Contribution of disease to Australian sea lion (<i>Neophoca cinerea</i>) pup mortality at Seal Bay, Kangaroo Island, 2016–2022

Abstract The endangered Australian sea lion ( Neophoca cinerea ) population is declining, and its conservation is threatened by the species' low fecundity and high pup mortality. This study details reasons for pup deaths and quantifies the impacts of disease and comorbidity. Of 284 dead pups reported from five breeding seasons (2016–2022) at the Seal Bay colony, Kangaroo Island, gross necropsy was conducted on 122, with fecal analysis for hookworm ( Uncinaria sanguinis ) detection, tissue histology, and bacterial culture performed on subsets. Determinable primary causes of mortality were conspecific trauma (± comorbidity, 41.0%), starvation (24.6%), and infectious disease (hookworm‐associated hemorrhagic enteritis [HAHE] ± bacterial infection, 19.7%). Allowing for comorbidity, 37.7% of pups died with an infectious disease, including 44.0% of deaths with trauma and 41.5% with starvation. In higher mortality summer‐autumn breeding seasons, pups were at greater risk of trauma ( p = .001) and subsequent bacterial infection ( p &lt; .001). Parasiticide treatment significantly reduced HAHE‐associated ( p = .005) and overall mortality ( p &lt; .001) risk, suggesting hookworm infection increases pup susceptibility to other causes of mortality. Although the exact causal pathways for this treatment‐related overall mortality benefit remain to be confirmed, intervention appears highly impactful and could benefit species conservation efforts.