Abstract
The New Zealand sea lion (NZSL), Phocarctos hookeri, was once widespread throughout New Zealand, however, Māori subsistence hunting and early commercial sealing extirpated the species from the mainland, Stewart Island, and the Chatham Islands. The remaining population was confined to the sub-Antarctic islands, at the southern edge of NZSL habitat. In 1993, a single female gave birth to the first pup born in Otago in over 150 years. In the last 3 decades, this single birth has grown into a small breeding population. This population marks the return of NZSLs to the core of their historical habitat and is key for the species recovery. To date, there have been few studies on the population demographics in Otago. Presented here is the first study to estimate stage-specific survival rates of NZSL and investigate the drivers of pup survival in Otago. Furthermore, this is the first study to investigate how NZSL pup survival is affected by environmental variables. This thesis uses long-term datasets of NZSL re-sightings in Otago, collected by the Department of Conservation (DOC) and the New Zealand Sea Lion Trust (NZSLT) volunteers. This was supplemented by re-sight data collected during beach surveys along the Otago coastline in 2021. Using these data, NZSL survival rates were estimated for pup (0yrs), juvenile (1-3yrs), and adult (4+yrs) age-classes. Survival rates were significantly lower for pups (0.708, 95% CI = 0.622-0.780), than juveniles (0.918, 95% CI = 0.870-0.95) and adults (0.954, 95% CI = 0.910-0.977). Compared to NZSL survival on the Auckland Islands, survival in Otago was much higher for pups and juveniles, marginally higher for adult females, and lower for adult males. High rates of pup and juvenile survival are likely due to the quality of habitat in Otago, where NZSL have less extreme foraging behaviour than the Auckland Islands, indicating an abundance of prey in nearshore habitat. Marginally higher adult female survival is likely due to fisheries by-catch in the Auckland Islands; by-catch is currently not a significant threat for NZSL in Otago. Using generalized linear mixed models, pup survival was modelled against a range of demographic and environmental factors. Demographic factors such as pup sex, and the number of pups a female had previously given birth to, significantly affected pup survival. Environmental factors describing sea surface temperature, chlorophyll-a concentration and the Southern Oscillation Index were found to impact pup survival at different seasonal and temporal scales. Environmental conditions are likely to affect pup survival indirectly by changing the foraging conditions and the availability of prey experienced by breeding females. As the Otago population continues to grow, it is likely that NZSL pup survival will decrease due to density-dependent causes of mortality (e.g., harassment disease and starvation). A growing number of NZSL in Otago may increase the impact of anthropogenic threats including resource competition and by-catch. The continued growth and expansion of NZSL in Otago, and across the species’ distribution, is key for its recovery. Therefore, pre-emptive strategies are crucial for the management of NZSL in Otago, to help the population continue to grow and flourish.