In a warming river, natural-origin Chinook salmon spawn later but hatchery-origin conspecifics do not

Median timing of reproduction in salmonid populations is generally consistent among years, reflecting long-term patterns of natural selection from characteristics of the local environment. However, altered selection from factors related to climate change or human intervention might shift timing over generations, with implications for the population's persistence. To study these processes, we modeled median timing of redd (nest) counts as an index of spawning timing by natural-origin Chinook salmon (Oncorhynchus tshawytscha) in the Skagit River system in Washington State, USA. Over the last 2-6 decades, natural-origin salmon have been spawning later by 0.03-0.52 days.year(-1), while a naturally spawning group that is influenced by strays from a hatchery has been spawning earlier by 0.19 days.year(-1). Trends in the spawning timing of hatchery-origin strays may reflect opposing selection from the hatchery, where egg take for propagation has become earlier by 0.58 days.year(-1). As mean August river temperatures have risen over the period of record, hatchery timing trends may be moving in the opposite direction from the plastic or adaptive patterns expressed by natural-origin fish.

Automated summary

Research shows that the median timing of reproduction in salmonid populations is generally consistent, but factors related to climate change or human intervention may shift this timing over generations, impacting the population's persistence. Natural-origin Chinook salmon in the Skagit River system have been spawning later, while hatchery-origin strays have been spawning earlier. Trends in spawning timing may differ between natural-origin and hatchery-influenced populations, reflecting opposing selection and potential impacts of rising river temperatures.