Brown trout as a potential biological control of signal crayfish

This study evaluates the potential of brown trout ( Salmo trutta ) as a biological control of a recently established signal crayfish ( Pacifastacus leniusculus ) population in a protected area (Baceiro River, Montesinho Natural Park, Portugal). Five sampling sites were monitored over an entire year. Results indicated that brown trout was able to predate on signal crayfish but in an infrequent manner since only 12.24 % of the samples showed signs of signal crayfish in their stomachs. The number of signal crayfish in the stomach contents of brown trout was low (only 2.13% of all prey items), but accounted for 17.70% of the total biomass of all retrieved prey items. Predation was higher in the warmer months and was size-dependent, with larger fish more able to predate on this non-native crayfish species. These findings highlight that, although brown trout can prey on signal crayfish, their effectiveness as a biological control agent is limited due to their lower abundance and predation rates. Given the complexity of trophic interactions, a multi-faceted management approach integrating other control strategies is recommended to decrease the abundance and biomass of the signal crayfish, and potentially their impacts, to maintain the ecosystem balance in this protected area.