Declining Body Condition of Territorial Male Australian Fur Seals: Correlations With Environmental Variables

ABSTRACT The Australian fur seal ( Arctocephalus pusillus doriferus , AUFS) population, currently less than 47% of its pre‐harvest size, represents the largest resident marine predator biomass in south‐eastern Australia. This region, characterized by low marine productivity and rapid warming, is home to Kanowna Island, the third‐largest AUFS colony. Territorial male AUFS, due to size sexual dimorphism, exert a greater influence on trophic dynamics than their conspecifics and serve as indicators of ecosystem health through their body condition index (BCI). Over a 12‐year period (2009–2021), BCI data from 547 adult males revealed a significant decline, suggesting deteriorating nutritional conditions. Lower BCI was associated with warmer sea surface temperatures (current‐year), higher spring Chlorophyll‐a concentrations in southern Tasmanian waters (1‐year lag), and stronger Bonney Upwelling westerly winds (2‐year lag). Additionally, negative correlations emerged with the Indian Ocean Dipole (current‐ and 2‐year lags), while positive correlations were linked to the Southern Annular Mode and Southern Oscillation Index (1‐year and 2‐year lags). These findings indicate that climate variability and warming may further impact AUFS male body condition, posing risks to broader demographic parameters and population stability. Long‐term monitoring is essential to evaluate ongoing nutritional stress and potential population‐level consequences under future climate scenarios.