In 1974, German biologist and angler Roland Lorkowsky introduced wels catfish, Silurus glanis, to the Segre, a tributary of the Ebro, after transporting juveniles from the Danube region. Technical and scientific summaries identify 32 juveniles as the initial release, though accounts differ. The fish were introduced to create a sport-fishing attraction. More than five decades later, the species is established in several Spanish river systems, including the Ebro, Tagus, Ter, Guadalquivir and Llobregat, as well as associated reservoirs. MITECO’s 2025 update is cited as documenting a broader distribution across eastern and southern Spain.
Later movements are reported to have extended the fish’s range within the Ebro system, including to the Mequinenza and Riba-roja reservoirs. Its spread beyond the Ebro unfolded over decades: records place the species in the Tagus by 2001, the Ter by 2003-2004 and the Guadalquivir by 2011. The expansion reflects both dispersal through connected waters and human-assisted transfers. A fishing introduction became a sustained management challenge.
From Hobby to Hazard
Silurus glanis is Europe’s largest freshwater fish, with reported maximum dimensions of about 2.5 metres and a weight of more than 100 kilograms. Its scaleless body, broad mouth and six sensory barbels suit it to hunting in deep water. The species is an opportunistic predator, feeding on fish and other prey from aquatic and terrestrial environments. Deep, warm, regulated reservoirs in the Ebro basin offer favourable conditions for large individuals.
As a large predator, the wels catfish can alter food webs, consume native fauna and compete with other species for food and habitat. The effects vary by site, and available material does not quantify a nationwide decline in native species attributable solely to the catfish. Concern is greatest where vulnerable native populations already face pressures such as drought, habitat loss or competition from other invasive species. In the Guadalquivir, researchers have raised concerns about potential risks to species including the sea lamprey.
Expansion Across Spain
From its initial foothold in the Ebro basin, Silurus glanis has been recorded in the Tagus, Ter, Llobregat and Guadalquivir systems. This expansion over several decades has brought sport-fishing interests into conflict with conservation priorities: the species was introduced for angling, but its predatory role has raised concern about native fauna, habitats and fisheries. The Ebro’s Mequinenza and Riba-roja reservoirs are associated with later spread within the basin.
Removal can be costly and difficult in large, interconnected river systems. Eradication is more feasible in isolated waters than across major basins, and further introductions can undermine control efforts. A similar pattern of individual actions with far-reaching consequences has featured in other high-profile disputes, including the superyacht legal battle reported by RUSSPAIN.
What Anglers Must Know
Spain lists Silurus glanis in its Catalogue of Invasive Alien Species under Royal Decree 630/2013. The regime prohibits keeping, moving, selling or releasing the species, as well as using it as bait; the article’s stated guidance also warns against returning captured specimens to the water. MITECO advises anglers to photograph a catch, record its location and notify environmental authorities. In large rivers, prevention and monitoring are basic because complete eradication is far less practical than in an isolated lake or small reservoir. Environmental DNA and acoustic tracking are among the techniques used to support detection.
The national aquatic conservation strategy, approved in July 2024, calls for coordinated action between administrations and protocols tailored to individual water bodies. Preventing new releases remains basic to that approach, alongside early detection and control.
Spain’s experience with Silurus glanis shows how a deliberate introduction for sport fishing can create a long-term invasive-species management problem. The challenge now is to limit further spread, manage impacts on native river life and protect vulnerable populations.
