The European catfish (Silurus glanis) is a non-native species with invasive character to Iberian freshwaters. Being the largest fish species in those invaded water bodies, with high fecundity rates, a large life expectancy and an extraordinary predatory potential, S. glanis has all the indicators that it could be exerting a dangerous pressure on native fish communities. Albeit there are some studies regarding the activity and depth use of this catfish, many of them are restricted to its native range and do not describe the circadian and annual behaviours in detail. Moreover, no studies have compared the differences in habitat use and movement ranges between adults and juveniles in a recently invaded territory. To fill these knowledge gaps, this study resorts to acoustic biotelemetry to track 25 fish (10 adults and 15 juveniles) in a Tagus river reservoir, the Belver dam, through an array of fixed acoustic receivers. The fish were internally tagged with acoustic transmitters, which, in the case of the adults, including a 3D-accelerometer and pressure sensors that allow obtaining information on activity and depth use for over a year. The results show that S. glanis is active throughout the year but with higher activity levels in summer and minimal in autumn, and with a crepuscular and nocturnal increase in activity. This species occupies shallower depths in spring/summer, while in autumn/winter roams at relatively deeper waters. Circadian vertical movement patterns were identified; however, they vary seasonally and have some individual variability. The areas used by the adults are larger than the juveniles’ and increase in warmer months. Adult preferences in the use of specific areas across the year and a possible migration to a spawning site were identified. Such findings will support the development of more effective control measures, for instance, by providing information on how to allocate the fishing efforts in space and time to maximize the efficiency of mass removal actions of this invasive fish.

Introduction: Biological invasions are a major driver of biodiversity loss, particularly in freshwater ecosystems. The Iberian Peninsula, a hotspot of endemic diversity, is increasingly threatened by invasive predatory fish, which may exert higher predatory rates under warmer environmental conditions, disrupting/endangering native fish communities. One such species is the European catfish (Silurus glanis), a large and voracious apex predator. Despite growing research, most telemetry studies have focused on lentic systems, limiting our understanding of its behaviour in lotic environments. Moreover, high-resolution biologging approaches remain largely unexplored. Objective: This study aims to characterize the habitat use and activity patterns of European catfish in a non-native lotic section of the lower Tagus River, and to identify key environmental drivers shaping its predatory behaviour. Methodology: Adult individuals were tagged with radio telemetry transmitters equipped with temperature, pressure (depth), and 3D-accelerometer archival sensors. A preliminary controlled experiment established activity thresholds to classify behaviours. Ten adult fish were then actively tracked over one year, combining spatial data with high-resolution biologging. Habitat use and activity patterns were analyzed across seasonal and circadian scales. Generalized Additive Models (GAMs) were used to assess the effects of environmental variables on activity levels and depth use, while Hurdle models were applied to identify the environmental drivers influencing the occurrence and frequency of burst activity events (predatory behaviour proxies). Results: Fish displayed strong site fidelity, frequently using structured habitats near riverbanks. European catfish also showed clear seasonal and circadian patterns in habitat use and activity, occupying deeper habitats in winter and shallower areas in warmer seasons. Activity occurred year-round, increasing in spring and summer and peaking at dusk, being influenced by temperature, river flow, season, and time of day. Burst activity occurred more often in spring and at dusk. Conclusions: This study unveils insights on European catfish behaviour in invaded lotic systems, highlighting consistent patterns linked to environmental conditions. These findings can support more targeted and effective management strategies for controlling this invasive species.

Inland wetlands are important biodiversity hotspots and amongst the most impacted ecosystems worldwide. Conservation management and restauration actions in wetlands are thus urgently needed to reverse trends in species loss and habitat degradation, particularly in regions harbouring already endangered endemic species. Inland wetlands may play an important role in supporting endemic endangered fishes in the Lower Tagus basin, where anthropogenic pressures have been increasing, but there is a lack of studies on fish communities, and few areas are identified as inland wetlands. Here, we aim to identify small inland wetlands in the Lower Tagus River and their potential role in supporting fish species, constituting the first study to identify and evaluate the most important fish communities. Inland wetlands were identified through the usage of remote sensing techniques and the calculation of a Normalized Difference Water Index (NDWI) with Sentinel-2 imagery for the Lower Tagus region. From a total of 486 locations identified, 31 were recognized as wetlands as having potential to host fish communities, with 11 being selected for sampling after in loco assessment. Fish sampling was conducted between 6 May and 11 June 2021. Furthermore, for each wetland, we evaluated anthropogenic stressors and land use changes between 2007 and 2018, using national land use data (i.e., Carta de Uso e Ocupação do Solo). A total of 7727 fishes from eight non-native and five native species were captured. Overall, fish communities were dominated by non-native species (97% catches), but both European eel (Anguilla anguilla) and Lisbon arched-mouth nase (Iberochondrostoma olisiponense), which are globally classified as critically endangered (CR), were found in at least two wetlands. Our results suggest that, over the last 10 years, intensive agriculture decreased (on average ≈ 3%) in the areas surrounding these wetlands, being replaced by extensive agriculture or natural uses. Despite non-native fish prevalence, some wetlands may act as refuge habitats for CR fish species. These results are important for guiding the restoration of inland wetlands and promoting conservation management actions to help reverse fish diversity loss.