The Wels catfish, European catfish Silurus glanis L.

LIFE PREDATOR tackles the most emblematic case of invasive alien fish species in western and southern Europe: the European catfish Silurus glanis, also called wels catfish. Native to eastern Europe and western Asia, it is now established in many countries out of its native range (Cucherousset et al. 2018). In Med countries, where temperatures are more favourable, it is considered highly invasive, threatening native fish and biodiversity mostly by direct predation and (out) competition (Cucherousset et al. 2018).  European catfish in a nutshell:

  • it is the largest freshwater fish species in Europe and in the introduced range, with adults being at least twice larger than native PREDATORs;
  • it is highly adaptable, tolerant, capable of securing and ingesting a wide range of prey, long lived, and fast growing.
  • it carries pests and diseases (SVC and ESV) that may adversely impact native fish.
  • due to its large size, very fast metabolism, aggressive behaviour in spawning period, it has a great advantage compared to native top PREDATORs.

Highly appreciated among anglers for its large size (Lyach & Remr, 2019), S. glanis was first introduced outside its native range in western and southern Europe in the XIX century in England and in the XX century in Spain, France and Italy mainly for fishery enhancement. In eastern Europe, it is also exploited as a food resource and hence the second main vector of its introduction was aquaculture. Due to its predatory habit, it was introduced in some cases as a biocontrol agent. Since then, it has established invasive populations in 12 European countries, exerting several negative impacts on local biodiversity, particularly in southern Europe. For instance,

Castaldelli (2013) studied the temporal variation in fish assemblages from 1991 to 2009 in 14 canals of the lower Po River basin in Italy. After its establishment, S. glanis increased from 6% up to 62% of the total biomass. In less than 20 years, some native species significantly declined in abundance and biomass (i.e., Alburnus arborella, Scaridinius hesperidicus) and 10 native species underwent local extinction (e.g., Rutilus pigus, Rutilus aula, Chondrostoma soetta, Tinca tinca). Thus, the success of exotic species, particularly of catfish which thrived in this degraded habitat, led to the decline of native fish fauna. Drastic native species decline, or extirpations, have been observed in Iberian Peninsula (Iberia nature, 2011; Invasiber, 2011). In one Iberic river, it has reduced abundance of most prey species. In L. Busco, may have caused the extinction of Phoxinus phoxinus (Kottelat et al., 2007). In the main stretch of the River Ebro in Spain, catfish has contributed to the extinction of endemic cyprinids, particularly Luciobarbus graelsii (Carol et al., 2009). In the lower Tagus River (Portugal), the European catfish exhibits a broad diet eating mostly fish prey in the lotic section, while in the Tagus reservoirs it predates mostly on crayfish and some non-natives (Ferreira et al., 2019). By feeding on previously unexploited resources, such as terrestrial prey (e.g., pigeons Columba spp., rats; Syvaranta et al., 2010) and larger native fishes, catfish can alter the trophic food webs of receiving ecosystems by creating novel trophic links or via novel predation pressure, often adapting easily to new local conditions. In France, the critically endangered European eel Anguilla anguilla L. constitute at least 50% of the diet of some specialised individuals (Syvaranta et al., 2010). Atlantic salmon returning to the Garonne River for spawning, which were safe from predation due to their large body size preventing predation by native PREDATORs, are now predated by S. glanis in fishways, with an estimated predation pressure of 35%, thus threatening conservation efforts (Boulêtreau et al., 2018). Similarly in the Garonne-Dordogne system, the catfish exerted a very high predation pressure on the anadromous sea lamprey Petromyzon marinus, where 80% of the surveyed lamprey were predated by catfish, 50% of which in only eight days after tagging (Boulêtreau et al., 2020). Predation on anadromous A. anguilla, P. marinus and shad Alosa sp. was also confirmed in the lower stretch of the Tagus River basin in Portugal, where catfish invasion is relatively more recent (2008) than France, Italy, and Spain, but it is already a conservation issue due to its broad PREDATORy impact (Ferreira et al., 2019). The extreme feeding plasticity and potential to establish new predation pressure was confirmed also for one large Italian lake, L. Maggiore, where larger individuals specialised upon pelagic prey fishes, including the native landlocked shad Alosa agone (De Santis & Volta, 2021). This was an unexpected discovery, since this fish is benthic, thus confirming the ability of the species to easily adapt to novel and local conditions and thrive while negatively impacting the local community.

