<p>The Golden Mahseer <i>Tor putitora</i>, is an endangered, migratory freshwater fish of ecological, commercial and cultural importance that inhabits foothill rivers in the Himalayas. This transboundary fish occurs across Bangladesh, Bhutan, India, Pakistan and Nepal, and is facing increasing pressures from multiple stressors including resource extraction and the rapid development of hydropower dams. The DRIFT ecosystem-based model was used to guide Environmental Flows assessments of a several hydropower dams in this region, with Golden Mahseer as a key indicator. Countless other hydropower dams continue to be developed in Golden Mahseer habitat without sufficient assessment of their impacts on this flagship species, thereby threatening its long-term survival. This study uses the decades of iterative work encapsulated in three DRIFT ecosystem-based models to calculate the contributions of individual causal mechanisms hypothesized to influence Golden Mahseer abundance in response to variations in natural resource management and hydropower operations. The resulting ranking of contributing mechanisms showed that the major mechanisms predicted to drive future change in Golden Mahseer abundance were those related to unsustainable levels of fishing, hydropeaking and barriers to migration. These impacts were assessed to be individually severe and cumulative such that focusing on one aspect without due consideration of the others was ineffective. On the basis of these findings five rules for hydropower are proposed to support Golden Mahseer conservation.</p>