Silurus glanis

The wels catfish ( or ; ''Silurus glanis''), also called sheatfish or just wels, is a large species of catfish native to wide areas of central, southern, and eastern Europe, in the basins of the Baltic, Black and Caspian Seas. It has been introduced to Western Europe as a prized sport fish and is now found from the United Kingdom east to Kazakhstan and China and south to Greece and Turkey. It is a freshwater fish recognizable by its broad, flat head and wide mouth. Wels catfish can live for at least fifty years.

Etymology

The English common name comes from Wels, the common name of the species in German language. ''Wels'' is a variation of Old High German ''wal'', from Proto-Germanic ''*hwalaz'' – the same source as for ''whale'' – from Proto-Indo-European ''*(s)kʷálos'' ('sheatfish').

Description

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 exhibits 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.

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.

Size

With a total length possibly exceeding and a maximum weight of over , the wels catfish is the largest freshwater fish in Europe and Western Asia (only exceeded by the anadromous beluga sturgeon). 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 long and in weight, and Vogt's 1894 report of a specimen caught in Lake Biel which was long and weighed . In 1856, K. T. Kessler wrote about specimens from the Dnieper River which were over long and weighed up to . (According to the Hungarian naturalist Ottó Hermann 835-1914 catfish of 300–400 kilograms were also caught in Hungary in the old centuries from the Tisza river.)

Most adult wels catfish are about long; fish longer than are a rarity. At they can weigh and at they can weigh .

Only under exceptionally good living circumstances can the wels catfish reach lengths of more than , as with the record wels catfish of Kiebingen (near Rottenburg, Germany), which was long and weighed . 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 long specimen weighing was caught on 21 June 2018 and the Danube river, where a catfish measuring 275 cm and weighing 117 kg was caught at Đerdap gorge in the same year), and Greece, where this fish was introduced a few decades ago. Greek wels grow well thanks to the mild climate, lack of competition, and good food supply.

Wels catfish have also been observed thriving in the cooling ponds of the damaged Chernobyl Nuclear Power Plant. Although popularly believed to have been 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, as well as being provided food by generous tourists visiting the area.

The largest accurate weight was for a 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 and one of , 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 , according to him.

Diet

Like most freshwater bottom feeders, the wels catfish lives on annelid worms, gastropods, insects, crustaceans and fish. Larger specimens have also been observed to eat frogs, snakes, rats, voles, coypu and aquatic birds such as ducks. They 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 behaviour 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 behaviour to forage on novel prey in response to new environments upon its introduction to the river Tarn in 1983 since this type of behaviour 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.

The wels catfish has also been observed taking advantage of large die-offs of Asian clams to feed on the dead clams at the surface of the water during the daytime. This opportunistic feeding highlights the adaptability of the wels catfish to new food sources, since the species is mainly a nocturnal bottom-feeder.

Distribution and ecology

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.

As introduced species

There are concerns about the ecological impact of introducing the wels catfish to regions where it is not native. Following the introduction of wels catfish, populations of other fish species have undergone steep declines. 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 that were once abundant, especially in the Ebro river, have disappeared due to competition with and predation by wels catfish. The ecology of the river has also changed, with a major growth in aquatic vegetation such as algae.

The wels catfish may have established a population in Santa Catarina, Brazil. They were imported from Hungary in 1988 and were washed into the Itajaí-Açu river after a flood caused their tanks to overflow. In 2006, a specimen weighing 86 kg (189.5 lbs) and 1.85 m (6 ft) long was captured in Blumenau, suggesting the catfish have survived and possibly be reproducing.

Conservation status

Although ''Silurus glanis'' is not considered globally endangered, the conservation status varies across the species native distribution range. In the northern periphery of the distribution, the species has been declining over the last centuries and was extinct from Denmark in the 1700s and from Finland in the 1800s. In Sweden it persists only in a few lakes and rivers, and is now considered as near threatened. Recent genetic studies have furthermore found that the Swedish populations harbors low genetic diversity and are genetically isolated and differentiated from each other, highlighting the need for conservation attention.

As food

Only the flesh of young wels catfish specimens is valued as food. It is palatable when the fish 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 specimens are not recommended for consumption, but are sought out as 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 a 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.
* Soldatov's catfish (''Silurus soldatovi'') from the Amur River, Russia

References

Further reading

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External links

:''This article includes information translated out of the German and French Wikipedias.''

{{DEFAULTSORT:Wels Catfish
Silurus
Catfish of Europe
Fish described in 1758
Taxa named by Carl Linnaeus
Articles containing video clips