Among the tapestry of top predators that forage within the Pacific Ocean, seabirds are some of the most conspicuous and wide-ranging. Although pelagic seabirds are atypical in their dependence on a terrestrial breeding colony, they spend most of their lives navigating thousands of kilometers of open ocean. Since the 1950’s, humans have become a top predator of the Pacific Ocean themselves and compete with seabirds, and other marine predators, for food. Scientists have recognized for decades that humans are in danger of overfishing some of the world’s largest and most remote ecosystems, and fishers have noticed rapid and drastic declines in formerly abundant catches. By the time humans recognized they had underestimated the ocean’s potential as a food source, they were dealing with the shocking realization that humans were capable of decimating such plentiful fish populations, potentially beyond recovery. Our understanding of fish stocks is often incomplete and is based on biased global marine catch statistics that offer little forewarning regarding fish abundances and distributions. Moreover, we have a limited understanding of how industrial-scale fish removal influences marine ecosystem integrity as a whole. As highly mobile top predators that occasionally become accessible on land, seabirds offer a unique opportunity to gain valuable insight into oceanic trophic dynamics. In addition to their potential to herald marine ecosystem change, their preserved tissues can provide a retrospective glimpse into a time that precedes catch statistics and inform long-term temporal shifts in oceanic food webs. This dissertation describes three lines of investigation into the foraging habits of three pelagic seabirds using either whole tissue or amino acid-specific isotope data. First it asks whether information on foraging habits of the Hawaiian Petrel derived from isotope analysis of a primary feather grown at the beginning of the nonbreeding season (primary 1, P1) and in the middle of the nonbreeding season (primary 6, P6) is the same. Secondly, it compares long-term (greater than 50 years) foraging habits of Newell’s shearwaters, Laysan albatross, and Hawaiian petrel using collagen-specific amino acid δ15N proxies for nutrient regime and trophic position. Thirdly, it evaluates seasonal changes in modern Newell's shearwater and Laysan albatross foraging habits and, for Laysan albatross, extends this seasonal analysis back a century using collagen- and feather-specific amino acid δ15N data. Chapter one asked whether whole tissue isotope data for two feathers (P1 and P6) yielded similar information on the location and biogeochemical regime in which three colonies of Hawaiian petrel foraged. Relative to Hawaii and Lanai birds, the low P6 δ15N values for Maui birds reflect foraging segregation and greater utilization of waters characterized by nitrogen fixation. There was no isotopic difference between P1 and P6, suggesting that either feather could be used to describe nonbreeding season foraging habits. This information increases our understanding of Hawaiian petrel foraging behavior over the nonbreeding season and informs sampling protocols for conservation managers who wish to understand nonbreeding season foraging. Chapter two compared the foraging habits of modern and historical populations of three ecologically distinct species using amino acid-specific isotope analysis. The data show persistent inter- and intra-specific foraging segregation among Newell's shearwater, Laysan albatross, and two populations of Hawaiian petrel. While our nutrient proxy showed no shift in nutrient regime use over time, a significant trophic decline occurred for Newell's shearwater and Laysan albatross within the past century, paralleling a similar trend previously observed in the Hawaiian petrel. This builds on current evidence of a basin-wide shift in trophic dynamics within the North Pacific Ocean. Chapter three uses amino acid-specific isotope analysis of feather and collagen to show that Newell's shearwater and Laysan albatross foraging habits (biogeochemical regime use and trophic position) differ between the breeding and nonbreeding seasons. In addition, within each season, each species utilizes a different foraging strategy despite the fact that they both breed on the Hawaiian Islands. While Laysan albatross have not altered their nonbreeding season foraging habits over the past century, the trophic decline they experienced occurred exclusively during the breeding season. Conservation management strategies for threatened seabirds like Newell's shearwater and Laysan albatross will require an understanding of at-sea risks—as well as threats on land—on seasonal timescales and may need to be individually tailored for each species.
Read
