Novel methods for managing freshwater refuges against climate change in southern Australia: Evaluating small barrier removal to improve refuge connectivity - a global review of barrier decommissioning and a process for southern Australia in a drying climate

Severe surface flow reduction is exacerbating the impacts of the existing stressors on aquatic ecosystems in southern Australia. River regulation via the creation of instream barriers such as dams and weirs is known to have wide ranging impacts on aquatic ecosystems and mitigating these barriers can greatly benefit aquatic fauna, particularly freshwater fishes. Given the continued reduction in surface flows and aging of barrier infrastructure, there will be an increasing need to assess, prioritise and decommission such structures particularly in southern Australia. Whilst considerable attention has been given to barrier mitigation and prioritisation in eastern Australia, there is less information on how barriers impact the ecosystems in the often intermittent rivers across southern Australia; particularly with regard to the relationship between barriers and refuge pools. There is also limited information on the impacts of complete barrier removal as opposed to barrier retrofitting and there is no consistent process for identifying and prioritising barriers for mitigation or removal across southern Australia. The current project aimed to review the global literature on the impacts of barriers and the processes that exist for prioritising their removal. It then aimed to develop and trial a process for instream barrier prioritisation tailored specifically for systems in southern Australian. Mitigation of instream barriers through the construction of fishways has been increasingly undertaken in Australia and internationally to reconnect fish communities however, whilst often partially effective, they are limited in terms of fully reconnecting fish communities and are also costly. Complete removal of barriers has therefore increasingly been undertaken particularly in the United States and Europe to completely reconnect river reaches. Both strong advocacy and opposition can exist to instream barrier removal projects and it is vital to have broad stakeholder involvement; particularly at a local level. Artificial instream barriers can also actually create important refuge habitats that may increase in significance particularly as surface flow reductions continue in southern Australia. However, spatial information on refuge pools across southern Australia is limited as are their relative ecological significance. Barrier prioritisation processes in Australia have usually utilised score and ranking systems but little regard has been given to optimising multiple barrier mitigations. The barrier identification and prioritisation process we developed is a stepwise protocol that is underpinned by broad stakeholder involvement and can be applied on multiple scales from single rivers to multiple catchments. It identifies existing information on barriers and aquatic fauna and also confirms barrier and refuge information using a cost-effective surveying protocol. It includes a score and ranking system that weights both the positive and potential negative impacts of removing instream barriers and incorporates information on species diversity, habitat availability, and spatial information on barriers and refuges. The process was trialled in three catchments in south-western Australia. Information on potential barrier locations and fish distributions was obtained by accessing GIS and distributional databases and undertaking local landholder surveys. The rapid aerial survey technique was found to be highly effective at confirming GIS information and identifying new barriers. The score and ranking system revealed that the least modified catchment had the highest scoring barriers. The information contained in this review will be of considerable interest to managers of fluvial ecosystems in temperate Australia and the prioritisation process will be a valuable and easily implemented tool in identifying and mitigating the impacts of in-stream barriers in southern Australia in a drying climate.