Marine top predators are often keystone species, having a considerable impact on community structure and ecosystem function due to their large population sizes, wide-ranging behavior, and high energy requirements. To effectively evaluate the susceptibility of marine predators and ecosystems to changing environmental conditions, whether natural or anthropogenic in origin, it is critical to understand predator diet and foraging behavior over space and time. Moreover, intraspecific variation in foraging behavior, including diet specialization and individual foraging strategies, may influence the ability of predator populations to respond to environmental change and may safeguard species during conditions when preferred prey are scarce or unavailable. However, in marine systems, especially the deep ocean, predator trophic dynamics, such as diet, food web interconnections, and predator-prey interactions, are poorly understood and logistically challenging to study. Studies on the diet and foraging strategies for wide-ranging marine predators are often highly localized in space and time and limited to small subsets of the total population. Advances in biologging technology have enabled the documentation of in situ foraging behavior, using animal-borne still and video cameras to obtain snap-shots of the diet for marine predators. Yet, while this approach provides invaluable data on the diet composition for a few individuals over short time-spans, it does not provide comprehensive diet information over broader spatiotemporal scales. In contrast, biochemical methods of diet determination, like fatty acid analysis, allow for the quantification of population diet trends over long time periods, while overcoming well-known biases of traditional methods, like stomach contents and fecal analyses. Combining diet composition data derived from biochemical analysis with concurrent movement and diving data collected by biologging instruments allows for a comprehensive approach in assessing the foraging ecology of deep ocean predators.
My dissertation is an extensive analysis of the diet and foraging behavior, from population-level to individual, of a deep ocean predator, the northern elephant seal (Mirounga angustirostris). Female northern elephant seals range across the entire eastern North Pacific, diving deeply into mesopelagic (200-1000 m) and bathpelagic (> 1000 m) zones to forage on prey in the deep acoustic scattering layers. While much is known about elephant seal physiology, diving behavior, and movement patterns, comprehensive information on their at-sea diet has been frustratingly difficult to obtain. In Chapter 1, I used quantitative fatty acid signature analysis (QFASA) to quantify the diet composition of 155 adult female northern elephant seals over five years. This is the first study to show that deep-sea fishes, particularly energy-rich myctophids, are a critical prey resource for northern elephant seals, refuting the long-held view that elephant seals are squid specialists. Moreover, I was able extend the applicability of the QFASA method for species with scant a priori diet data and large variety of potential prey available in their foraging habitat. In Chapter 2, using the QFASA-derived diet data, I tested for spatiotemporal differences in diet composition. I found evidence of seasonal, spatial, and interannual diet differences for female seals and that these differences are likely driven by the spatiotemporal dynamics of the oxygen minimum zone in the eastern north Pacific and interannual variation in climate phenomena. Finally, in Chapter 3, I quantified vertical foraging strategies for female elephant seals and tested whether those foraging strategies were correlated with diet composition or diet specialization. I found that female seals are generalists in both their vertical foraging behavior and individual diet composition. Further, their flexibility in foraging tactics appears to be tied to in to broad-scale spatiotemporal variation in their deep ocean habitat, reflecting differences in the distribution and availability of their prey. Overall, my dissertation provides critical insights to the trophodynamics of a deep-diving top predator in the North Pacific deep-sea ecosystem.