Abiotic environmental conditions provide important cues for critical life-history behaviours in aquatic organisms. In river ecosystems, seasonal changes in temperature and discharge are widely recognised as key drivers of freshwater fish spawning, shaping the timing of breeding events and potentially signalling suitable rearing conditions for young. Anthropogenic pressures, including climate change, are altering river abiotic conditions and therefore may shift the timing, duration or reliability of these cues, with consequences for early life history and recruitment. Understanding changes in breeding phenology is therefore important for assessing species vulnerability to a changing climate and future population resilience. For example, species with a broader range of appropriate conditions for spawning (i.e. a wide spawning window) may be better able to accommodate future environmental change; whereas species with narrower spawning windows and environmental requirements may be more susceptible to climate-driven mismatches between environmental conditions and reproductive timing.
In this study, we analysed 30 years of larval fish survey data from several northern Victoria rivers and investigated temporal trends in the onset of spawning for multiple species. We found a disconnect between increasing spring/summer air temperatures and water temperatures. Variation in annual water temperature resulted in mixed long-term trends in spawning onset dates between species and rivers. By describing the thermal conditions in which larvae have been observed we built a distribution of appropriate conditions for this critical life history event and discuss the impact that future temperature changes may have on the timing of spawning in these rivers. Understanding, and if possible managing environmental conditions that support successful breeding, is critical for maintaining native fish populations into the future under a changing climate.