Pink Salmon sightings are increasing at their southern edge in California

Objectives Our objectives were to compile observations of Pink Salmon Oncorhynchus gorbuscha in California rivers over the past three decades, to evaluate whether sightings are increasing, and to understand the factors associated with their patterns of occurrence. Methods We compiled Pink Salmon records from 1996 to 2023 from agency monitoring programs, gray literature, and outreach to more than 30 organizations monitoring salmonids across California. We then modeled the probability of Pink Salmon presence as a function of time (year), watershed characteristics, proximity to potential source populations, and timing (month) of monitoring efforts. Results We recorded Pink Salmon observations in 18 watersheds and subbasins in California. Our findings indicate an increase in Pink Salmon detections corresponding to increasing Pink Salmon abundances across the whole North Pacific. Both adults and fry were observed in California rivers, indicating successful spawning. All 18 watersheds had odd-year observations of Pink Salmon, reflecting the dominance of odd-year Pink Salmon in their southern range. However, five of these watersheds also had observations of Pink Salmon during even years. Overall, Pink Salmon were more likely to be detected in California when monitoring began earlier in the spawning season (i.e., late summer to early fall). Conclusions Our observations raise the possibility that Pink Salmon have a growing presence in California, in contrast to severe and widespread declines of most other Pacific salmon runs in California. Our work highlights the importance of inclusive monitoring frameworks to detect rare or emerging salmonid populations at range boundaries and to inform species management strategies.