Biologging an invader: habitat use and activity patterns of the European catfish in the lotic Tagus River (Portugal)

Abstract

The European catfish (Silurus glanis), an invasive species recently introduced to the Iberian Peninsula, presents a significant ecological threat due to its large size, high fecundity, and strong predatory potential. As a voracious apex predator, it can disrupt fish assemblages across the region. While biotelemetry studies have examined its habitat use and activity patterns in nonnative areas, most focused on lentic systems using passive telemetry, leaving its lotic behaviour understudied. Since its first record in Portugal (2014) and subsequent establishment in the Tagus and Douro rivers, understanding its habitat use and activity patterns in these dynamic environments has become crucial for effective management. To address this, 12 adult catfish were tagged with radio telemetry archival tags equipped with temperature, pressure (depth), and 3Daccelerometer sensors to assess habitat use and activity patterns in a lotic stretch of the lower Tagus River. A controlled experiment with two individuals validated acceleration-derived activity thresholds, classifying behaviours as immobile (<0.03 g), mobile (0.03–0.78 g) or burst movement (>0.78 g). The remaining 10 fish were actively tracked for a year. Results showed that catfish occupied deeper habitats in winter (mean depth: 3 m) and moved to shallower areas in spring and summer (mean depth: 1.6 m). Activity persisted year-round but was lower in winter and autumn and higher in warmer seasons. Circadian depth use patterns remained stable, with fish preferring shallower depths during the day and deeper habitats at night. Activity peaked at dusk and was lowest during daylight hours. Individuals exhibited strong site fidelity, consistently occupying small areas near riverbanks. These patterns were strongly correlated with several environmental predictors, possibly linked to prey availability and reproduction cycles. Findings provide valuable insights for targeted management strategies, including optimizing timing and location of fishing efforts to improve mass removal actions aimed at controlling this invasive species.