Wels catfish

With a total length possibly exceeding 3 m (9.8 ft) and a maximum weight of over 200 kg (440 lb),[3] 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 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).[4] 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).[5] (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.)[6]

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[7][8] and the Danube river, where a catfish measuring 275 cm and weighing 117 kg was caught at Đerdap gorge in the same year[9]), 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 144 kg (317 lb) for a 2.78 m (9 ft 1 in) long specimen from the Po Delta in Italy.[10]

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.[11]

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, mice, rats and aquatic birds such as ducks and can be cannibalistic. A study published by researchers at the University of Toulouse, France, in 2012[12] documented individuals of this species in an introduced environment lunging out of the water to feed on pigeons on land.[13] 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[14] 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.[15]

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.[17] 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.[18] In Sweden it persists only in a few lakes and rivers, and is now considered as near threatened.[19] Recent genetic studies have furthermore found that the Swedish populations harbors low genetic diversity and are genetically isolated and differentiated from each other,[20][21] highlighting the need for conservation attention.