Association between a declining population of Humboldt penguins and changes in sea surface temperatures within their foraging range

In the Humboldt Current System, recurrent episodes of ocean warming drive large shifts in the abundance and accessibility of small pelagic prey, with widespread consequences for seabird populations. For Humboldt penguins (Spheniscus humboldti, HUPE), this threat is magnified given the recent reduction of one of its historically most important colonies, Punta San Juan (PSJ). We evaluated the influence of local warming events on the HUPE adult population at PSJ from 2012 to 2024 at two temporal resolutions, monthly and interannually. We estimated monthly average sea surface temperature (SST) values from 2012 to 2024 within the HUPE foraging range off PSJ as a proxy of local fluctuations in environmental conditions (source: MODIS). We then used GLMs at a monthly resolution to estimate SST association with two HUPE population parameters: adult population and reproductive success (ratio of chicks/adults). Reproductive success declined during the warm period of 2015, increased following the onset of cooler conditions in 2018, peaked in 2021–2022 during a particularly cold phase, and declined again in 2023–2024 as temperatures rose. Therefore, our study shows that the reproductive success of HUPE had an immediate response to SST trends (negative association) explaining the decrease in breeding failure with increased SST. In contrast, the adult population continuously decreased from 2016 until 2024 despite colder conditions between 2018 and 2022. Results highlight the complexity of HUPE response to SST anomalies as well as contrasting results when evaluated at monthly or interannual resolutions. We found differences between short-term reproductive responses (i.e. breeding failure) and long term demographic responses (i.e. adult dispersal or mortality). We recommend future studies to elucidate how rapidly changing climate scenarios can affect trophic food webs and other HUPE demographics, such as survival, in synergy with other potential threats (e.g. fishery pressure) at different temporal and spatial scales.