but the forms with sparse gill-rakers move on mainly to bottom fauna. The feeding migrations of the anadromous, or migratory, form may be very long. Whitefish spawning in the rivers of the northern Gulf of Bothnia may migrate all the way to the Åland Sea, a distance of over 1000 km there and back. Sea-spawning whitefish tend to migrate only between shallow and deep water in the sea. In lakes the length of the feeding migration depends on the size of the catchment. Whitefish usually avoid warm water, so in summer they move into deeper waters.
Vulnerability, threats and management: Natural whitefish stocks have decreased in many areas because of deteriorating reproduction conditions. The breeding grounds of river-spawning stocks have been dredged, and migrations have been blocked by power-plant dams. Regulation of the water level may be fatal to lake spawning stocks. Most vulnerable to regulation are stocks that spawn in shallow water. Whitefish have been reared in Finland for over 100 years. Due to easy rearing and stocking obligations, the management of whitefish waters and stocks now consists almost entirely of releases. About 30 million newly hatched or one-summer-old whitefish are released annually. Many economically valuable whitefish stocks are dependent on releases. In the northern Gulf of Bothnia, fishing is oriented too much towards young, immature migrating whitefish. The need for fishing regulation is strongest for the bottom-net fishery aimed at anadromous whitefish. An increase in the mesh size for sea trout (Salmo trutta) and pikeperch (Sander lucioperca) fishing would also benefit whitefish stocks. Over large areas of the Gulf of Bothnia, the smallest allowable mesh size is already 50 mm. Of Finland's six whitefish forms, the anadromous whitefish and the plankton whitefish are classified as vulnerable (VU) and the lake whitefish as near threatened (NT).