Wels catfish

With a total length possibly exceeding 3 m (9.8 ft) and a maximum weight of over 200 kg (440 lb),[4] the wels 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 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).[5] 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).[6] (According to the Hungarian naturalist Ottó Hermann [1835-1914], catfish of 300–400 kilograms were also caught in Hungary in the old centuries from the Tisza river.)[7]

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). Even larger specimens have been caught in 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 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[8][9] and the Danube river, where a catfish measuring 2.75 m (9.0 ft) and weighing 117 kg (258 lb) was caught at Đerdap gorge in the same year[10]), 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.

The heaviest authenticated specimen, captured from the river Po by a Hungarian fisherman in 2010,[11] weighed 134.97 kg (297.6 lb), although there are recent anecdotal reports of larger wels exceeding 140 kg (300 lb).[12][13][14] Meanwhile, the longest wels on record was an unweighed specimen from the Po measuring 2.85 m (9.4 ft), captured in 2023 [15].

An albino Wels catfish.

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 he attempted to grab the fish in a hold. The man barely escaped from the fish, which he estimated to have weighed over 100 kg (220 lb).[16]

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, even cannibalising on other catfish.[17] Researchers at the University of Toulouse, France, in 2012[18] documented individuals of this species in an introduced environment lunging out of the water to feed on pigeons at water edge.[19] 28% of the beaching behaviour observed and filmed in this study 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[20] 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.[21]

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.[23] 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.

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.[24] In Sweden it persists only in a few lakes and rivers, and is now considered as near threatened.[25] Recent genetic studies have furthermore found that the Swedish populations harbors low genetic diversity and are genetically isolated and differentiated from each other,[24][26] highlighting the need for conservation attention.