The bleached bones of seabirds are telling us a new story about the far-reaching impacts of industrial fisheries on today’s oceans. Looking at the isotopes of 250 bones from Hawaiian petrels (Pterodroma sandwichensis), scientists have been able to reconstruct the birds’ diets over the last 3,000 years. They found an unmistakable shift from big prey to small prey around 100 years ago, just when large, modern fisheries started scooping up fish at never before seen rates. The dietary shift shows that modern fisheries upended predator and prey relationships even in the ocean ocean and have possibly played a role in the decline of some seabirds.
“Hawaiian petrels spend the majority of their lives foraging over vast expanses of open ocean. In their search for food, they’ve done what scientists can only dream of. For thousands of years, they’ve captured a variety of fish, squid and crustaceans from a large portion of the North Pacific Ocean, and a record of their diet is preserved in their bones,” Anne Wiley, lead author of the new paper in the Proceedings of the National Academy of Sciences (PNAS), explains.
 A Hawaiian petrel flies over part of its Pacific Ocean foraging grounds. Photo by: Jim Denny. |
To conduct their study the researchers turned to a vast collection of Hawaiian petrel bones found in cave colonies, some of them preserved from long before the first humans arrived on the Hawaiian Islands. By comparing the ratio of nitrogen-15 and nitrogen-14 in the bones’ isotopes, Wiley and colleagues were able to tell what the generalist predators were eating. The larger the ratio, the bigger the prey. For thousands of years the birds’ diet was heavily composed of bigger prey, until nitrogen ratios shrunk around a century ago denoting a suffen shift of small prey species. According to the paper, this “[suggested] a relatively rapid change in the composition of oceanic food webs in the Northeast Pacific.”
Co-author Peggy Ostrom with Michigan State University calls the findings “alarming,” noting that the study is one of the first to discover that industrial fishing has impacted non-target species and food webs even as far as the open ocean.
“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,” explains Wiley. “Understanding the influence of fisheries on open-ocean food webs has been one of the great mysteries of biological oceanography.”
 400- to 1,000-year-old bones from an endangered seabird, the Hawaiian petrel. These bones represent a small fraction of those collected from the species. They offer a window into the lives of seabirds before and after the arrival of humans in the open ocean environments of the Pacific. Photo by: Brittany Hance, Smithsonian Institution. |
The scientists believe this forced dietary change may have conservation impacts as well. They theorize that the shift may have limited total population number in addition to possibly shrinking the size of the seabirds. Hawaiian petrels are currently listed as Vulnerable by the IUCN Red List and is protected under the U.S. Endangered Species Act (ESA).
“Conservation efforts for most seabirds focus on breeding grounds where habitat loss and predation from introduced species are obvious hazards. However, rapidly shifting or disappearing prey bases may be a hidden threat to Hawaiian petrels and other marine species,” the authors write.
Seabirds are among the world’s most endangered birds: 28 percent of the world’s sea birds are threatened with extinction, while 48 percent are seeing population declines. In addition to prey decline, seabirds have also suffered heavily from industrial fisheries as bycatch. Longline fisheries alone are estimated to kill 160,000-320,000 seabirds every year.
CITATION: Anne E. Wiley, Peggy H. Ostrom, Andreanna J. Welch, Robert C. Fleischer, Hasand Gandhi, John R. Southon, Thomas W. Stafford, Jr., Jay F. Penniman, Darcy Hu, Fern P. Duvall, and Helen F. James. Millennial-scale isotope records from a wide-ranging predator show evidence of recent human impact to oceanic food webs PNAS 2013 ; published ahead of print May 13, 2013, doi:10.1073/pnas.1300213110
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