Development of a Trait-Based Risk Assessment to Minimise the Impacts of Trout on New Zealand Native Fishes

The management of introduced species to protect native fauna in the face of their rapid decline often presents a costly, technically complex, and sometimes socially controversial challenge. Prioritising when and where conservation efforts are best applied for the greatest efficacy is difficult. Using a systematic risk assessment framework effectively prioritises options among the range of management alternatives. Tensions exist between managing trout in New Zealand as a highly valued sport fishery and conserving native freshwater fish populations. We developed a trait-based risk assessment matrix to identify the native fish most vulnerable to trout pressures and prioritise river reaches where those species are abundant for conservation focus. The trait-based assessment indicated that all species identified as highly vulnerable to trout pressures were either non-migratory galaxiids or mudfish. Trout overlapped in occurrence with approximately 10% of the entire extent of species deemed high risk to trout pressures, equating to 1626 km of river length, largely in central Otago and Hawkes Bay. While 1626 km of river is small compared to the entire riverine network (0.4%), it is still a substantial area for resource conservation interventions. Improving habitat quality to bolster native fish resilience to trout pressures, removing or reducing trout abundance are, however, potentially effective options. The approach adopted here will allow conservation managers to direct these actions to areas where they result in the greatest likelihood of positive outcomes.