Oceanographic drivers of interannual habitat use in breeding rhinoceros auklets (Cerorhinca monocerata)

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Seabirds exhibit flexibility in foraging distributions in response to changes in the marine environment and prey availability. From 2015 to 2019, we studied foraging distributions and behaviors of chick-rearing rhinoceros auklets ( Cerorhinca monocerata , n = 42; 6–12 bird per year) on Southeast Farallon Island using GPS loggers and time depth recorders (2018 and 2019 only). We hypothesized that auklets would adjust their foraging behavior in response to interannual oceanographic variability, predicting that auklets would exploit shelf break habitats under stable conditions but shift their foraging distributions and increase foraging effort in years with anomalous conditions. Surface water conditions varied significantly and were warmer than average in 2015–2016 and 2019, normal in 2017, and cooler in 2018. Auklet foraging distances varied accordingly (20.2–55.3 km), with SST a strong driver from 2015 to 2017, and bathymetry from 2018 to 2019. Core foraging ranges (50% KDE) exhibited high overlap with June cool waters from 2015 to 2018 (69.7–90.4%), but little overlap in 2019 (23%), likely reflecting a spatial displacement of cool water areas beyond auklet foraging ranges as prey shifted to nearshore habitats. Our results are consistent with auklets selecting thermal habitats associated with prey availability, though this association weakened in 2019. Diet composition varied significantly with Northern anchovy dominating in 2015, 2018 and 2019, and juvenile rockfish in 2016 and 2017. These shifts may reflect the effects of forage fish recruitment from the 2014–2016 marine heat wave, which appeared to support higher numbers of Northern anchovy. Auklets exhibited similar diving patterns between 2018 and 2019, diving to similar maximum depths (38.6 ± 8.8 vs 37.3 ± 7.3 m, respectively) for similar durations (39.8 ± 25.1 vs 38.3 ± 19.4 s, respectively) despite contrasting oceanographic conditions. This study highlights the complexity in marine predator foraging ecology and their responses to changing marine environments.