Spatial patterns of mercury (Hg), selenium (Se), and Se:Hg molar ratios in blood and lanugo in California sea lion (Zalophus californianus) pups from the central and southern Gulf of California within a One Health framework.

Environmental and biological factors influence element bioaccumulation in top predators, potentially impacting wildlife health. This study evaluated whole blood (n = 60) and lanugo (n = 61) total mercury concentration ([Hg]), total selenium concentration ([Se]), and whole blood Se:Hg molar ratios of California sea lion (Zalophus californianus) pups from central (Granito, Partido/Rasito) and southern (Los Islotes) Gulf of California rookeries. The influence of location, sex, and dorsal standard length (DSL) were assessed. Significant spatial variation was observed, with higher lanugo [Hg] in central rookeries compared to the southern region (p < 0.05). Overall, 11.5% of pups exceeded the toxicological threshold for lanugo [Hg] (20 mg kg-1 dry weight), with higher prevalences in Partido/Rasito (15%) and Granito (14.3%) compared to Los Islotes (5%). For all individuals, a strong correlation between lanugo and blood [Hg] (r = 0.78, p < 0.05) was detected, but not for [Se]; therefore, lanugo does not reliably reflect circulating [Se], possibly due to homeostatic regulation. Biological drivers influenced element dynamics in whole blood, but not in lanugo. A growth effect was observed, with [Hg] decreasing as DSL increased (p = 0.01); [Se] and Se:Hg molar ratios increased with DSL (p < 0.001). Males had higher [Hg] (p = 0.01); females showed higher [Se] and Se:Hg molar ratios (p < 0.01). These results highlight rookery-specific exposure and suggest effects of sex and growth on element accumulation. California sea lions serve as sentinels of chemical pollution that may affect marine biodiversity and human communities reliant on shared resources in the Gulf of California.