The influence the oceanographic variability has on the top predators of the sub-Antarctic domain

Evans, Sean

The influence the oceanographic variability has on the top predators of the sub-Antarctic domain

Master Thesis

2021

Abstract

Historically, a lack of small-scale physical oceanographic (hours to days, 1-10 km) and behavioural (<10 sec, ~1 m) observations in the relatively inaccessible Southern Ocean has led to poor quantification of marine mammal foraging behaviour and the physical upper ocean processes that may influence them. In situ temperature and depth profiles from 2009-2015 were obtained from devices fitted to 39 adult Subantarctic fur seal (SAFS) (Arctocephalus tropicalis) females inhabiting the Prince Edward Islands (PEI). This provided a unique opportunity to study the fine-scale effects of thermal water column structure and upper ocean submesoscale processes on the diving behaviour and vertical foraging effort of SAFS. Seasonal and diel trends of foraging effort were investigated and compared to upper ocean thermal structure. Dives were distinguished using the Clustering for Large Applications algorithm according to vertical movements made by the seals. Shallow, high effort dives differentiated from deep, low effort dives, primarily based on bottom effort. High effort dives, associated with high vertical foraging effort, bottom effort, and dive efficiency, were more numerous when the seals were in well-mixed water columns. Generalised additive mixed-effects models showed that thermal water column structure plays a significant role in modulating dive types made by seals. The probability of high effort dives decreased with increasing stratification, while the relationship with the stability and mean temperature of the water column was complex, yet significant. Overall, seals are predicted to enhance vertical foraging effort and dive shallower in well-mixed, warmer water columns, with a strong association to diel and seasonal trends in mixing found. However, seals do not appear to align their distribution of foraging depth with the MLD in either season. Investigation into what upper ocean processes may be driving variation in thermal water column structure surrounding the islands led to investigations into the downstream effects of the small-scale topography of the islands on ocean variability. Results show that the PEI act as a solid obstruction to the relatively laminar flow of the Antarctic Circumpolar Current and coherent eddy structures moving through the Archipelago region. The topographic influence of the PEI on multiple scales of local ocean variability in the region, from mesoscale (10-100 km, days to weeks) to submesoscale (1-10 km, hours to days) and vertical mixing (<1m, <10 sec), is illustrated. Downstream enhanced squeezing and stretching of mesoscale and submesoscale gradients intensify restratification and stability in the lee of the Islands with the potential to enhance biological productivity. Seals do not appear to adjust their fine-scale foraging behaviour to these downstream processes. By improving our understanding of SAFS habitat use, we can more accurately predict how regional and global change may affect populations in the future, linking to more effective conservation management and policy. Furthermore, this study emphasizes how concurrent measurements of oceanographic and behavioural data collected from diving samplers can be used to study the downstream effects on both physical oceanography and foraging ecology surrounding small islands.

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