Giant Invader: How the European Catfish Conquered the Continent’s Rivers

Few freshwater predators inspire as much fascination and alarm as the European catfish, Silurus glanis. Capable of growing to three metres in length and thriving in waters far beyond its native range, this giant bottom-dweller has been quietly expanding across the continent for decades. Now, a comprehensive review published in the journal NeoBiota has pulled together the scattered evidence on what makes this species such a successful invader, how it reshapes the ecosystems it enters, and which tools scientists can use to detect it before it becomes impossible to remove. The verdict is sobering: the catfish’s success stems from a potent combination of biological traits and human activity, and our ability to track and manage it remains frustratingly fragmented.

The review, which systematically screened 126 scientific publications, documents a clear acceleration in the species’ spread. Over the past seven years alone, Silurus glanis has established itself in new territories including central Italy, Croatia, the Netherlands and small catchments in France. Much of this expansion has been driven not by accident but by deliberate human action. Recreational anglers, drawn by the prospect of catching exceptionally large fish in warmer non-native waters, have repeatedly moved the species into new basins. Escapes from aquaculture and the ornamental trade have added further pathways. This sustained propagule pressure, combined with habitat modifications and a warming climate, has created ideal conditions for a predator that seems almost engineered to colonise novel environments.

The catfish’s biological toolkit is remarkable. It is a long-lived, large-bodied species with rapid growth, reaching lengths of up to three metres, which allows individuals to dominate size-structured communities and escape predation. Its fecundity, typically between 10,000 and 30,000 eggs per kilogram of body weight, remains consistent across native and invaded ranges, indicating that reproductive performance is maintained regardless of environment. In southern Europe, individuals mature earlier and at smaller sizes than in their native eastern range, and a protracted spawning season stretching from February to June extends the window for successful recruitment. Males guard and ventilate nests, improving offspring survival, while social recognition among conspecifics reduces stress under resource limitation and predation risk, promoting persistence after introduction.

Physiological tolerance is equally impressive. The species thrives at water temperatures between 25 and 27 degrees Celsius, with peak activity in late spring and summer. Below 12 degrees Celsius, feeding declines, and it ceases entirely below 7 degrees. Yet in systems where temperatures remain mild, such as the Belver Reservoir on Portugal’s Tagus River, the catfish remains active year-round, shifting seasonally between shallow and deep habitats. Tolerance to low oxygen levels, typically down to 3.0 to 3.5 milligrams per litre and occasionally as low as 1.5, gives it a decisive competitive advantage in degraded or eutrophic waters where other large predators cannot survive. This tolerance, supported by high haemoglobin concentrations, enables persistence in hypoxic conditions and may even drive seasonal aggregations.

Perhaps the most consequential trait is trophic plasticity, the ability to exploit a wide range of prey across diverse habitats. The catfish’s diet spans freshwater fishes, invasive and native crayfish, migratory species, semi-aquatic vertebrates, amphibians and even aquatic insects. It has been observed beaching itself to capture terrestrial prey and can digest otherwise unpalatable items such as perch egg strands. Its hunting strategies shift with life stage, season and diel cycle. Large individuals in deep lakes adopt a pelagic diet focused on coregonids, shad and roach, while juveniles forage in littoral zones on crayfish and small fish. In rivers like the Lower Tagus, smaller individuals feed on crustaceans while larger ones become increasingly piscivorous, sometimes exhibiting aggregative hunting during prey spawning events. This flexibility reduces competition and increases resilience to fluctuating prey availability, allowing the species to establish successfully in almost any freshwater system it encounters.

Yet the ecological consequences of its arrival are far from uniform. In the Torrejón Reservoir in Spain, where the species was first recorded in 2010, native barbel populations experienced dramatic declines and one native species became locally extinct within a year, with the catfish soon becoming dominant by catch-per-unit-effort. In Italy’s Po River lowlands, the catfish increased from 6 to 62 percent of fish biomass since the early 1990s, with several native species becoming locally extinct. In the Garonne River in south-western France, predation on migratory species has been severe: up to 35 percent of Atlantic salmon passing through a monitored fishway were consumed, 80 percent of tagged sea lampreys were eaten within a month, and allis shad constituted nearly 90 percent of stomach contents during peak migration. Stable isotope analysis confirmed that anadromous fish made up between 53 and 65 percent of the catfish diet in that system.

