The decline of the endangered Australian sea lion (Neophoca cinerea) population is partly attributable to high pup mortality, with endemic hookworm (Uncinaria sanguinis) infection a significant primary or contributing factor in many deaths. Furthermore, mortality rate shows a strongly alternating seasonal pattern, higher during summer breeding seasons at the Seal Bay colony, Kangaroo Island. To explore the associations of hookworm infection and season with pup health (assessed by growth and haematologic parameters) a controlled field trial recruited young pups to one of two age cohorts during consecutive 2019 winter-spring (n=158) and 2020-1 summer-autumn (n=164) breeding seasons, then randomised pups within each to hookworm treatment (topical ivermectin 500ug/kg) or control groups. Regardless of pup age at treatment, ivermectin significantly increased hookworm clearance (P<0.001). Hookworm elimination resulted in significantly higher growth rates and improvements in haematologic parameters, especially in the first month after treatment, regardless of recruitment age or season (P<0.001). Greatest benefit occurred with earlier treatment. A growth benefit persisted to the study conclusion (5 months of age). The higher mortality season was associated with higher pup stress, based on neutrophil:lymphocyte ratio. Important implications for hookworm treatment protocols, including treatment age and seasonal interaction, will be outlined for this endangered species.