The tench or doctor fish (Tinca tinca) is a freshwater and brackish water fish of the cyprinid (commonly called 'carp) family found throughout Eurasia from Western Europe including the British Isles east into Asia as far as the Ob and Yenisei Rivers. It is also found in Lake Baikal. It normally inhabits slow-moving freshwater habitats, particularly lakes and lowland rivers.
The tench is most often found in still waters with a clay or muddy substrate and abundant vegetation. This species is rare in clear waters across stony ground, and is absent altogether from fast-flowing streams. It tolerates water with a low oxygen concentration, being found in waters where even the carp cannot survive.
Breeding takes place in shallow water usually among aquatic plants where the sticky green eggs can be deposited. Spawning usually occurs in summer, and as many as three hundred thousand eggs may be produced. Growth is rapid, and fish may reach a weight of 0.11 kg within the first year.
Source: en.wikipedia.org/wiki/Tench - 18.09.2011
In the winter the Tench lives in bigger groups in deeper waters or it digs oneself into the mud.