Assessing the impacts of infectious disease on reproductive success in New Zealand sea lions (Phocarctos hookeri) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science in Wildlife Health at Massey University, Palmerston North, Manawatū, New Zealand
Poor reproductive success is one factor that may be perpetuating the population decline of the
threatened New Zealand (NZ) sea lion (Phocarctos hookeri). The species has a severely restricted
distribution, with 99% of breeding occurring on the remote NZ sub-Antarctic islands and amongst
the lowest expected reproductive outputs compared to other otariids. Infectious disease,
particularly septicaemia caused by the bacterium Klebsiella pneumoniae is known to be a major
mediator of early pup mortality, but the role of infectious disease in impairment of reproductive
success has not been investigated.
This thesis aimed to fill this knowledge gap by investigating three areas of concern. Firstly, the role of
infectious disease in stillbirth of NZ sea lion pups was examined with a histopathological study of
archived necropsy tissues. Secondly, the seroprevalence of adult and juvenile NZ sea lions to
Toxoplasma gondii, a known cause of reproductive failure, at several locations was evaluated.
Finally, a survival analysis was conducted to model the long term survival and reproductive success
of pups that were treated with ivermectin as pups, to assess ongoing benefits of early hookworm
In contrast to the mass mortalities seen with bacterial disease in NZ sea lion colonies, at least in the
topics covered in this thesis, the role of infectious disease contributing to poor reproductive success
is apparently minimal. No specific infectious agents were identified to have caused the death of the
stillborn pups examined, however pneumonia was diagnosed in four animals. A low seroprevalence
to T. gondii was found in mainland but not sub-Antarctic colonies, however those animals with
strongly positive titres showed no clinical signs and had reproduced normally. Finally, although the
survival analysis was limited by small sample size and very poor juvenile survival, it depicted
promising trends for improved survival for those pups treated with ivermectin as pups. All studies
have generated areas for future research and recommendations for further conservation
management of this vulnerable species.