| Summary: | New Zealand (NZ) sea lions (Phocarctos hookeri) are an endangered and endemic species to NZ. Following historic extirpation from their original range around mainland NZ, now nearly all breeding occurs at islands in the NZ sub-Antarctic between latitudes 50-53°S. Following a period of decline in pup production at the Auckland Islands for a decade to near threshold low numbers in 2015, around half of that at the apparent peak, the New Zealand Sea Lion Threat Management Plan was developed with several streams aiming to guide research and active management to improve the species’ population trajectory. NZ sea lions at the most studied site, Enderby Island in the Auckland Islands have been subject to several disease events, with ongoing impacts on pups due to a now endemic hypervirulent (HV) strain of Klebsiella pneumoniae following epizootic emergence at the site in 2001-02. Pup mortality due to HV K. pneumoniae has been modelled as a primary perceived threat to NZ sea lions that if mitigated, could aid in increasing population growth rate. As a result, this thesis aimed to determine the prevalence of all causes of pup mortality, identify important contributing risk factors and investigate potential avenues of active management by conducting a case-control study and a prospective cohort study over two austral summer field seasons (2016-18) at Enderby Island. A nested randomised controlled treatment trial with the anthelmintic ivermectin was run concurrently to assess the contribution of hookworm (Uncinaria spp.) to mortality. Evidence for feasible management options based on demonstrated risk factors for pup mortality is developed and explored.
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