Shrinking Age Distribution of Spawning Salmon Raises Climate Resilience Concerns
Published:12 Apr.2023    Source:University of California - Santa Cruz
By returning to spawn in the Sacramento River at different ages, Chinook salmon lessen the potential impact of a bad year and increase the stability of their population in the face of climate variability, according to a new study by scientists at UC Santa Cruz and NOAA Fisheries.Unfortunately, spawning Chinook salmon are increasingly younger and concentrated within fewer age groups, with the oldest age classes of spawners rarely seen in recent years. The new study, published February 27 in the Canadian Journal of Fisheries and Aquatic Sciences, suggests changes in hatchery practices and fishery management could help restore the age structure of the salmon population and make it more resilient to climate change.
The researchers focused on Sacramento River fall-run Chinook salmon, which contribute heavily to the salmon fisheries of California and southern Oregon. This population is particularly susceptible to the effects of increasingly severe drought conditions driven by climate change.
Carvalho developed a life cycle model of the Sacramento River fall-run Chinook salmon population to simulate the effects of different drought scenarios and other variables on the population. The model was grounded in data from field studies, such as research by NOAA Fisheries scientists that quantified the relationship between river flows and survival rates of juvenile salmon.The model allowed the researchers to assess the effects of different mechanisms that can affect the age structure of the population. A century ago, most of the spawning salmon returning to the Sacramento River watershed were four years old, and some were as old as six years. Today, however, six-year-old fish are rarely observed and most of the spawners are three years old.

Overall, the results show that maintaining or increasing the age structure through reduced mortality and delayed maturation improves the stability of the salmon population, buffering against the adverse effects of drought and making the population more resilient in an increasingly variable climate.