Wels catfish

Habitat

The wels catfish lives in large, warm lakes and deep, slow-flowing rivers. It prefers to remain in sheltered locations such as holes in the riverbed, sunken trees, etc. It consumes its food in the open water or in the deep, where it can be recognized by its large mouth. Wels catfish are kept in fish ponds as food fish.

An unusual habitat for the species exists inside the Chernobyl exclusion zone, where a small population lives in abandoned cooling ponds and channels at a close distance to the decommissioned power plant. These catfish appear healthy, and are maintaining a position as top predators in the aquatic ecosystem of the immediate area.

Diet

Like most freshwater bottom feeders, the wels catfish lives on annelid worms, gastropods, insects, crustaceans, and fish. Larger specimens have been observed to also eat frogs, mice, rats, aquatic birds such as ducks and can be cannibalistic. A study published by researchers at the University of Toulouse, France in 2012, documented individuals of this species in an introduced environment lunging out of the water to feed on pigeons on land. Out of the beaching behaviors observed and filmed in this study, 28% were successful in bird capture. Stable isotope analyses of catfish stomach contents using carbon 13 and nitrogen 15 revealed a highly variable dietary composition of terrestrial birds. This is likely the result of adapting their behavior to forage on novel prey in response to new environments upon its introduction to the Tarn River in 1983 since this type of behavior has not been reported within the native range of this species. They can also eat red worms in the fall, but only the river species.

Physical characteristics

The wels catfish's mouth contains lines of numerous small teeth, two long barbels on the upper jaw and four shorter barbels on the lower jaw. It has a long anal fin that extends to the caudal fin, and a small sharp dorsal fin relatively far forward. The wels relies largely on hearing and smell for hunting prey (owing to its sensitive Weberian apparatus and chemoreceptors respectively), although like many other catfish, the species is characterised with a tapetum lucidum, providing its eyes with a degree of sensitivity at night, when the species is most active. With its sharp pectoral fins, it creates an eddy to disorient its victim, which the predator sucks into its mouth and swallows whole. The skin is very slimy. Skin colour varies with environment. Clear water will give the fish a black color, while muddy water will often tend to produce green-brown specimens. The underside is always pale yellow to white in colour. Albinistic specimens are known to exist and are caught occasionally. Wels swim in a fashion similar to eels, and so can swim backwards.

Size

With a total length possibly up to 5 m (16 ft) and a maximum weight of over 300 kg (660 lb), the wels catfish is by some margin the largest true freshwater fish (as opposed to anadromous or catadromous) in its region (Europe and parts of Asia). Such lengths are rare and unproven during the last century, but there is a somewhat credible report from the 19th century of a wels catfish of this size. Brehms Tierleben cites Heckl's and Kner's old reports from the Danube about specimens 3 m (9.8 ft) long and 200–250 kg (440–550 lb) in weight, and Vogt's 1894 report of a specimen caught in Lake Biel which was 2.2 m (7 ft 3 in) long and weighed 68 kg (150 lb). In 1856, K. T. Kessler wrote about specimens from the Dnieper River which were over 5 m (16 ft) long and weighed up to 400 kg (880 lb). (According to the Hungarian naturalist Ottó Hermann, catfish of 300–400 kilograms were also caught in Hungary in the old centuries from the Tisza river.)

Most adult wels catfish are about 1.3–1.6 m (4 ft 3 in–5 ft 3 in) long; fish longer than 2 m (6 ft 7 in) are a rarity. At 1.5 m (4 ft 11 in) they can weigh 15–20 kg (33–44 lb) and at 2.2 m (7 ft 3 in) they can weigh 65 kg (143 lb).

Only under exceptionally good living circumstances can the wels catfish reach lengths of more than 2 m (6 ft 7 in), as with the record wels catfish of Kiebingen (near Rottenburg, Germany), which was 2.49 m (8 ft 2 in) long and weighed 89 kg (196 lb). This giant was surpassed by some even larger specimens from Poland (2,61 m. 109 kg.), the former Soviet states (the Dnieper River in Ukraine, the Volga River in Russia and the Ili River in Kazakhstan), France, Spain (in the Ebro), Italy (in the Po, where a record 2.78m long wels catfish was caught, and Arno), Serbia (in Gruža Lake, where a 2.4 m (7 ft 10 in) long specimen weighing 117 kg (258 lb) was caught on 21 June 2018. and catfish with length of 275cm and weight 117kg at Đerdap gorge on Danube River during the same year), and Greece, where this fish was released a few decades ago. Greek wels grow well thanks to the mild climate, lack of competition, and good food supply.

