Risk assessment and mitigations of the potential impacts of trout predation on New Zealand’s indigenous fish species : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Manawatū, New Zealand

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Trout were introduced into New Zealand freshwater ecosystems ~150 years ago as a prized sports fish, despite unknown potential impacts of this introduction. New Zealand’s native fish are experiencing widespread decline, with trout implicated as one potential key stressor. The concurrent introduction of trout with large-scale land use change have made attributing decline to specific stressors difficult. However, understanding the effect of specific stressors is necessary for management of healthy populations of both the native fauna and valued introduced species. This thesis unpacks the predatory impacts of trout on native fish by (1) using the literature to develop a risk assessment matrix to identify fish species at high risk of detrimental impact from trout predation, and (2) experimentally examine whether trout presence alters the habitat preferences of a small non-migratory galaxiid. Risk assessments provide an avenue for wildlife managers to prioritise conservation and remediation efforts towards reducing the impacts of trout predation on the most at-risk native fish. The risk assessment matrix created in this thesis has triaged species most at-risk of detrimental population impact from trout predation. Perhaps unsurprisingly, the risk assessment identified non-diadromous galaxiids and mudfish as the most vulnerable groups to trout predation. Nationally, approximately 10% of river reaches occupied by trout also overlap with locations of at least one native fish species at high risk of trout impact. These reaches should be a focus for future investigation and mitigation efforts, such as physical habitat restoration, water quality improvements, restoration of hydrological regimes, and active population control. Dwarf galaxiids (a non-migratory native fish) were identified as highly vulnerable to trout predation. I investigated potential impacts of trout on the habitat preference of dwarf galaxiids in replicated mesocosms. The experiments examined two contrasting habitats, comparing sand and cobble substrate, and vegetation and no vegetation. Potential changes in refuge seeking behaviours were examined in the physical presence of trout, the presence of trout odour, and the absence of trout. In the gravel substrate experiment, galaxiids were noted in refuge more frequently when exposed to trout odour, no other significant differences were observed. This indicates that dwarf galaxiids are unlikely to alter their refuge seeking behaviour in the presence of trout, which may leave them exposed to trout predation, and further indicates the importance of instream and riparian refuge for dwarf galaxiids.