But the picture is not uniformly grim. A national analysis of over 10,000 electrofishing records in France found no consistent reductions in species richness or biomass associated with catfish presence, and some sites even showed slightly higher richness, possibly due to habitat differences or other non-native introductions. In the Tarn River, the species acted as an opportunistic predator with limited impact, likely because most resident species were themselves non-native and had historically co-existed with the catfish, unlike migratory species lacking evolved defences. This variability is reflected in the species’ formal impact classification, where it has been assigned a Major rating for predation on native communities but with low confidence, owing to the largely descriptive nature of the evidence and the prevalence of data-deficient sources.

The review’s authors argue that this inconsistency stems partly from methodological shortcomings. Many studies rely on short-term data, lack pre-invasion baselines or fail to include uninvaded control sites, limiting causal inference. In many invaded systems, native species were already under pressure from hydrological regulation, pollution and habitat degradation before the catfish arrived, making it difficult to isolate its specific contribution to observed declines. The authors call for standardised, long-term monitoring programmes and experimental studies that move beyond correlative evidence, noting that most current research is based on simple correlations between catfish increase and native species decline rather than rigorous causal analysis.

Detection methods present their own challenges. Direct approaches such as gill nets, trammel nets, hoop nets and long-lines each carry distinct trade-offs. Standard gill nets introduce size-selectivity bias, with larger catfish frequently evading capture, while long-lines, though highly selective and effective for eradication, depend on active feeding behaviour that decreases below 12 degrees Celsius and raise ethical concerns around live bait use. Electrofishing can be effective for adults in small flowing systems during warmer periods but struggles in large, deep or low-conductivity waters. Indirect methods offer promising alternatives. High-frequency acoustic sonars can detect large catfish specimens over one metre in length across extensive water volumes without physical disturbance, though they cannot reliably identify smaller individuals. Environmental DNA analysis, particularly quantitative PCR, provides sensitive, non-invasive and cost-effective detection with precise taxonomic identification, but faces challenges from incomplete DNA databases, non-standardised primers and the risk of false positives from contamination.

The review proposes an ideal early warning system combining eDNA-based methods with hydroacoustics, supplemented by citizen science and social media mining for initial screening of high-risk sites. In areas where the species is already established, long-term monitoring strategies should be adapted to the environment, with electrofishing and passive nets suited to smaller waterbodies and hydroacoustic surveys necessary for large, deep systems. The authors also stress the importance of engaging anglers, who often underestimate the species’ impacts or view it as beneficial for recreational fishing, and suggest that promoting the catfish in gastronomy or as raw material could create economic incentives for removal. Several European projects, including LIFE PREDATOR, which combines eDNA monitoring, selective removal and public engagement across Italy, Portugal and the Czech Republic, are already testing these integrated approaches. What remains clear is that without coordinated transnational frameworks, standardised protocols and a shift from descriptive to experimental research, Europe’s largest freshwater fish will continue its quiet, relentless advance.

Subject of Research: Invasion biology and ecological impacts of the European catfish Silurus glanis in European freshwater ecosystems

Article Title: Silurus glanis in Europe: A review of invasive traits, ecological impacts and detection methods

Article References: Silurus glanis in Europe: A review of invasive traits, ecological impacts and detection methods. (n.d.). https://doi.org/10.3897/neobiota.108.158198

Image Credits: AI Generated

DOI: 10.3897/neobiota.108.158198

Keywords: invasive species, European catfish, Silurus glanis, freshwater ecosystems, ecological impacts, eDNA, hydroacoustics, trophic plasticity, early detection, biological invasions, river ecology, species management