The current study reports the first occurrence and the spread of the European catfish Silurus glanis (Family: Siluridae) in the Portuguese section of the Douro River, suggesting a potential expansion of its distribution in Portugal either via westward dispersal across international rivers and/or human-assisted introductions into new reservoirs and drainages. The European catfish has unique features (e.g., opportunistic predator, hunting, and aggregation behaviour) that make it highly suitable for establishing self-sustaining populations in new areas and likely contribute to its invasion success. The species may severely affect native prey communities and modify food web structure and ecosystem functioning. Efficient and sustainable management actions are needed to prevent further introductions in the future.

The early detection of invasive species in aquatic ecosystems is particularly challenging with most records of new species made years after their initial invasion, by which time they are already widespread. Recently, early detection tools such as citizen science and environmental DNA have emerged, significantly improving early warnings in aquatic ecosystems. However, there is limited understanding of how these new tools complement each other and how consistent they are. In this study, we present a case study combining social media data mining and environmental DNA (eDNA) to detect new records of the invasive European perch (Perca fluviatilis) in mainland Portugal. From 2021 to 2024, we analyzed online angling groups to identify potential new areas of invasion. Later, water samples were collected from several reservoirs and tested for the European perch presence using real-time quantitative PCR. This combined approach detected four new locations of European perch. Moreover, eDNA analysis revealed three new potential populations, while data mining appears to offer near real-time tracking of the species’ spread. This work showcases the improved efficiency and early detection benefits of this integrated approach for monitoring freshwater invasive fish, with broader applicability to other invasive species.

The introduction of non-native species poses a critical threat to biodiversity in Mediterranean freshwater ecosystems. This study documents the first record of non-native Squalius hybrids in the Ave River (Iberian Peninsula), combining morphological and molecular data to confirm their presence and assess their potential origin. The specimens exhibit hybrid characteristics, with most morphological and meristic traits overlapping those of both S. cephalus and S. squalus. Almost all individuals analysed showed an admixed genomic background for both species, suggesting a hybrid origin likely within the species’ native range before their introduction into Portugal through human-mediated activities, likely related to recreational fishing. Furthermore, molecular data also supports hybridization with the native S. carolitertii, raising conservation concerns given the negative impacts associated with hybridization with non-native species. By combining data from social media and citizen science platforms, we were able to map the rapid expansion of this invasive chub in the Ave River from 2018 until now. Our findings emphasize the urgent need for population control measures, while enhancing monitoring, genetic studies, and increased public awareness to mitigate the impacts of invasive species on Iberian freshwater biodiversity.

In recent decades, the European catfish Silurus glanis has spread across Europe and elsewhere, significantly impacting the native fauna of the recipient ecosystems. Studies on its reproductive traits from its non-native range are limited, although this is a critical step in developing effective management plans. Here, we explore the reproductive characteristics of invasive European catfish populations in the Lower River Tagus in Portugal, focusing on its sex ratio, size at maturity, spawning period, fecundity and oocyte diameter. European catfish were collected monthly by electrofishing, gill nets, baited hooks, and professional fishermen from January 2022 to November 2023. A total of 217 females, 156 males and 301 immature fish were analysed. The female-to-male sex ratio was 1.4: 1, with females and males reaching size at first maturity (TL50) at 72.9 and 68.8 cm total length, respectively. The spawning season was protracted, from February to June, showing asynchronous oocyte development patterns. The absolute fecundity was positively correlated with total length and total weight and ranged from 8,364 to 319,000 oocytes. Relative fecundity varied from 4.39 to 23.53 oocytes, with the mean diameter of the matured oocytes ranging from 1.50 to 3.21 mm. The results obtained in this study enhance our understanding of the reproductive biology of invasive species and can contribute to the development of effective management strategies, such as population control and density-dependent fecundity strategies.

The current study reports the first occurrence and the spread of the European catfish Silurus glanis (Family: Siluridae) in the Portuguese section of the Douro River, suggesting a potential expansion of its distribution in Portugal either via westward dispersal across international rivers and/or human-assisted introductions into new reservoirs and drainages. European catfish has unique features (e.g., opportunistic predator, hunting, and aggregation behaviour) that make it highly suitable for establishing self-sustaining populations in new areas and likely contribute to its invasion success. The species may severely affect native prey communities and modify food web structure and ecosystem functioning. Efficient and sustainable management actions are needed to prevent further introductions in the future.

Environmental DNA (eDNA) metabarcoding has revolutionized ecological and environmental research by describing communities without relying on direct observations, making it a powerful, non-invasive, and cost-effective tool in biodiversity monitoring. However, implementation of eDNA as a standard protocol in long-term monitoring programs, that have traditionally relied on capture-based methods, poses challenges in terms of data comparability. Here, we compared freshwater fish communities assessed through eDNA metabarcoding and electrofishing, across 35 sampling sites in the lower Tagus River basin, Portugal. For most species or species-groups analyzed individually (13 out of 17), there was a significant correspondence between electrofishing and eDNA metabarcoding detections. The correspondence was weaker when comparing the number of specimens captured by electrofishing with the number of eDNA metabarcoding reads, with seven out of 13 taxa showing significant relationships. Species richness estimates based on the two methods were very similar at the basin level. The methods yielded significantly different species compositions, although these differences were driven by samples collected in the Tagus main channel, which is wider and has higher flow rates than tributaries. Benthic and shoreline fish communities showed similar species composition in the two methods, but this was not the case for pelagic communities, probably due to the higher water turnover of the pelagic zone and electrofishing inefficiency. Our results highlight the high potential of eDNA metabarcoding as a complementary method to electrofishing for freshwater fish monitoring, though further validation is needed to assess biases related to site-specific hydrological conditions and the ecology of the target species.