The wels catfish is the largest freshwater fish in European inland waters. It has a long, cylindrical, scaleless body, a broad, rather flat head and a wide mouth.
The upper jaw has two whiskers or barbels reaching as far as the pelvic fins. The lower jaw has four shorter barbels. The anal fin stretches back until it almost reaches the tail and the small sharp dorsal fin is positioned relatively far forward.
The coloration of the wels can vary from fish to fish but normally they have an olive-grey to blue-black body with creamy yellowish sides and abdomen. The sides also have large dark spots creating a mottled effect. The wels can have a length of up to three metres and reach a maximum weight of 150 kg. However, these dimensions are extremely rare and the last credible report of a catfish that size was in the 19th Century. Usually the wels reaches a length of between 1.30 m and 1.60 m. Specimens longer than two metres are extremely rare. Under optimal conditions, however, there are such specimens as the record wels catfish of Kiebingen (near Rottenburg, Germany), with its length of 2.49 m and weight of 89 kg This has only been surpassed by specimens from France, Spain, Italy and Greece. The wels was released in these countries some years ago and, in virtue of the favourable climate, lack of competition, and good food supply, the conditions were optimal for propagation and growth.
For a long time Eastern Europe had the best catfish waters. In the 80s large wels weighing over 100kg were found in Romania's Danube Delta.
Wels feed mainly on fish but also on insects, worms, snails and crabs. With increasing size, however, they also hunt frogs, mice, rats, and various water birds such as ducks.
It prefers to remain in sheltered locations such as overhanging riverbanks, sunken trees and plant populations where it can dig itself into the mud.
Towards dusk, the wels begins to search for food. It possesses highly developed senses of touch and smell due to its long, moveable whiskers. This allows it to detect food in the dark. In addition, the catfish has a strong pressure sensibility which, even during a thunderstorm, signals it to be active during the day.
The spawning period of the wels is from May to July. At that time, they are found in pairs on shallow shores or wetlands. The large, slightly yellowish eggs are deposited on water plants with a temperature of at least 18 °C. Once fertilised the colour changes and the eggs lose their yellow tint. The eggs, as well as the hatched larvae, are guarded by the parent fish. After about 3 days the larvae hatch at a length of 5 mm and feed from their yolk sac until their 6th day. Only then do the inconspicuous, transparent larvae start out in the open water, actively feeding on animal plankton.