Managing Isolation: Implementing In‐Stream Barriers to Exclude Introduced Trout From Fragmented Native Freshwater Fish Refuges

ABSTRACT Fish invasions are a key driver of change in freshwater systems, impacting biodiversity and economies. The use of in‐stream barriers to prevent or mitigate such invasions and protect native fish populations, known as isolation management (IM), has been increasingly reported. However, despite the need to secure vulnerable populations, trade‐offs between isolation and invasion are likely, and it is unclear if current knowledge supports optimal long‐term outcomes. Aotearoa New Zealand has a history of using exclusion barriers to prevent incursions of introduced sports fish into native river‐resident galaxiid (RRG) refuges predominantly within the South Island. We assessed 37 structures and predicted successful exclusion would be due to a combination of barrier parameters and habitat. Additionally, we assessed global exclusion knowledge, conducting a systematic review and meta‐analysis of peer‐reviewed studies of trout barriers to examine the prevalence of quantitative barrier parameters in the literature. We found that drop height had the strongest relationship with exclusion in both field survey and meta‐analysis, and downstream pool depth was also negatively correlated with exclusion in the field data. Less than a third (27%) of trout barrier studies identified by systematic review reported quantitative barrier parameters, and barrier height was the only parameter found to have a relationship with exclusion. Only six (11%) of the quantitative studies described deliberate exclusion of fish for ecological gain. Trout barriers had successful outcomes in New Zealand, with galaxiid densities higher above successful barriers than any other position; therefore, there is evidence supporting barrier use in conservation.