Wels catfish

The wels catfish, known scientifically as *Silurus glanis*, is a massive freshwater fish. It is found throughout central, southern, and eastern Europe. Its range also includes the basins of the Baltic, Black, and Caspian Seas. It has been introduced to parts of Western Europe, the Mediterranean, and Asia as a prized sport fish. Today, its range stretches from the United Kingdom in the west to western China in the east.

This species is one of the largest freshwater fish in Europe and Western Asia. Only the Atlantic and beluga sturgeon are larger in these regions.

This predator has a very specialized way of hunting. It relies heavily on hearing and smell to find prey. This is possible because of its sensitive Weberian apparatus and chemoreceptors. These organs help it detect movement and chemical signals in the water. The wels catfish is also nocturnal, meaning it is most active at night. Its eyes have a tapetum lucidum, which is a reflective layer that helps it see in low light. When it finds a victim, it uses its sharp pectoral fins to create an eddy, or a swirling current. This movement disorients the prey. The catfish then sucks the victim into its large mouth and swallows it whole.

The physical appearance of the wels catfish varies based on its environment. Its skin is very slimy and its color changes depending on the water. In clear water, the fish often appears black. In muddy water, specimens tend to be green-brown. The underside is always a pale yellow or white. Occasionally, albino specimens are caught, which lack normal skin pigment.

The fish has an elongated body shape. This allows it to swim backwards, much like an eel. Its mouth contains many small teeth and several barbels. It has two long barbels on the upper jaw and four shorter ones on the lower jaw.

Fossil evidence shows that the wels catfish has a long history. The earliest identifiable fossils are from the late Miocene, about 5.3 million years ago, in Ukraine. Some less well-preserved remains might be 6 to 7 million years old. During that time, the Soldatov's catfish was the dominant species in eastern Europe. That species was better adapted to the warm climate. However, the wels catfish was more adaptable to cooling and drying trends. During the Pliocene, the wels catfish expanded its range. It eventually replaced the Soldatov's catfish in the region.

These fish are capable of incredible growth and longevity. Most adults are about 1.5 meters long and weigh around 20 kilograms. However, they can grow much larger under excellent conditions. A record fish in Germany was 2.78 meters long and weighed 137 kilograms. In Poland, a specimen reached 2.61 meters and 109 kilograms. The heaviest authenticated specimen was caught in Italy's Po River in 2010, weighing 110 kilograms. They are also very long-lived. A study in Sweden recently identified a specimen that was 70 years old.

The diet of the wels catfish is highly opportunistic. It usually eats worms, insects, crustaceans, and other fish. Larger fish may eat eels, frogs, snakes, rats, and even aquatic birds.

In France, researchers documented catfish lunging out of the water to catch pigeons. This behavior was observed in an introduced environment. In their native range, they are mainly nocturnal bottom-feeders. However, they can adapt to new food sources. They have even been seen eating dead Asian clams at the water's surface during the day.

Because they are top predators, they can impact local ecosystems. When introduced to new areas, they can cause steep declines in other fish populations. In the Ebro basin, they have caused the disappearance of certain Iberian barbels. They also change the environment by increasing the growth of aquatic vegetation like algae.

In Sweden, the species is considered near threatened due to declining numbers. Some populations are also genetically isolated. While they are not globally endangered, their status varies by region. People often catch them for sport, but they are only recommended for food when they are under 15 kilograms.