Abstract
Juveniles have limited foraging ability compared to adults, due to physiological, morphological and behavioural constraints that ultimately affect their fitness and survival. Therefore, constraints on juvenile foraging ability may have implications for population dynamics. The objective of this thesis was to study the foraging behaviour of juvenile New Zealand (NZ) sea lions (Phocarctos hookeri) at the subantarctic Auckland Islands (AI). The endemic NZ sea lion is one of the rarest pinnipeds in the world and is mainly found in AI. This species has declined since 1998, with bycatch and resource competition with the arrow squid (Nototodarus sloani) trawl fisheries hypothesized to be the main causes for the decline. Understanding how juveniles are constrained in foraging ability may help us predict the degree to which they are further compromised by anthropogenic impacts that change prey availability.
I investigated intrinsic (e.g. mass, age and sex) and extrinsic (e.g. habitat characteristics) factors influencing the foraging behaviour of juvenile NZ sea lions from 2007-2010 at Sandy Bay, Enderby Island, AI. Platform transmitting terminals (PTTs) and time-depth recorders (TDRs) were deployed on juvenile females (1-3 years-old; n = 22) and males (1-5 years-old; n = 18) to describe diving behaviour, at sea movements and potential spatiotemporal overlap with squid trawl fisheries. Mass was negatively related to yearling dive depth; this is likely because bathymetry constrained diving behaviour by dictating possible dive depths. Heavier, older juveniles dove to greater depths for longer durations than lighter, younger individuals. However, only 5-year-old males were capable of adult female dive performance. Despite males (the larger sex) having greater diving ability (i.e. dove to greater maximum depths for longer maximum durations) than juvenile females, on average, females expended greater foraging effort (i.e. dove deeper and spent more time diving). Juvenile males and females also foraged in different areas, with female foraging ranges having higher overlap with fisheries activities. Habitat differences strongly influenced foraging behaviour. AI juvenile females expended more foraging effort than Otago juveniles on the NZ mainland. Combined with the smaller body size of AI juveniles, these contrasts support the hypothesis that AI are a marginal foraging environment.
Results from this thesis suggest juvenile NZ sea lions are likely more susceptible than adults to environmental and anthropogenic impacts that alter prey distribution. The restricted foraging ability of juveniles may limit their available foraging habitat and ability to acquire food, especially in a low resource environment. For juvenile females, their higher foraging effort and fisheries-overlap likely further increase their susceptibility to external impacts. Greater vulnerability to external impacts may influence juvenile survival, particularly females, and ultimately, may have consequences for NZ sea lion population dynamics. For management to be effective, population models need parameters that incorporate the higher vulnerability of juvenile NZ sea lions (especially females), resource competition with fisheries and suboptimal habitat, in order to accurately estimate population growth rates. Management can also directly mitigate sea lion bycatch by enforcing fisheries closures or gear restrictions in areas of high sea lion-fisheries overlap identified in this thesis.