Hawaiian petrels forced to eat lower on food chain, industrial fishing blamed

Hawaiian petrels are eating far lower on the food chain, and a new study blames that on industrial fishing.

The takeaway from that is that the entire Pacific food web may be changing as a result of human activities. 

(Image, a Hawaiian petrel or `a`o. Credit Brenda Zaun, USFWS.)

Says the study: “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.”

The petrels, known in Hawaiian as `a`o, are endangered seabirds that, like the Newell’s shearwaters, nest in mountain burrows in the Hawaiian Islands. 

An abstract of the study is here. An American Bird Conservancy news release on the study is here. 

The study is fascinating. Researchers went back through fossil bird bones dating to as far as 4,000 years ago. They looked at isotopes in the bones, and were able to make conclusions about what the birds were eating. 

“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,” the authors wrote.

The research shows that up until about 100 years ago, the diet of the birds was the same for centuries. And then it changed significantly. The changes in isotopes suggest a switch from larger fish to smaller ones, from higher on the food chain to lower.

Which in turn suggests there weren’t as many bigger fish around for the petrels to eat. It’s clear that the diet change occurred. Precisely how is not so clear.

Say the authors: “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.”

The study is Millennial-scale isotope records from a wide-ranging predator show evidence of recent human impact to oceanic food webs, in PNAS, the Proceedings of the National Academy of Sciences. Authors include 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.

Several of them are with Hawai`i research organizations, including Pacific Cooperative Studies Unit, University of Hawaii; National Park Service in Honolulu, and the Hawai`i Department of Land and Natural Resources.

More on Hawaiian petrels at this Fish and Wildlife Service website.

© Jan TenBruggencate 2013