Small population sizes above barriers suggest conservation trade-offs between isolation and invader impacts for vulnerable native fish populations

Interventions using exclusion barriers to mitigate invasions in freshwaters and protect native species are known as isolation management. Such barriers are often used to secure vulnerable populations of native river-resident galaxiids, threatened by non-native salmonids (trout), however effectiveness and riskd of this strategy have not been fully assessed both generally and for galaxiids. We quantified fish abundance and habitat size above barriers preventing trout incursion for four species of threatened galaxiid, with upstream population limits determined by electrofishing and environmental DNA sampling. Across all populations habitat extent ranged from 1.57 km to 152m, meaning habitats above such barriers are generally small, making populations occupying them vulnerable. Above-barrier population sizes were also small and generally related to habitat length. Combining these population size estimates with temporal trends revealed evidence of severe population declines above barriers due to both trout invasion and drought in some cases. Total estimated population sizes varied but were never large and many have undergone dramatic fluctuations as indicated above. Overall, our study indicates that barriers play a key role in threatened species protection, but these populations remain inherently vulnerable to future invasion and stichastic events associated with isolation. Careful planning, rapid non-desirable species removal and post intervention monitoring are required to ensure population persistence. Moreover, isolation management is only a stopgap to secure fragments and unlikely capable of reversing population trends unless habitat availability is not limitted. It must be considered as part of more holistic management actionssuch as extended reserves, habitat restoration, trout removal, captive breeding and translocations.