New Zealand sea lion (Phocarctos hookeri) is a pinniped endemic to New Zealand and is among the rarest of sea lion species. New Zealand sea lions are incidentally caught in the trawl fishery for squid around the Auckland Islands, and a sea lion catch-limit or Fishing Related Mortality Limit (FRML) is used to manage this interaction. Since 2003 such limits have been calculated using an age-structured Bayesian population model. One problem with this approach is that several key demographic parameters have had to be assumed, or are based on very few data. Archaeological and other historical records demonstrate that New Zealand sea lions were substantially more widespread before the arrival of humans to New Zealand than they are today (Chapter 2 published as Childerhouse & Gales 1998). The present population size is clearly reduced, with subsistence and commercial hunting the most likely cause of historical changes in distribution and abundance. Campbell Island, the only significant breeding site outside the Auckland Islands, was thoroughly surveyed for New Zealand sea lions for the first time in 2003. An estimated 385 pups were born there, comprising 13% of the total pup production for the species for 2003 (Chapter 3 published as Childerhouse et al. 2005). This thesis provides the first robust estimates of several demographic parameters for New Zealand sea lions. These data were gained via the capture, tagging and ageing of 865 individual females, which had come ashore to pup between 1999 and 2001. This research was underpinned by the development of a novel and robust ageing technique for live New Zealand sea lions (Chapter 5 published as Childerhouse et al. 2004). Chapters 6, 7 and 8 used analyses of the age structure of these females, and of subsequent resightings of them, and of known-age females between 1998 and 2005, provided the first estimates of individual growth, mean reproductive rate (0.67, SE = 0.01), mean adult survival (0.81, SE = 0.04), and maximum age (28 years) for females. These data show that New Zealand sea lions are among the slowest growing, slowest reproducing, and longest lived sea lion species. Significant differences in the age structure of the two largest breeding colonies highlight flawed assumptions of the current management approach. The application of this new demographic information has the potential to significantly alter the existing management advice relating to the setting of FRMLs and the impact of the squid fishery on the New Zealand sea lion population. Taken alone, these results suggest a dim outlook for an already threatened species. In the context that pup production is in significant decline (e.g. 32% since 1998 Chilvers et al. 2007), the species� foraging environment is thought to be marginal (Costa & Gales 2000), and that resource competition may also be impacting on the population (Chapter 4 published as Childerhouse et al. 2001a), the picture darkens further. Taken as a whole, these data suggest that current management is insufficient to ensure population stasis, let alone meet the Government�s statutory goal of recovery.

New Zealand sea lion (Phocarctos hookeri) is a pinniped endemic to New Zealand and is among the rarest of sea lion species. New Zealand sea lions are incidentally caught in the trawl fishery for squid around the Auckland Islands, and a sea lion catch-limit or Fishing Related Mortality Limit (FRML) is used to manage this interaction. Since 2003 such limits have been calculated using an age-structured Bayesian population model. One problem with this approach is that several key demographic parameters have had to be assumed, or are based on very few data. Archaeological and other historical records demonstrate that New Zealand sea lions were substantially more widespread before the arrival of humans to New Zealand than they are today (Chapter 2 published as Childerhouse & Gales 1998). The present population size is clearly reduced, with subsistence and commercial hunting the most likely cause of historical changes in distribution and abundance. Campbell Island, the only significant breeding site outside the Auckland Islands, was thoroughly surveyed for New Zealand sea lions for the first time in 2003. An estimated 385 pups were born there, comprising 13% of the total pup production for the species for 2003 (Chapter 3 published as Childerhouse et al. 2005). This thesis provides the first robust estimates of several demographic parameters for New Zealand sea lions. These data were gained via the capture, tagging and ageing of 865 individual females, which had come ashore to pup between 1999 and 2001. This research was underpinned by the development of a novel and robust ageing technique for live New Zealand sea lions (Chapter 5 published as Childerhouse et al. 2004). Chapters 6, 7 and 8 used analyses of the age structure of these females, and of subsequent resightings of them, and of known-age females between 1998 and 2005, provided the first estimates of individual growth, mean reproductive rate (0.67, SE = 0.01), mean adult survival (0.81, SE = 0.04), and maximum age (28 years) for females. These data show that New Zealand sea lions are among the slowest growing, slowest reproducing, and longest lived sea lion species. Significant differences in the age structure of the two largest breeding colonies highlight flawed assumptions of the current management approach. The application of this new demographic information has the potential to significantly alter the existing management advice relating to the setting of FRMLs and the impact of the squid fishery on the New Zealand sea lion population. Taken alone, these results suggest a dim outlook for an already threatened species. In the context that pup production is in significant decline (e.g. 32% since 1998 Chilvers et al. 2007), the species� foraging environment is thought to be marginal (Costa & Gales 2000), and that resource competition may also be impacting on the population (Chapter 4 published as Childerhouse et al. 2001a), the picture darkens further. Taken as a whole, these data suggest that current management is insufficient to ensure population stasis, let alone meet the Government�s statutory goal of recovery.

Biological management of populations plays an indispensable role in all areas of population biology. In deciding between possible management options, one of the most important pieces of information required by population managers is the likely population status under possible management actions. Population dynamic models are the basic tool used in deriving this information. These models elucidate the complex processes underlying the population dynamics, and address the possible consequences/merits of management actions. These models are needed to guide the population towards desired/chosen management goals, and therefore allow managers to make informed decisions between alternative management actions. The reliability that can be placed on inferences drawn from a model about the fate of a population is undoubtedly dependent on how realistically the model represents the dynamic process of the population. The realistic representation of population characteristics in models has proved to be somewhat of a thorn in the side of population biologists. This thesis focuses in particular on ways to represent environmental stochasticity and density dependence in population models. Various approaches that are used in building environmental stochasticity into population models are reviewed. The most common approach represents the environmental variation by changes to demographic parameters that are assumed to follow a simple statistical distribution. For this purpose, a distribution is often selected on the basis of expert opinion, previous practice, and convenience. This thesis assesses the effect of this subjective choice of distribution on the model predictions, and develops some objective criteria for that selection based on ecological and statistical acceptability. The more commonly used distributions are compared as to their suitability, and some recommendations are made. Density dependence is usually represented in population models by specifying one or more of the vital rates as a function of population density. For a number of reasons, a population-specific function cannot usually be selected based on data. The thesis develops some ecologically-motivated criteria for identifying possible function(s) that could be used for a given population by matching functional properties to population characteristics when they are known. It also identifies a series of properties that should be present in a general function which could be suitable for modelling a population when relevant population characteristics are unknown. The suitability of functions that are commonly chosen for such purposes is assessed on this basis. I also evaluate the effect of the choice of a function on the resulting population trajectories. The case where the density dependence of one demographic rate is influenced by the density dependence of another is considered in some detail, as in some situations it can be modelled with little information in a relatively function-insensitive way. The findings of this research will help in embedding characteristics of animal populations into population dynamics models more realistically. Even though the findings are presented in the context of slow-growing long-lived animal populations, they are more generally applicable in all areas of biological management.