Sexual Segregation in Juvenile New Zealand Sea Lion Foraging Ranges: Implications for Intraspecific Competition, Population Dynamics and Conservation

Abstract

Sexual segregation (sex differences in spatial organisation and resource use) is observed in a large range of taxa. Investigating causes for sexual segregation is vital for understanding population dynamics and has important conservation implications, as sex differences in foraging ecology may affect vulnerability to area-specific human activities. Although behavioural ecologists have proposed numerous hypotheses for this phenomenon, the underlying causes of sexual segregation are poorly understood. We examined the size-dimorphism and niche divergence hypotheses as potential explanations for sexual segregation in the New Zealand (NZ) sea lion (Phocarctos hookeri), a nationally critical, declining species impacted by trawl fisheries. We used satellite telemetry and linear mixed effects models to investigate sex differences in the foraging ranges of juvenile NZ sea lions. Male trip distances and durations were almost twice as long as female trips, with males foraging over the Auckland Island shelf and in further locations than females. Sex was the most important variable in trip distance, maximum distance travelled from study site, foraging cycle duration and percent time at sea whereas mass and age had small effects on these characteristics. Our findings support the predictions of the niche divergence hypothesis, which suggests that sexual segregation acts to decrease intraspecific resource competition. As a consequence of sexual segregation in foraging ranges, female foraging grounds had proportionally double the overlap with fisheries operations than males. This distribution exposes female juvenile NZ sea lions to a greater risk of resource competition and bycatch from fisheries than males, which can result in higher female mortality. Such sex-biased mortality could impact population dynamics, because female population decline can lead to decreased population fecundity. Thus, effective conservation and management strategies must take into account sex differences in foraging behaviour, as well as differential threat-risk to external impacts such as fisheries bycatch.

Preliminary assessment of the foraging behaviour and population dynamics of a cryptic population of the endangered New Zealand sea lion

“…obs.). These groups likely have some foraging area overlap with females, particularly juvenile males (as seen at Enderby Island; Leung et al 2012Leung et al , 2014. In addition, there are large numbers of other marine carnivores which forage in these areas, including New Zealand fur seals Arctocephalus fosteri (>20 000 individuals; Lalas 2008) that could have an overlap or be competing for prey (Lalas & Webster 2014) whereas fur seal numbers are limited at the Auckland Islands (pers.…”

Foraging Behaviour of Juvenile Female New Zealand Sea Lions (Phocarctos hookeri) in Contrasting Environments

“…Due to the large scale of differences found in at sea movements between AI and Otago juvenile females, using different filters did not affect the validity of the comparison between the sites. Satellite locations from animals at AI were corrected and interpolated with a state-space model, fitted into a hierarchical Bayesian context [47], [48]. We ran the models using WinBUGS [49] and R [50].…”

“…We ran the models using WinBUGS [49] and R [50]. The analyses were conducted hierarchically by grouping tracks from multiple individuals within the same age group [48]. To fit the model, two Markov Chain Monte Carlo (MCMC) chains were run at a four hour time step for 40 000 iterations, with a burn-in of 20 000.…”

A Simulation-Based Study on Bayesian Estimators for the Skew Brownian Motion

“…First, prey aggregations may change location due to variations in oceanographic features, thus obligating predators to change their foraging areas, see [22]. Second, individuals usually segregate and change their foraging locations in order to decrease intra-specific resource competition ( [23]). Finally, SASL are considered to be a generalist predator, which means that they consume different prey species [24].…”