What is an Invasive Alien Species (IAS)?

The project area

Previous conservation efforts in Italy and Portugal

In a risk assessment study done by Almeida et al. (2013), the FISK tool scored a high-risk impact for S. glanis, given the enormous predation potential of this species. Moreover, S. glanis is in the high-risk score category (lower region) of potential pests in ENSAR (European non-native species aquaculture risk assessment) according to its risk of introduction, establishment, dispersal, and impacts, although this may be variable according to context (Copp et al., 2009). Because it is in part native to Europe, S. glanis is not included in the Invasive Alien Species of Union concern, however, Invasive Alien Species Regulation (Regulation (EU) 1143/2014) invite each member state to develop their own list of IAS of national concern.

In Italy, the first record of S. glanis was in 1957 in the Adda River (northern Italy). It was a single individual, probably escaped from a put and take lake in the occasion of an exceptional flooding. The first self-established population was then recorded in the Po River at the end of the 1960s (Gandolfi & Giannini, 1979). Since then, the species has spread in all major river basins of north Italy (De Santis & Volta, 2021), central Italy and has invaded also the largest southern Italian basin, the Volturno river basin (De Bonis et al. 2015). Due to its wide distribution in Italian peninsula and the decline of several prey species detected after its invasion, the species poses a serious risk for the national biodiversity and therefore several regulatory and conservation efforts have already took place., the species has been included in project-supporting legislative tools started to consider this and other invasive species as a national priority. For instance, LIFE 15 GIE/IT/001039 ASAP developed and sent to the Italian Ministry of the Environment a proposal for the IAS national blacklist, including s. glanis and classifying it with the maximum score regarding invasiveness and with 14/15 points as overall level of priority. Also, LIFE GESTIRE2020 included it among the worst invasive alien species in freshwaters (Tamborini et al., 2019). From a regulatory point of view, to prevent this unwanted invader from expanding, Lombardy (LR 31/2008; RR 2/2018) and Piedmont (LR 37/2006) Regions have regulated catfish as prohibited, imposing a ban on both introduction and release of the species, making it mandatory to kill the caught catfish.

Control and eradication campaigns have been carried out in Italy for instance in L. Comabbio and L. Varese but most of the conservation efforts have been targeted so far riverine ecosystems in Italy, while no other projects specifically targeted catfish in large lakes and reservoirs.

In Portugal, the species was first introduced in 2008, in the Tagus River, as indicated by citizen science records (Gago et al. 2016). Nevertheless, the first official record was in 2014, when local fishermen captured two individuals in the Tagus River (Gkenas et al. 2015). At present, the species is restricted to the Tagus River Basin, where it inhabits both lotic and lentic environments (Ferreira et al 2019). However, given the increasing number of new introductions and range expansions of the species in the Iberian Peninsula, and the growing interest by recreational fishermen (Gago et al. 2016), it is highly likely that other basins will be colonised in the near future. For example, citizen science data indicate the presence of catfish already in the Douro River (Martelo et al., 2021). Moreover, the Guadiana River basin may be colonised by individuals from neighbouring drainages, such as the Guadalquivir River basin, where the species has recently been recorded (Sáez-Gómez and Prenda, 2019), or from the Alqueva reservoir, which has been the recipient of several new non-native fishes (Banha et al., 2017).Because of its fast spread and the already reported impacts on Portuguese native biodiversity, S. glanis has been included in the national blacklist (Annex II) of the Decreto-Lei nr. 92/2019, forbidding its transport and detention, and within the legal framework of the Portuguese inland recreational fisheries law (Decreto-Lei nr. 112/2017), anglers and professional fishermen are requested to euthanize this IAS once caught.. Currently, control and other management actions are not carried out systematically in Portugal, but a National Plan for the Catfish Control is under developing, which aims to propose management actions for the species control and communication approaches to prevent the further spread of the species and mitigate its impacts.

Despite regulatory and management actions took already place, and despite the invasion history is different between Italy (long) and Portugal (recent), in both countries the species keeps spreading in new ecosystems and, considering the strong fragmentation of the main rivers of northern Italy and of the Tagus River in Portugal, it is highly likely that the colonization of lakes and reservoirs is mainly attributable to voluntary releases by recreational fishermen, since this fish is considered a valuable resource among them due to the large body size reached.