Evaluating brown trout as a potential biological control agent 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 throughout 2023. Results indicated that brown trout were able to predate on signal crayfish but did so infrequently as only 12.24% of the sampled fish showed signs of signal crayfish in their stomachs. The number of signal crayfish in the stomach contents of brown trout was also 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. However, this situation may change in the future since brown trout may increasingly consume signal crayfish as they become more familiar with this novel prey. Therefore, it is important to conserve key native populations, and even reinforce their abundance, to allow communities to develop effective resistance to non-native species. In the meantime, a multifaceted management approach that incorporates additional control strategies, such as mechanical removal, is recommended to reduce the abundance and biomass of signal crayfish, potentially mitigating their impact and helping to maintain ecosystem balance in this protected area.