Taxonomy & naming
Silurus glanis was formally described by Carl Linnaeus in 1758 in the tenth edition of Systema Naturae, making it one of the earliest fish to receive a binomial name. It is placed in the family Siluridae — the sheatfishes or glass catfishes of Europe and Asia — within the order Siluriformes. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats Silurus glanis as the valid name for this species.
The genus Silurus holds a handful of large Eurasian catfishes. Silurus glanis is by far the most widespread and best studied, but smaller congeners such as S. aristotelis (Greece) and S. triostegus exist. The species has accumulated a wealth of vernacular names reflecting its distribution: wels in German and English, sheatfish or European sheatfish in English, som in Slavic languages, and silure in French. 'Wels' derives from old Germanic roots for the fish; 'sheatfish' likely refers to the torpedo-like body shape.
Morphology
The wels is unmistakable: an enormous, scaleless, torpedo-shaped catfish with a broad, flat head, an enormous gape, and a laterally compressed body tapering to a long anal fin that extends nearly to the tail. The skin is entirely naked — no scales, no bony plates — and is covered in thick mucus that gives the fish a slimy, smooth appearance. Colouration is variable but typically olive-green to brown-black on the dorsum fading to cream or yellowish on the belly, sometimes with irregular blotching.
There are six barbels: one long pair from the upper jaw and two shorter pairs on the lower jaw, used to detect prey in turbid or dark water. The pectoral fins carry stiff, serrated, locking spines on their leading edges; these spines can lock erect and tangle viciously in nets — always move a wels in a container, never net it. Reported maximum total length is 9 ft (verified; historical claims of 16 ft are considered measurement errors), with weights to 130 kg. Aquarium specimens purchased as juveniles can reach this scale given time, space, and nutrition.
Habitat
Silurus glanis is native to large, slow-flowing and still waters across a broad swathe of Europe and western Asia: the Baltic, North Sea, Black Sea, Caspian Sea and Aral Sea drainages, from as far north as southern Sweden and Finland south and east through central Europe and into central Asia. It occupies large rivers, lakes, reservoirs and oxbow lakes, preferring deep, slow reaches with abundant cover — undercut banks, sunken wood, deep holes — where it shelters by day.
This is a temperate species, not a tropical one. Water temperatures in its native range span roughly 39–68 °F through the year, and it is active across that range, feeding most energetically in the warmer months. pH in its native waters spans approximately 6.0–8.0. The wels has been widely introduced outside its native range — throughout western Europe, into North Africa and elsewhere — where it can become invasive, displacing native predatory fish.
Feeding
The wels is a generalist, ambush predator. Juveniles take invertebrates — insect larvae, crustaceans, worms — and small fish; as the fish grows it shifts almost entirely to larger prey. Adults prey on fish of all sizes they can engulf, as well as frogs and other amphibians, and there are documented records of birds (waterfowl) and small mammals taken at the surface. The engulfment method is key: the wels rushes its prey and sucks it in through that enormous gape.
In aquaria and pond settings, large specimens will accept dead foods — strips of fish, prawns, mussels — and will learn to associate the keeper with feeding. Any tankmate small enough to be engulfed will be eaten sooner or later. Companions must be of comparable or larger body size, and even then aggression over space and feeding can occur. Live feeder fish are not recommended as they can introduce pathogens; high-quality carnivore pellets, whole prawns and oily fish pieces are better choices.
Mating
Spawning in the wild occurs from May through August, timed to rising water temperatures in the warmer months. Males move into shallow, vegetated areas to construct a rudimentary nest from aquatic plant material. A female is attracted to the nest site, and spawning is overt and vigorous — the pair can be observed thrashing in shallow water.
Females are prolific: clutch sizes range from approximately 10,000 to 350,000 eggs depending on body size, with larger females producing far more. After spawning the female's role ends and the male remains to guard the egg mass.
Breeding
After the eggs are deposited in the plant nest, the male guards them attentively, fanning the clutch to oxygenate it and driving off intruders. Incubation is temperature-dependent and takes approximately 2–10 days. Fry are free-swimming shortly after hatching and initially feed on invertebrates before transitioning to larger prey.
Breeding Silurus glanis in a domestic aquarium is not a realistic prospect given the adult size and the space required for natural spawning behaviour. Commercial production for the food and stocking trade is conducted at outdoor pond facilities, often with hormone induction to synchronise spawning. In the aquarium context this species must be regarded as essentially not bred in captivity by hobbyists; any specimens encountered in the trade are almost certainly farm-raised for the food fish or stocking market.
In the aquarium
The wels catfish is one of the most extreme size-trap species a hobbyist can encounter. Juveniles are sold at 2–6 in and look manageable; the fish will reach 23.5–35.5 in within two or three years under good conditions and, given decades, can approach 7 ft or beyond. Only public aquaria with tanks measured in thousands of litres, or keepers with very large outdoor ponds in temperate climates, can appropriately house an adult. Anyone purchasing a juvenile should have a concrete plan for that adult animal before the fish enters the building.
Beyond size, several care points are non-negotiable. The skin is entirely scaleless — there are no protective scales or bony armour — making the wels significantly more sensitive to copper-based medications and many proprietary treatments than scaled fish. Halve the standard dose at minimum and watch for stress; consult sources that account for scaleless sensitivity (as with loaches, puffers and eels). The pectoral-fin spines lock erect and are heavily serrated; they will tangle irreversibly in any net and can cause serious cuts. Always transfer this fish in a container or bucket, never with a net.
The wels is a temperate species and does not want tropical temperatures; 50–68 °F is appropriate for most of the year, with seasonal cooling beneficial for long-term health. It is a solitary, territorial predator and must be the only wels in a system unless the tank is very large. Any tankmate must be too large to swallow — which, for a mature wels, means companions of 23.5 in or more. The fish is nocturnal and needs deep, dark hides. Water quality must be excellent; a large, heavily fed predator produces a substantial bioload.
Conservation
The IUCN Red List assesses Silurus glanis as Least Concern, reflecting its broad native range across Europe and western Asia and its status as a commercially farmed species. Populations across its native range are generally stable or recovering in regions where water quality has improved.
Conservation concerns for the wels are largely the inverse of most threatened species: in many parts of western Europe where it has been introduced (France, Spain, Italy, the UK), the wels is an invasive predator of concern, suppressing native fish communities and displacing top predators. Responsible keeping means never releasing any specimen into open water — a fish that has outgrown its aquarium must be rehomed to another keeper or a suitable facility, not released. Trade fish are farm-raised and do not create significant pressure on wild populations in the native range.