Modelling energy pathways and spatio-temporal drivers of change for the Kerguelen Plateau food web

The Kerguelen Plateau is a biological hotspot and an important region for Southern Ocean fisheries. However, its remoteness and harsh environmental conditions make consistent sampling difficult. Therefore, significant knowledge gaps remain regarding regional-scale impacts of fishing and climate change. Very little is understood about the transfer of energy from prey to predators on the Kerguelen Plateau and even less is understood about how the food web is being impacted by fishing pressure and will be impacted by future climate change. Understanding the combined impacts of bottom-up and top-down processes on food web dynamics can help inform ecosystem-based management on the Kerguelen Plateau. The research in this thesis compiles current knowledge of the Kerguelen Plateau food web and uses Ecopath with Ecosim to explore ecosystem dynamics and identify the potential impacts of climate change and fishing over time and space. Previously published work to develop the first food web model with a balanced energy system for the Kerguelen Islands. This model forms the basis for the research in this thesis. Next, the entire plateau food web was incorporated by extending the domain of the model to include not only the waters surrounding the Kerguelen Islands but also the region further south surrounding Heard and McDonald Islands. This model was used to undertake a thorough exploration of food web structure and function, including answering questions regarding the transfer of energy through the food web and interactions between the fishery and the food web. Results from this work indicate that killer whales, cephalopods (squid) and myctophids (lanternfish) are key pathways for the transfer of energy from lower trophic levels to top predators while fishing activity (in the form of catch and by-catch) had little impact on overall food web dynamics. This work provides the first quantitative description of the food web on the Kerguelen Plateau. The Ecopath model was calibrated using time-series data, and the resulting Ecosim model was applied to explore climate and fishing impacts on the Kerguelen Plateau food web over the last 32 years (1986‚Äö-2018). Model results again showed that fishing (fishing mortality) was not a driver of food web dynamics. Instead, analyses identified sea surface temperature, zonal wind speed and the Southern Annular Mode as drivers of biomass trends in the food web. Additionally, results indicate that Patagonian toothfish recruitment may be sensitive to surface warming. These results highlight the importance of considering environmental change in ecosystem-based management. Spatial differences in environmental processes, fishing pressure and marine protected areas (MPAs) on the Kerguelen Plateau could have local-scale impacts on the food web. The Ecosim model was spatially resolved by incorporating habitat maps in Ecospace. Food web dynamics were compared with fishing and without fishing present to evaluate spatial fishing impacts and food web distribution was evaluated in relation to the current locations of MPAs. Observed spatial distributions were replicated in the model and findings showed limited direct impacts of fishing on Patagonian toothfish and the food web. Modelled distributions showed that groups in the food web were well distributed within MPAs, indicating that current ecosystem-based management is representative of the food web. This thesis presents the first comprehensive analysis of food web dynamics on the Kerguelen Plateau. Results from this thesis identified key energy pathways, potential environmental drivers and evaluated the degree of protection provided by MPAs, all of which are important for conservation management and predicting Southern Ocean ecosystem response to change in the future. Findings suggest that current fishing for Patagonian toothfish is operating at sustainable levels and spatial management is representative of the food web. This work provides an important foundation for considering future ecosystem-based approaches, especially those that include climate change.