UC Santa Cruz

In a rapidly changing ocean that remains largely undersampled, physical and biological observations are crucial to understanding, predicting, and mitigating effects of anthropogenic stressors. Animals instrumented with oceanographic sensors offer valuable supplements to datasets from more traditional oceanographic methods while simultaneously offer information about the oceanography of areas significant to the animals. Marine predator foraging behavior relative to physical or biological features such as fronts, eddies, and phytoplankton blooms can be used to infer oceanographic influences on the distribution of pelagic prey. This dissertation applies tracking data from adult female northern elephant seals (Mirounga angustirostris) and southern elephant seals (Mirounga leonina) with in situ temperature, salinity, and chlorophyll fluorescence collected by instruments carried by the seals to investigating relationships between oceanographic features and the foraging behavior of these wide-ranging mesopelagic predators at basin- to submesoscales. Chapter 1 investigated the behavior of northern elephant seals when they encountered eddies. This project used a 17-year dataset of time-depth recorders and concluded that while eddies are a minor feature of their habitat, seals do derive foraging benefits from both cyclonic and anticyclonic eddies. Our observations suggest that physical prey aggregation is a more likely mechanism making eddies beneficial to foraging seals than bottom-up energetic enhancement of the food web resulting from nutrient injection. Chapter 2 combined tracking data from northern and southern elephant seals to compare their behavior relative to the oceanographic conditions they encountered. This first direct comparison between the at-sea behavior of these two closely related species showed comparable movement and diving behavior and further, very similar relationships between behavior and temperature, salinity, and mixed layer depths encountered. Both seal populations were more responsive to horizontal variability in physical conditions during the post-molt trip than during the post-breeding trip and showed these inter-trip differences in behavior despite the seasonal offset in when the trips occur in their relative hemispheres. We conclude that these species employ similar strategies in two contrasting ocean basins, indicating that the mesopelagic prey field may be driven by similar oceanographic properties and seasonal resource pulses in both ocean basins. Chapter 3 used in situ chlorophyll fluorescence data collected by seal-borne instruments and remotely sensed chlorophyll data to test whether elevated chlorophyll concentrations were associated with enhanced foraging behavior in northern elephant seals. We found that real-time chlorophyll data, despite having the advantage of containing subsurface data, did not predict seal foraging behavior. Instead, remotely sensed chlorophyll data from 2-4 months prior to seal presence was associated with elevated foraging behavior. This effect was especially strong during the post-molt season when less of the seals’ range was contained elevated chlorophyll but had several months prior. These chapters illuminate the interplay between intrinsic and extrinsic drivers of behavior and the role of spatiotemporal scale linking physics to biology in the open ocean.