Fish or pisces are aquatic vertebrates with gills. There are more than 30,000 different species worldwide. 'Fish' is understood to be a group of morphologically similar animals and not a self-contained class as is the case with mammals, amphibians, reptiles and birds.
There are basically two different types of fish- cartilaginous fish, such as sharks and rays, which have a skeleton made of cartilage, while bony fish, such as salmon, have a skeleton made of bone.

Ein Fischschwarm. © W. Wichmann
Fish are sociable animals and often come together to form large schools.

In the flowing and still waters of the Schaalsee region, 30 species of fish and cyclostomes (lampreys) have been recorded so far. 17 species were found in flowing waters and 28 species in still waters. The overlaps are partly due to the migratory behaviour of certain stillwater species, which migrate to the confluent flowing waters during the spawning season. The waters in the Biosphere Reserve are thus relatively rich in fish species; 28 species were found in Schaalsee alone.

These include European whitefish (Coregonus lavaretus), vendace (Coregonus albula), spined loach (Cobitis taenia), weatherfish (Misgurnus fossilis). This also includes such highly endangered fish species as smelt (Osmerus eperlanus), common dace (Leuciscus leuciscus), burbot (Lota lota) and European bullhead (Cottus gobio). In the case of smelt, a distinction is made between a migratory form (Osmerus eperlanus eperlanus), which occurs in the North Sea and Baltic Sea, and a stationary form, the inland smelt (Osmerus eperlanus m. spirinchus). The landlocked smelt has good populations in Schaalsee. The Schaalsee is one of the few lakes in northern Germany where ice-age relict crustaceans live. For example, the amphipod Pallasea quadrispinosa is found in the deep waters of Schaalsee, between 8 and 40 metres. This species is eaten there by vendace, eels and burbot, for example.

Probably the best-known fish species in the Schaalsee is the vendace. This fish became the heraldic animal of the town of Zarrentin. But how did vendace get into the Schaalsee? An old legend tells us:
"In the Cistercian nunnery once lived an abbess who came from Lake Constance. The tasty vendace, a common species of fish in Lake Constance, was one of her favourite foods. Once, during Lent, in the solitude of Zarrentin, she was so irresistibly overcome by greed for the much desired fish that she called upon the devil and asked him to bring her some vendace from Lake Constance. In return, she wanted to pledge her soul to him. But he had to be back before 12 o'clock at night. The enterprising master of evil agreed and immediately set off. No sooner had he disappeared than the abbess' conscience struck her. In her perplexity, she called the convent of nuns together and penitently confessed her offence. Then one of the nuns got up, calmed the pious mother, climbed up the tower and set the clock an hour forward. By now it was 11 o'clock in the evening and the devil was over Techin with his fish. Then the Zarrentin monastery clock struck with twelve thunderous strokes. In his rage, the devil threw the whitefish into the Schaalsee. That's how they catch these fish there today, which have made the Schaalsee so famous for gourmets."