Trophic response of brown trout to signal crayfish invasion in a protected mountain river

We investigated the dietary response of brown trout ( Salmo trutta ) to the invasion of the signal crayfish ( Pacifastacus leniusculus ) in the Baceiro River (Montesinho Natural Park, Portugal). We analyzed 1012 stomach contents across six sites (five invaded and one non-invaded) over the year of 2023. A total of 49 distinct dietary taxa were identified. Results reveal strong spatial and temporal variation in trout diets. While aquatic invertebrates (e.g., Baetidae, Heptageniidae, Simuliidae , oligochaetes, and signal crayfish) dominated in the spring and early summer, a shift toward terrestrial insects was observed during summer. Generalized Linear Models indicated that signal crayfish biomass significantly influenced prey richness, biomass, and diversity, while multivariate analyses confirmed distinct prey assemblages across space and time. Our study also showed that signal crayfish were consumed occasionally (2.09 % of all prey items) but accounted for 17.68 % of total prey biomass. This situation suggests limited, but non-negligible, trophic incorporation of the invasive crayfish on the brown trout diet, with possible implications for brown trout condition and food web structure. This study highlights the dynamic nature of trophic interactions following biological invasions and underscores the need for integrated management strategies to mitigate the impacts of P. leniusculus in protected freshwater ecosystems. • Brown trout diets showed spatial and temporal variation. • A shift from aquatic to terrestrial invertebrates was observed. • Crayfish biomass strongly affects trout prey richness, biomass and diversity. • Signal crayfish were consumed occasionally by brown trout. • Management actions are necessary to mitigate the impacts of P. leniusculus .