Insights into anthropogenic threats and early mortality in the Guadalupe fur seal from a plausible mixed colony on the San Benito Archipelago, Mexico

In Mexico, the Guadalupe fur seal ( Arctocephalus townsendi , GFS) is listed as Risk of Extinction, with one breeding site on Guadalupe Island, and another potential colony proposed in the late 1990s on the San Benito Archipelago (SBA). Although there are some health and mortality studies regarding GFS on Guadalupe Island, the SBA colony remains understudied. This study aimed to characterize the causes of neonatal GFS morbidity and mortality, and to assess potential threats to species conservation associated with entanglement, alopecia syndrome, and marine pollution on the West Island of the SBA during the 2024 breeding season. Field data on total births and neonate mortalities were collected during the first 30 days of life. Entanglement and alopecia syndrome incidents were recorded and analyzed to determine age class and sex predilection using Bayesian analyses. Also, marine pollution was classified and quantified by environmental debris type. The neonatal mortality rate in GFS was 29.52%. Perinate deaths were considered natural and attributed to trauma (50%) and emaciation/infectious disease (50%). The prevalence of the alopecia syndrome in non-pup GFSs was high (21% to 26%) during the 2024 breeding season. Clinically, there were bilateral and asymmetrical presentations, with a higher probability of detection of this syndrome in yearlings and juveniles, particularly in males. Entanglements were related to marine debris (100%), affecting juvenile individuals (80%) and adults (20%). A field survey of the composition of marine debris along the SBA beaches revealed that 92% was plastic, predominantly bottles (65%) and fishing gear (16%). Local or regional sources appear to contribute to pollution in the SBA based on the composition of the debris and its proximity to human habitation. Mortality and alopecia rates in immature GFSs were much higher compared to Guadalupe Island, identifying potential challenges in health and survival during early development stages in the SBA. The establishment of the GFS population in the SBA spans almost three decades; however, key demographic factors remain undetermined. Continued efforts to characterize the factors limiting the transition of the SBA from a plausible mixed to a breeding colony are warranted.