Tench

(Tinca tinca)

Order: Cypriniformes Family: Cyprinidae

Foto: Lauri Urho

Description: The tench is a cyprinid fish with very small scales embedded in thick skin, soft fins and tiny barbels in the corners of the mouth. The overall colour is brown with an olive-green tint; the back and fins are darker, and the belly is yellowish. The eyes are small, with red rings in adult individuals. The caudal peduncle is quite high, and the caudal fin is truncate. The pelvic fins of males are bigger and more sturdily built than those of females; a difference that is already noticeable in juvenile fish.

Origin and distribution: The native range of the tench extends from east of England and the Iberian Peninsula, throughout Europe to the River Yenisei in Siberia. The tench has been introduced into Ireland and the Balkans, and also into other continents. The tench is a warm water species that thrives in small, shallow lakes with soft bottoms and rich submerged vegetation, as well as in slow-running river sections and eutrophic bays of the Baltic. In larger lakes it is confined to sheltered areas of the littoral zone.

After the latest glacial period, the tench probably reached Finland and the neighbouring countries during the warm Ancylus Lake stage of the Baltic. Bones of tench have been found as far north as 62°15´N in excavations on Finnish prehistoric settlements. Later, as the climate cooled, the northern limit of its range withdrew. At the end of the 19th century, the few occurrences of tench were restricted to some lakes and the coastal archipelago of southern Finland. In the early 20th century, pond rearing and translocations from brackish to fresh waters were started. Adult fish for translocations were also brought from the area of present-day Russian Karelia. In addition, a brood fish stock of Polish origin was imported in 1936. New tench populations were established in the south of the country as far north as the cities of Joensuu, Kuopio and Vaasa.

Reproduction: Tench reach maturity at a length of 20−27 cm, in Finland probably at 4−6 years of age. In early summer, adults gather into shallow spawning grounds with abundant submerged plants and sheltered from the wind. Spawning begins once the water temperature has risen to at least 20 ºC, and is at its most intense at 22−24 ºC, though it may easily cease if the temperature falls. Tench spawn in small groups. Each female is followed by some males, which in turn fertilize the eggs released by the female. The eggs are small and sticky, and adhere to mosses and other plants. Spawning is intermittent, in Finland normally taking place in 2 or 3 acts within a month or so. A few days after fertilization, the larvae hatch when not more than 5 mm long. They start feeding within 4−7 days of hatching. The demand for a relatively high spawning temperature restricts the success of tench reproduction in Finland. Usually, only in autumns following very warm summers are one-summer-old tench big enough to escape the mortality caused by winter starvation and predators such as perch.

Diet, growth and migrations: Larval and young-of-the-year tench feed mostly on zooplankton. As one-year-olds, tench still prey on cladocers and copepods, but they also start to feed on chironomid larvae. Bigger tench feed mainly on the larvae of aquatic insects and tadpoles, gastropods and small mussels. Tench dig for food into the bottom sediment, deeper than bream (Abramis brama) or crucian carp (Carassius carassius). They become active at twilight, whereas during the day they hide among macrophytes. Feeding stops in the autumn when the water temperature drops below 8 ºC. The length of young-of-the-year tench ranges from 2 to 8 cm, depending on the summer temperature. At 10 years, tench may weigh about 1.5 kg. There are clear local variations in growth rate; in some small bodies of water tench are stunted because of high density. They often weigh as much as 2−3 kg and can live for up to 15−20 years. In summer, they thrive in a fairly restricted nearshore area. In winter they live rather passively in deeper layers.

Fishing and catches: Tench are caught with fyke nets, gill nets and by angling. Nowadays they are mostly caught for household use and by sport fishermen. The flesh is firm and white and tasty when smoked or fried.

Vulnerability, threats, and management: The tench is species of least concern (LC) in Finland. The majority of the stocks are now self-sustaining. During the last two decades, many tench populations have increased as a result of warm summers and eutrophication of waters. On the other hand, in some small lakes tench, along with other species, have died because of depletion of oxygen in winter. Since 1995, there has been no pond rearing of brood fishes. Stocks from different origins are probably mixed because of many translocations.

Other issues: The relative oxygen consumption of the tench is low, and it survives in waters with low oxygen levels. It can withstand a relatively wide range of pH levels (5 to10.5), and thus can survive in both fairly acidic and very eutrophicated lakes. The tench can also tolerate high temperatures (30–33 ºC) for some time by stiffening its body until it is motionless.

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