Decline of one of the southernmost northern elephant seal (Mirounga angustirostris) colonies and its relationship with a warm sea environment

Over several decades, air temperature and sea surface temperature (SST) have increased in the northeastern Pacific, as has the frequency of large-scale warm anomalies in SST. The objective of this study was to evaluate the impact of these warm anomalies on the production of northern elephant seal (Mirounga angustirostris; NES) pups from the colony of the San Benito Archipelago, located in the central-western region of the Baja California Peninsula. Pup and adult female counts from the 2002 to 2019 breeding seasons were compiled to determine the trend in the abundance of the colony and its current state. In addition, birth rate variations during the study period were assessed. Likewise, the presence of warm anomaly events in the SST in the potential foraging area of adult females (PFA) was analyzed. The San Benito colony decreased at an average annual rate of 3.6% from 2002 to 2019, with an estimated abundance of 4,723 individuals (95% CI: 3,821–5,615) in 2019. However, the birth rate remained constant (mean 0.68 ± 0.08), even though several warm anomaly events associated with El Niño and The Blob phenomena were detected in the PFA. Our results suggest that large-scale warm anomalies in the SST do not severely impact NES pup production, thus the decline of the San Benito colony since the late 1900s could be due to other environmental factors, such as the increase in air temperature associated with climate change.