Millennial-scale isotope records from a wide-ranging predator show evidence of recent human impact to oceanic food webs.

Human exploitation of marine ecosystems is more recent in oceanic than near shore regions, yet our understanding of human impacts on oceanic food webs is comparatively poor. Few records of species that live beyond the continental shelves date back more than 60 y, and the sheer size of oceanic regions makes their food webs difficult to study, even in modern times. Here, we use stable carbon and nitrogen isotopes to study the foraging history of a generalist, oceanic predator, the Hawaiian petrel (Pterodroma sandwichensis), which ranges broadly in the Pacific from the equator to near the Aleutian Islands. Our isotope records from modern and ancient, radiocarbon-dated bones provide evidence of over 3,000 y of dietary stasis followed by a decline of ca. 1.8‰ in δ(15)N over the past 100 y. Fishery-induced trophic decline is the most likely explanation for this sudden shift, which occurs in genetically distinct populations with disparate foraging locations. Our isotope records also show that coincident with the apparent decline in trophic level, foraging segregation among petrel populations decreased markedly. Because variation in the diet of generalist predators can reflect changing availability of their prey, a foraging shift in wide-ranging Hawaiian petrel populations suggests a relatively rapid change in the composition of oceanic food webs in the Northeast Pacific. Understanding and mitigating widespread shifts in prey availability may be a critical step in the conservation of endangered marine predators such as the Hawaiian petrel.

Foraging segregation and genetic divergence between geographically proximate colonies of a highly mobile seabird.

Foraging segregation may play an important role in the maintenance of animal diversity, and is a proposed mechanism for promoting genetic divergence within seabird species. However, little information exists regarding its presence among seabird populations. We investigated genetic and foraging divergence between two colonies of endangered Hawaiian petrels (Pterodroma sandwichensis) nesting on the islands of Hawaii and Kauai using the mitochondrial Cytochrome b gene and carbon, nitrogen and hydrogen isotope values (δ(13)C, δ(15)N and δD, respectively) of feathers. Genetic analyses revealed strong differentiation between colonies on Hawaii and Kauai, with Φ(ST) = 0.50 (p < 0.0001). Coalescent-based analyses gave estimates of <1 migration event per 1,000 generations. Hatch-year birds from Kauai had significantly lower δ(13)C and δ(15)N values than those from Hawaii. This is consistent with Kauai birds provisioning chicks with prey derived from near or north of the Hawaiian Islands, and Hawaii birds provisioning young with prey from regions of the equatorial Pacific characterized by elevated δ(15)N values at the food web base. δ(15)N values of Kauai and Hawaii adults differed significantly, indicating additional foraging segregation during molt. Feather δD varied from -69 to 53‰. This variation cannot be related solely to an isotopically homogeneous ocean water source or evaporative water loss. Instead, we propose the involvement of salt gland excretion. Our data demonstrate the presence of foraging segregation between proximately nesting seabird populations, despite high species mobility. This ecological diversity may facilitate population coexistence, and its preservation should be a focus of conservation strategies.