Hawaiian Petrel foraging changes suggest rapid change in oceanic food web. Photo by Brenda Zaun, USFWS

(Washington, D.C., June 12, 2013) A new study on the Endangered Hawaiian Petrel by scientists from Michigan State University (MSU), the Smithsonian Institution, and eight other organizations, has documented a dramatic and unprecedented shift in foraging habits that is likely linked to industrial fishing in the Pacific.

“The question is, have the effects of open-ocean fishing gone beyond targeted species, like tuna,” said Anne Wiley, lead author, Smithsonian postdoctoral researcher and former MSU doctoral student. “Our study is among the first to show that it has, and because Hawaiian Petrels eat such a wide variety of prey over a large area, our results suggest that fishery influence may be widespread and profound in the Pacific.”

The research, which was just published in the Proceedings of the National Academy of Sciences, looked at the chemistry of feathers and bones of modern Hawaiian Petrels and bones from subfossils as old as 4,000 years to determine where on the food chain and where in the Pacific Ocean the birds have been foraging, and how that may have changed over many centuries. Scientists extracted protein from bones and feathers and studied stable isotopes of carbon and nitrogen in the protein. What they found in older samples were nitrogen isotope ratios that were consistently high, indicating a diet of relatively large prey high on the food chain. Those less than a century old, after industrial fishing had started, had low ratios, revealing a shift to smaller fish, squid, and other prey lower on the food chain overall.

According to the report, earlier studies in coastal marine environments have shown that the combined effects of habitat loss and harvest of marine organisms have caused regional loss of entire ecosystems. Seabirds, such as the Hawaiian Petrel, and other top predators, are often viewed as indicators of their food webs. Because they forage over such wide expanses of the Pacific, changes in their foraging habits have the potential to reflect widespread changes in food chain dynamics.

The study examined isotope data from more than 250 individuals, including birds from every known modern and ancient Hawaiian Petrel population. Furthermore, the authors studied a chronology of samples that extends back over 4,000 years, reflecting conditions both prior to and following human presence in the oceanic northwest Pacific.

The nitrogen isotope ratio declined in the petrel following the onset of large-scale industrial fishing, which could have affected the petrel diet through several mechanisms. Many seabirds such as the Hawaiian Petrel forage in association with schools of large predatory fish, such as tuna and billfish that drive prey to the ocean surface. Depleted numbers of these schools, therefore, may reduce the availability of prey for the petrel. Additionally, it is possible that that petrel prey species have been depleted by direct harvest or bycatch in fisheries.

"This is a ground-breaking study that highlights the need for further research to explore the conservation implications of this shift for the Hawaiian Petrel, other seabirds, and the marine food webs that support them,” said Dr. George Wallace, Vice President for Oceans and Islands for American Bird Conservancy.

The Hawaiian Petrel (Pterodroma sandwichensis) is a crow-sized oceanic bird that ranges widely over the northeast Pacific. This species was once numerous and widespread throughout the entire Hawaiian archipelago but now numbers only about 15,000 birds distributed in isolated breeding colonies on Kauaʻi, Lanaʻi, Maui, and Hawaiʻi Island. The birds spend most of their time at sea and return to land only to breed in barren alpine areas and steep forested slopes, where they come and go nocturnally from underground burrows.

“Conservation efforts for endangered seabirds take place mainly on land at breeding colonies where there are obvious threats like introduced predators,” said Helen James, coauthor and research zoologist at Smithsonian's National Museum of Natural History. “Our study suggests we should pay more attention to the lives of these birds at sea.”