Injury Risks to a Migratory Freshwater Fish During Downstream Passage Through a Regulated Floodplain Outlet: A Case Study on Golden Perch ( Macquaria ambigua )

ABSTRACT River infrastructures such as weirs, dams, inlet and outlet regulators often impair connectivity, leading to degradation and loss of key habitats for riverine fishes. This also holds true for golden perch ( Macquaria ambigua Richardson), a migratory species in Australia's Murray–Darling Basin. Regulated outlets between main stem and floodplain habitats can restrict dispersal, and fish may be exposed to harmful hydraulic forces during downstream passage. Associated injury risk for fish mainly relates to rapid decompression (barotrauma), strike and shear stress. In this study, laboratory barometric chamber experiments on juvenile golden perch were conducted to determine decompression thresholds that can cause barotrauma‐related injuries. For field validation under realistic operating conditions, autonomous sensors were used to quantify decompression, strike and shear forces golden perch face during passage through a floodplain outlet regulator. The measured decompression was below the threshold that led to serious injuries in the laboratory testing, and also shear forces remained below known injury thresholds. Conversely, mechanical strike was the main risk during passage at the investigated outlet regulator, with severe strike events (> 95 g ‐force) capable of causing injuries in 73% of runs. The bio‐physical thresholds identified can help inform safer river infrastructure design and operation. Targeted modifications to the existing structures could reduce injury and support golden perch persistence across their range.