Intact coastal nursery rearing amid climate-driven phenological shifts in threatened salmon

Literature linking phenology to climate has proliferated in recent decades and shows that climate change has reorganized seasonal opportunities worldwide. However, this literature often quantifies changes in point measures of timing (e.g., beginnings and peaks) without context about duration (e.g., rearing season length) or performance (e.g., juvenile development). Consequently, phenological studies often cannot discern whether climate change truncates biological processes or merely recenters them intact around different dates, which limits the utility of these studies for climate vulnerability assessments. Showing how research may bridge this gap, we quantified links between temperature and coastal nursery rearing phenology in threatened Chinook salmon (Oncorhynchus tshawytscha, Walbaum 1792). Long nursery rearing periods are crucial to enabling growth that promotes survival and life-history diversity that spreads risk. Because salmon can express diverse, climate-influenced phenologies and our study region warmed but remained temperate, we sought to discern whether nursery rearing was intact or truncated amid potential phenological change. Leveraging ∼170,000 observations across three decades and three basins, we found that warm years generally advanced populations' seasonal trajectories of habitat use and growth and that they did not reduce annual 95th percentile body lengths. Additionally, warm years did not consistently compress or expand habitat use in rivers or deltas but consistently expanded habitat use in nearshore marine waters. Overall, there was substantial evidence that warm years advanced nursery rearing intact and little evidence of truncation. Thus, climate change shifted when populations reared but did not prevent populations from completing rearing. Global research on phenological change has begun to transcend descriptions of patterns to consider biological consequences. Simultaneously, efforts to assess populations' climate vulnerabilities have become widespread. Studies that inspect phenological change to distinguish intact processes from truncated processes could provide insights into climate vulnerabilities. Coastal nursery rearing is one process that may especially befit this framework.