Wels catfish in the Ebro river: the invasive species introduced in 1974

The wels catfish, a fish of the species Silurus glanis, was deliberately introduced into the Ebro river basin in 1974, where it found favourable conditions for breeding. Half a century later, this large predator has spread through Spanish rivers and reservoirs, disrupting food chains and threatening native fish, invertebrates and other animals.

How did the wels catfish reach the Ebro river in 1974?

The introduction is linked to Roland Lorkowsky, a German biologist and angler, who brought juveniles from the Danube region into the Ebro basin to establish a new attraction for sport fishing. Accounts of the fish's arrival at the Mequinenza reservoir refer to thousands of fry.

The original number remains disputed, however. Technical records and studies of the species' spread state that 32 juveniles were released into the Segre river, part of the Ebro basin. Despite this difference, the key details of the event are agreed:

• The introduction was intentional and took place in 1974;
• The fish came from the Danube region;
• The purpose was connected with sport fishing;
• Further transfers helped extend their distribution;
• The species reached reservoirs and rivers connected to the Ebro.

Why did the fish spread so quickly?

Silurus glanis can withstand deep, murky and low-oxygen waters, conditions found in many reservoirs and slow-flowing stretches of rivers. Once established, it moves through connected waterways, while illegal transfers to new fishing locations speed up its arrival in other river basins.

What makes the wels catfish a “freshwater titan”?

Regarded as Europe’s largest freshwater fish, the wels catfish can grow to around 2.5 metres and weigh more than 100 kilograms. Its elongated body, broad mouth and sensory barbels enable it to find food even in dimly lit, muddy water.

Its diet changes as it grows. Young specimens feed on plankton and invertebrates, whereas adults begin catching larger prey, chiefly at night and around dusk. Animals recorded in its diet include:

• Fish of varying sizes;
• Crustaceans and other aquatic invertebrates;
• Amphibians found near riverbanks;
• Water birds caught on occasion;
• Small mammals approaching the water.

What harm does the invasive species cause to native wildlife?

As an invasive species, the wels catfish occupies the role of a major predator in environments that did not evolve alongside it. Its pressure on fish and macroinvertebrates reshapes aquatic community structures, increases competition for food and may worsen the position of native species already affected by dams, pollution and habitat loss.

Why has controlling the invasion become so difficult?

Removal is challenging in long rivers and connected reservoirs, as individuals sheltering in deep areas can recolonise controlled stretches. Its large size, effective reproduction and the difficulty of detecting small populations also make complete eradication less likely once the fish has become established.

Preventing further introductions has become the main tool for safeguarding the biodiversity of Spanish rivers. Measures include preventing the transport of live specimens, enforcing action against illegal releases, monitoring new areas and removing fish from locations with high densities. The story that began in 1974 demonstrates how releasing a small number of animals outside their natural range can alter a river basin for decades.