Thermal bottlenecks constrain early-life survival of native fish in regulated rivers.

Climate change is reshaping thermal regimes worldwide, threatening species whose early development depends on narrow temperature ranges. Yet directly linking warming to reproductive failure in wild populations remains intractable, particularly in aquatic ecosystems. We developed an empirical framework integrating stable isotope thermometry of archival tissues, landscape-scale temperature modeling, and field surveys to identify when and where successful reproduction occurs. Applying this approach to endangered winter-run Chinook salmon confined below a large dam, we revealed a narrow "critical thermal window" during early development. Surviving juveniles experienced temperatures below 12.5°C within ∼15 days of fertilization, and modest warming during this period sharply reduced survival. In warm years, dam-released cold-water overlapped only briefly with this window, restricting successful reproduction to a small fraction of the total spawning season. These results show how climate warming and river regulation interact to constrain population persistence and provide a framework to guide research on assessing climate vulnerability in temperature-sensitive species.