Flow‐Regulated Migration Phenology of Potamodromous Fishes in a Large Regulated Australian River

Phenological shifts in animal migration are widely reported under climate warming, yet responses of warm‐temperate river fishes in regulated systems remain poorly resolved. Understanding how hydrological and thermal cues interact to structure migration timing is central to predicting ecological responses in large rivers and informing environmental water management.

We analysed 31 years (1994–2025) of daily fishway monitoring data from the Murray River, south‐eastern Australia (5460 sampling days; 349,305 individuals), to quantify long‐term variation in migration phenology of five potamodromous fish species across juvenile and adult life‐stages, and to evaluate the relative roles of river discharge and water temperature in structuring migration timing.

River water temperatures increased significantly over the study period, particularly during summer and autumn. Despite this warming, migration timing remained largely stable across species and life‐stages, with strong interannual variability driven primarily by episodic flow events rather than gradual temperature change. Only Murray cod ( Maccullochella peelii ) exhibited a modest advancement in migration timing, associated with low‐flow conditions.

These results indicate that, in warm‐temperate regulated rivers, migration phenology is structured primarily by hydrological variability rather than temperature trends alone. Importantly, the apparent absence of directional phenological change represents an informative ecological signal, demonstrating that life‐history plasticity, facultative migration and flow regulation can buffer migratory timing against observed moderate climatic warming.

Our findings highlight the context‐dependence of climate–phenology relationships in freshwater ecosystems and have direct implications for the timing of environmental flows and fish passage management in regulated rivers.