A rare study on the impact of industrial fishing on open-ocean food supplies has found that the Hawaiian petrel, a crow-sized oceanic bird that flies above the northeast Pacific, isn’t eating nearly as well as it used to.

A team of scientists from the Smithsonian Institution and Michigan State University undertook an extensive study of ancient and modern Hawaiian petrel bones and found that since the advent of industrial fishing a century ago, the bird’s diet had diminished from larger prey to smaller fish, squid and other relatively meager fare.

The findings are published in the current edition of the science journal Proceedings of the National Academy of Sciences.

Drawing from a collection of more than 17,000 Hawaiian petrel remains, some as old as 4,000 years, the scientists extracted protein from bones and feathers and studied stable isotopes of carbon and nitrogen contained therein. This allowed the scientists to see how the bird’s diet had changed due to human incursion into its feeding areas.

According to the study, bones older than 100 years had relatively high nitrogen isotope ratios, which indicates that petrels had a diet that included larger varieties of fish, squid and crustaceans. More recent remains yielded lower nitrogen isotope ratios, indicating a shift to smaller prey.

The Smithsonian-MSU study represents one of the rare instances in which scientists have been able to significantly deepen understanding of so-called food webs in the open ocean, an area of research that has traditionally been hampered by the sheer size of open-ocean feeding areas and a relative lack of ocean species remains dating back further than a half-century.

Hawaiian petrel populations have been found in Haleakala crater, Mauna Loa volcano, Waimea Canyon, Lanaihale — a forest on Lanai — and the Galapagos Islands. The birds breed in burrows and caves, providing ample and accessible bone records. They travel over a massive area, flying as far as Alaska and Japan during periodic feeding trips.

The petrels typically feed by grabbing prey while sitting on the water or by dipping for prey while flapping just above the ocean surface, according to a species profile prepared by the state Department of Land and Natural Resources.

Given that the petrels eat such a wide variety of prey over such a large area, the results of the study indicate that “fishery influence may be widespread and profound in the Pacific,” according to lead author Anne Wiley, a Smithsonian postdoctoral researcher.

Wiley said the study is among the first to suggest that open-ocean fishing has affected more than just targeted species like tuna.

The findings raise concern for the Hawaiian petrel and other potentially affected species since shifts down the food chain are associated with decreases in overall population.

The International Union for Conservation of Nature and Natural Resources has classified the Hawaiian petrel as a “vulnerable” species — one that is likely to become extinct if threats to its survival and reproduction continue unabated.