Evaluating threats to New Zealand sea lions Phocarctos hookeri with respect to life history strategy
To reverse population declines, effective management actions are required. Identifying the causes of population declines is particularly challenging if no reference data from non-declining opulations are available. The 'endangered' New Zealand sea lion (NZSL) Phocarctos hookeri is New Zealand's (NZ's) only endemic pinniped. Originally, the NZSL occurred from NZ's sub-Antarctics up to the northernmost of the NZ mainland, but was extirpated during the early 19th century. The extant sub-Antarctic NZSL is mostly restricted to the Auckland Islands (50°30' S, 166°17' E) accounting for 73% of NZSL's total pup production. However, the NZSL population at the Auckland Islands declined by approximately 50% since 1998. Effective management has been hampered by a lack of data prior to the population decline or comparably-sized reference populations. My thesis focuses on four knowledge gaps in NZSL conservation: (1) the contribution of life history traits to NZSL population growth (Chapter 2); (2) the existence of density dependence (further referred to as density feedback) (Chapter 3), a key assumption in fishery management; (3) the demographic effects of disease (Chapter 4), which killed a large number of pups since 1998; (4) the population-level effects of NZSL mortality in the commercial fishery, which is spatially and temporally overlapping with the foraging behaviour of female NZSLs (Chapter 5). I used a multistate state-space Cormack-Jolly-Seber (CJS) model to analyse mark-recapture data of 2,928 individual female NZSLs that were tagged as pups at Sandy Bay, Auckland Islands, between 1998 and 2012 (Chapter 2). Using this model, I fitted a sub-model for pup survival against pup census data (as an index of population size) (Chapter 3) and data for the presence and absence of disease-related mass mortalities (Chapter 4). Furthermore, demographic estimates were used to parameterise a matrix population model in order to quantify the responsiveness of the population growth rate to variability in demographic parameters (Chapter 2). I then used this model to predict the improvement in adult female survival that is required to reach population equilibrium (Chapter 5). The latter was quantitatively compared against fishing-related NZSL mortality estimated using a Bayesian generalised linear model, which was fitted against bycatch data (Chapter 5). I provide quantitative evidence based on the NZSL life history that indicates NZSLs offset large variability in pup survival by maintaining constant and high adult survival, however adult survival appeared to be low compared to other pinnipeds (Chapter 2). This finding is underpinned by two important results. First, I found that disease caused no net change in pup survival and was thus compensatory to other sources of pup mortality (Chapter 4). Second, I found a high probability that bycatch mitigation, if effective, should have averted the population decline (Chapter 5). I found no clear evidence for density feedback in NZSL pup survival, underpinning that the population is facing deterministic extinction instead of showing density feedback dynamics (Chapter 3). My study provides the first comprehensive analysis of NZSL population dynamics and its relationship to life history and contributes important findings for the future species management.
Advisor: Robertson, Bruce C.; Krkošek, Martin
Degree Name: Doctor of Philosophy
Degree Discipline: Department of Zoology
Publisher: University of Otago
Keywords: New Zealand sea lion; Matrix population modelling; Mark-recapture modelling; Multistate modelling; State-space modelling; Population growth rate; Bycatch; Elasticity
Research Type: Thesis