A study of the Hawaiian petrel, a seabird that spends the majority of its life foraging for food across the open ocean, has revealed that over time the bird has been eating lower down into the food chain, rather than higher, a surprising find that researchers are correlating with the growth of industrialized fishing.

An analysis of both modern and ancient petrel bones enabled the researchers to determine that the menu for the seafaring bird has drastically changed and prompted concern that other species are facing the same situation.

"Our bone record is alarming because it suggests that open-ocean food webs are changing on a large scale due to human influence," said Peggy Ostrom, co-author and Michigan State University zoologist.

"Our study is among the first to address one of the great mysteries of biological oceanography - whether fishing has gone beyond an influence on targeted species to affect non-target species and potentially, entire food webs in the open ocean."

The birds' diet is recorded in the chemical composition of their bones. By studying the bones' ratio of nitrogen-15 and nitrogen-14 isotopes, researchers can tell at what level in the food chain the birds are feasting; generally, the larger the isotope ratio, the bigger the prey. Bone analysis of Hawaiian petrels from 4,000 to 100 years ago indicated that the birds one had a high isotope ratio, indicating their diet consisted of larger varieties of squid, crustaceans and fish.

The researchers state that after the onset of widespread industrial fishing in the 1950s, the isotope ratio in the Hawaiian petrel declines, indicating that the birds underwent a species-wide shift in diet to smaller fish and other prey.

A collection of more than 17,000 ancient Hawaiian petrel bones was available for sampling.  

"The petrels breed in burrows and caves [in Hawaii] where, if they die, their bones are likely to be preserved for a long time," said co-author Helen James of the Smithsonian Institution. "It's fortuitous to find such a rich bone record for a rare oceanic predator."

Ranging from the equator to near the Aleutian Islands, the Hawaiian petrel has a feeding range that is larger than the continental United States.

"What you choose to put on your dinner plate - that's your connection with the endangered Hawaiian petrel, and with many other marine species," said lead author Anne Wiley, formerly a Smithsonian Institution postdoctoral researcher.