A statistical investigation of the behavioural responses of a deep diving predator to mesoscale oceanographic features

Master Thesis


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University of Cape Town

Marion Island lies directly in the path of the ACC and to the south-west of the island is an extensive region of high sea level anomaly variability (eddy field). Previous research has shown how southern elephant seals from Kerguelen Island and Peninsula Valdés have targeted eddies for foraging. These findings combined with the observed post-moult migrations of tagged female southern elephant seals from Marion Island which traverse the nearby eddy field suggest that they may exploit the eddy field for foraging. Overall the seals undertook significantly more of their dives (18.5) within the eddy field than predicted (p < 0.01). Eleven of the 16 seals dived within the eddy field region significantly more often than predicted. Dives were generally shallower and shorter within the eddy field. Mixed models estimated that a maximum depth reduction of 53.03 ± 5.30 m (LME: t=10.00, DF=24986, p < 0.01) and a dive duration reduction of 6.17 ± 0.58 min (LME: t=10.70, DF=24986, p < 0.01) could be attributed to diving within the eddy field. Within the eddy field U-dives increased by 6.97, W dives decreased by 7.18 and O-dives increased by 0.21. Seven individuals’ maximum dive depths were shallower and nine individuals dive durations were shorter as a result of diving within the eddy field. Ten individuals showed significant changes in the types of dives which they undertook inside of the eddy field with eight seals undertaking more Uand fewer W-dives and two seals undertaking more W- and fewer U-dives. Overall the effect of diving within the eddy field appeared to be biologically important to ten seals. While dive durations were significantly affected by changing sea level anomaly (SLA) (LME: 0.15 ± 0.01 min, t=11.72, DF=12372, p < 0.01), maximum depths were not. The log-odds of switching from U- to W-dives within the eddy field were significantly affected by changing SLA (Multinomial: 0.00 ± 0.00, z=2.97, p < 0.01). However all significant effects of SLA on dive parameters within the eddy field were very small in comparison to those of day-stage. The low magnitudes of the effects of SLA were similar for all individuals.

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