Wels have also been observed thriving in the cooling pond of the damaged Chernobyl Nuclear Power Plant. Although believed by tourists to have mutated into large sizes as a result of radioactive fallout, in reality the fish are growing to such proportions due to the absence of humans, hunting and fishing having been outlawed in the exclusion zone following the accident.

The largest accurate weight was 144 kg (317 lb) for a 2.78 m (9 ft 1 in) long specimen from the Po Delta in Italy.

Exceptionally large specimens are rumored to attack humans in rare instances, a claim investigated by extreme angler Jeremy Wade in an episode of the Animal Planet television series River Monsters following his capture of three fish, two of about 66 kg (145 lb) and one of 74 kg (164 lb), of which two attempted to attack him following their release. A report in the Austrian newspaper Der Standard on 5 August 2009, mentions a wels catfish dragging a fisherman near Győr, Hungary, under water by his right leg after the man attempted to grab the fish in a hold. The man barely escaped with his life from the fish, which must have weighed over 100 kg (220 lb), according to him.

Ecology

There are concerns about the ecological impact of introducing the wels catfish to non-native regions. These concerns take into account the situation in Lake Victoria in Africa, where Nile perch were introduced and rapidly caused the extinction of numerous indigenous species. This severely affected the entire lake, destroying much of the original ecosystem. The introduction of foreign species is almost always a burden on the affected ecosystem. Following the introduction of wels catfish, some fishes' numbers are in clear and rapid decline. Since its introduction in the Mequinenza Reservoir in 1974, it has spread to other parts of the Ebro basin, including its tributaries, especially the Segre River. Some endemic species of Iberian barbels, genus Barbus in the Cyprinidae, were once abundant especially in the Ebro river but due to competition from and predation by wels catfish have since disappeared in the middle channel Ebro. The ecology of the river has also changed, as there is now a major growth in aquatic vegetation such as algae. Barbel species from mountain stream tributaries of the Ebro that wels catfish have not colonized are not affected.

Breeding

The female produces up to 30,000 eggs per kilogram of body weight. The male guards the nest until the brood hatches, which, depending on water temperature, can take from three to ten days. If the water level decreases too much or too fast the male has been observed to splash the eggs with its tail in order to keep them wet.

As a food fish

Only the flesh of young Silurus glanis specimens is valued as food. It is palatable when the catfish weighs less than 15 kg (33 lb). Larger than this size, the fish is highly fatty and additionally can be loaded with toxic contaminants through bioaccumulation due to its position at the top of the food chain. Large S. glanis are not recommended for consumption, but are sought out as a sport fish due to their combativeness.

Attacks on people

Tabloids regularly report attacks caused by various catfish that primarily affected animals (often only the role of the catfish was presumed). In April 2009, an Austrian fisherman was allegedly attacked by an catfish in one of the fishing lakes in Pér, near Győr, Hungary. However, the man reportedly managed to break free.

Similar stories occur in the works of older natural history writers. Alfred Brehm (1829–1884), a German naturalist, published his famous work The World of Animals in the 19th century. It was also translated into Hungarian at the beginning of the 20th century. In this, Brehm or the compiling Hungarian scientists write the following:

“Old Gesner’s (Conrad Gessner swiss naturalist, 1516–1565) claim that catfish doesn’t spare humans either doesn’t just belong in the realm of tales, as we know of several cases that confirm that. Thus, Heckel and Kner mention that a catfish was caught at Bratislava, in the stomach of which the remains of a child corpse were found. Fishermen credible to Antipa (probably Romanian zoologist Grigore Antipa, 1866–1949) told me that children bathing in the stomachs of catfish were caught in the bones of their hands and feet. - Communicates Vutskits (probably Hungarian zoologist hu:Vutskits György, 1858–1929). - A Romanian fisherman penetrated the middle of the Danube with his boat because he wanted to take a bath. While bathing, a catfish caught his legs, which he could no longer pull out of the mouth of this big-mouthed monster, and so he got to the bottom of the water. A few days later, they came across the corpse of a dead fisherman whose legs were still in the catfish's mouth, but even the greedy robber could not release his victim's legs and drowned because of it.”

Related species

• Aristotle's catfish (Silurus aristotelis) from Greece, the only other native European catfish species beside Silurus glanis.
• Amur catfish (Silurus asotus), introduced to European rivers
• Giant lake biwa catfish (Silurus biwaensis) from Japan endemic to Lake Biwa.
• Silurus soldatovi from Russia