The missing link : pelagic prey field prediction for Southern Ocean predators

Bottom-up processes affecting the availability of prey play a fundamental role in driving the distribution of higher marine predators. Yet, adequate representation of environment – prey – predator linkages remains a major barrier to understanding marine predator responses to environmental change. A key limitation is the difficulty in obtaining synoptic prey observations at spatial and temporal scales relevant to foraging predators because the micronekton groups that dominate the prey of diving/higher predators are notoriously difficult to observe and sample. Simulated prey fields, derived from environmentally forced models are an emerging alternative approach for representing biomass and spatial dynamics of hard-to-observe mid-trophic prey. One such model, SEAPODYM (Spatial Ecosystem and Population Dynamics Model), has been used to skilfully represent the spatial dynamics and biomass of marine biota at multiple trophic levels. My thesis considers SEAPODYM’s utility in filling the mid-trophic prey gap between marine predators and the biophysical environment in the rapidly changing Southern Ocean.
I first explored the relationship between modelled estimates of mid-trophic biomass derived from SEAPODYMand foraging distribution, behaviour and success of two cosmopolitan Southern Ocean predators, the southern elephant seal (Mirounga leonina) and macaroni penguin (Eudyptes chrysolophus). I compared model-derived mid-trophic prey metrics with the spatial distribution of tracked elephant seals to identify important seal habitat. Next, I considered whether interannual variability in modelled prey biomass could be related to predator foraging success, as indicated by average arrival mass of macaroni penguins at the onset of breeding. Results from these studies indicated SEAPODYM’s spatially explicit prey field estimates could provide useful insights into both predator foraging behaviour and success, highlighting potential for their use in identifying current foraging behaviour and forecasting the impacts of climate-driven change in the Southern Ocean.
Having established that SEAPODYM can effectively represent Southern Ocean prey fields, I then present a modified SEAPODYM model framework to represent the life history of a single, dominant Southern Ocean mid-trophic prey species, Antarctic krill (Euphausia superba). Capturing the spatial dynamics of this species will become increasingly important in the face of anthropogenic climate-related change, and extractive pressures from likely expansion of the krill fishery. As the first step, I produced a model estimating circumpolar potential spawning habitat for the krill population and used this to identify regions that likely serve as population sources. Then, identifying how the structural framework of SEAPODYM may be adapted, I detail how the spawning habitat index could feed into a krill population model that incorporates critical life stages, habitats, and spatial processes. Output from such an implementation should provide unique circumpolar estimates of krill densities at different developmental stages, as well as critical insight into how the magnitude and distribution of krill biomass could shift under environmental change.
This thesis significantly advances knowledge on how coupled environmental and biological models, such as SEAPODYM, can provide useful representations of Southern Ocean midtrophic prey. It demonstrates empirical relationships between modelled prey fields and predator foraging ecology and lays the structural groundwork for representing a key prey species (krill) within a mechanistic modelling framework. In doing so it makes a foundational contribution toward predictive capacity in assessing ecosystem responses under future climates.