Sergio Bedmar, a doctoral researcher at the Doñana Biological Station, with one of the juvenile Wels catfish captured during a sampling session in the Rivera de Huelva. Photo credit: Miguel Clavero

Researchers from the Doñana Biological Station – CSIC have captured three juvenile Wels catfish during a monitoring survey in the Rivera de Huelva. This discovery confirms that the species is reproducing in the area and highlights the serious threat it poses to aquatic ecosystems in the Lower Guadalquivir, including those of Doñana.

The Wels catfish (Silurus glanis) was first detected in the Guadalquivir basin in 2011, when anglers caught it in the Iznájar reservoir on the Genil River, which borders the provinces of Córdoba, Málaga, and Granada. By 2015, Wels catfish were being caught in another Guadalquivir tributary, the Rivera de Huelva, in a location so far from Iznájar that experts believe it must have been introduced independently. Since then, reports of its presence in the Lower Guadalquivir have only increased. The recent capture of juvenile individuals confirms that the species is not only present but is now established and reproducing.

These captures were part of the CRAYMAP project, in which researchers at the Doñana Biological Station study two invasive crustacean species: the red swamp crayfish (Procambarus clarkii) and the blue crab (Callinectes sapidus). The Rivera de Huelva is one of the project’s monitoring sites.

“It’s a particularly interesting stretch of river, as it remains ecologically connected to the sea despite being about 120 kilometers from the mouth of the Guadalquivir,” explains Miguel Clavero, lead researcher on the project. It was during the most recent sampling effort that three juvenile Wels catfish—each just over 30 centimeters long—were captured. “These individuals were born last year, confirming that the species is reproducing in the lower reaches of the Rivera de Huelva,” he adds.

The Wels catfish: a threat to ecosystems

The Wels catfish is one of the largest freshwater fish in the world, capable of growing over 2.5 meters in length and weighing more than 100 kilograms. Native to large rivers in Eastern Europe, it has been introduced widely for sport fishing. It first arrived in the Iberian Peninsula in 1974, when juveniles were released into the Mequinenza reservoir on the Ebro River. Over the past 50 years, the species has gradually spread to new rivers and further within rivers where it was already present—always with human assistance, as people transport these fish from place to place. Today, Wels catfish are found in nearly all Iberian rivers, and their expansion shows no sign of slowing.

A generalist predator, the Wels catfish can eat virtually any animal that fits in its massive mouth—from crustaceans to birds, fish to mammals. “Its ability to alter the ecosystems it invades is enormous,” says Clavero. “This is due not only to the sheer size of the largest individuals, but also to the fact that established populations tend to be dense and include individuals of a wide range of sizes—from just a few centimeters to over two meters. In such a scenario, almost no animal is safe from predation.”

Impacts in the Rivera de Huelva and the future of Doñana

The stretch of the Rivera de Huelva studied by the Doñana Biological Station is unique within the Guadalquivir basin. “The stretch downstream from the El Gergal dam, built in 1979, is the best remaining habitat for migratory fish species in the entire basin,” says Clavero. This section supports a particularly high density of European eel (Anguilla anguilla), a critically endangered species. “It’s especially notable for hosting many large females—over 70 centimeters long—which are now extremely rare and are believed to play a crucial role in the species’ life cycle due to their higher likelihood of reaching the Sargasso Sea and producing a large number of eggs.” But even these large eels are vulnerable to Wels catfish predation.

This stretch also supports populations of mullet (Chelon ramada) and likely even sea bass (Dicentrarchus labrax). In the not-so-distant past, the Rivera de Huelva saw large numbers of sea lampreys (Petromyzon marinus) making their spawning runs, along with allis shad (Alosa alosa) and twaite shad (Alosa fallax). It may have even once hosted spawning by the now nearly extinct sturgeon (Acipenser sturio). The river is also home to freshwater shrimp (Atyaephyra desmarestii) and at least three species of freshwater mussels, a group of animals that is highly threatened. “A stable Wels catfish population in this area would be disastrous for the incredible natural value that remains and a major setback to restoring what has been lost,” warns Clavero.

The expansion of the Wels catfish in the Lower Guadalquivir threatens to bring the species into Doñana. In particularly wet years, the aquatic ecosystems of Doñana become more connected to the Guadalquivir River, making it easier for invasive species to arrive. This is what happened with the black bullhead (Ameiurus melas), which was first detected in the Guadalquivir in 2007 and reached Doñana by 2010. “A similar scenario with the Wels catfish is highly likely, so it's essential to have early detection systems in place and to prepare a rapid response plan for when the species arrives. If we wait too long, the problem will become unmanageable,” Clavero concludes.