Emperor penguins (Aptenodytes forsteri) and Weddell seals (Leptonychotes weddellii) are two abundant Antarctic mesopredators, yet there remain a number of unknowns regarding many aspects of their fundamental ecology, including their population dynamics, foraging behaviour and what drives their spatial distributions. There is considerable overlap in the ecological niche of these two species in the Southern Ocean — they both rely on a stable fast ice platform for breeding and raising their young, they display similar diving behaviour, and target many of the same prey. Despite these similarities, and the potential for competitive exclusion, the Ross Sea is home to the largest breeding aggregations of both species. Yet, much is still unknown about how emperor penguins and Weddell seals may interact in the Ross Sea ecosystem. It was once thought that Weddell seals were capital breeders, provisioning their young from body stores gathered prior to pupping and fasting throughout lactation. Our understanding on this has now changed, Weddell seals instead display a mixed capital-income reproductive strategy, in which many Weddell seal mothers resume foraging while still caring for a pup. This development has important consequences for understanding both their habitat requirements during this energetically-expensive period and how they may interact with potential competitors during the spring.

In the first data chapter, I investigate the foraging ecology of lactating Weddell seals in Erebus Bay to determine prey targeted and the frequency of foraging. We attached 26 seals with seal-mounted cameras and time-depth recorders in November and December of 2018 and 2019. I found crustaceans (Mysida, Decapoda, and Amphipoda) were the most common prey encountered, representing 46% of the prey observed in the video footage. Seals also preyed upon on Antarctic silverfish (Pleuragramma antarcticum), crocodile icefish (Channichthyidae), juvenile Antarctic toothfish (Dissostichus mawsoni), bald notothen (Trematomus borchgrevinki), eelpout (Zoarcidae), and two unidentified Octopod species. I found seals were more likely to forage with increasing pup age. Closely tied to the first, in the second data chapter, I present an opportunistic observation made during analysis of video captured by seal-mounted camera, in which four female Weddell seals were observed searching for prey within the cavities of Rossella sp. glass sponges. This new-to-science observation highlights one of the unique foraging tactics employed by the species when facing restrictions imposed by lactation. I explore potential drivers of this observation and suggest it may be an adaptation acquired by some individuals driven by decreased availability of prey due to intraspecific competition during the pupping season.

In the third data chapter, I explore emperor penguin population dynamics using remote sensing and a Bayesian State-Space model to estimate the abundance at each Ross Sea emperor penguin colony between 2005 and 2018. I investigate relationships between metapopulation growth rates and changes in the environment and identify 1-y lagged El Niño and 5-y lagged winter sea ice concentration anomalies as important drivers of population change in the Ross Sea. I also identified patterns of covariation between colonies that could indicate inter-colony movement and present evidence to support metapopulation dynamics as an important contributor to population change in the Ross Sea. These observations have important implications for understanding how this species may respond to future environmental change and highlight the importance of understanding the frequency and drivers behind inter-colony movement when investigating emperor penguin population dynamics.

In the final data chapter, I applied a series of species distribution models to investigate drivers of distribution and abundance for emperor penguins, Weddell seals, and another important mesopredator, Adélie penguins (Pygoscelis adeliae), in the Ross Sea. I assessed the importance of covariates, representing food, open water access, shelter, and interspecific associations. The main factors that determined species distributions in the Ross Sea were fast ice extent and distance to coast. Both penguin species were found in areas with reduced fast ice extent, while Weddell seals displayed more plasticity in habitat selection, indicating a much broader niche. Although I hypothesised that these species may compete with each other for resources, the only negative interspecific co-occurrence was between Adélie penguins and Weddell seals; Adélies were less likely to be observed near large Weddell seal aggregations. All other interspecific relationships were positive. My observations in this chapter suggest differences in ecological adaptation, rather than competition for prey, may be the main driver behind mesopredator distributions in the Ross Sea. I highlight the ability of Weddell seals to occupy areas of greater fast ice extent, relatively safe from predators, as a key factor that shapes their distributions. I suggest especially productive breeding areas may be utilised by many species and this may be indicative of the productive Ross Sea marine environment and temporal and spatial differentiation between how each species partitions prey resources.

Herein I present new insight into fundamental aspects of emperor penguin and Weddell seal ecology that had previously remained elusive. I concur with previous suggestions that, despite their ecological similarities, emperor penguins and Weddell seals may not be in direct or exploitative competition and I highlight the role that the productive Ross Sea marine environment may play. I further show the importance of a holistic approach to ecological research and highlight the important developments that can be achieved when a broad-scale perspective is applied to an ecosystem.