Seabird bones reveal changes in open-ocean food chain

A research team, led by Michigan State University and Smithsonian Institution scientists, analyzed the bones of Hawaiian petrels – birds that spend the majority of their lives foraging the open waters of the Pacific. They found that the substantial change in petrels’ eating habits, eating prey that are lower rather than higher in the food chain, coincides with the growth of industrialized fishing.

The birds’ dramatic shift in diet, shown in the current issue of the Proceedings of the National Academy of Sciences, leaves scientists pondering the fate of petrels as well as wondering how many other species face similar challenges.

“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 MSU 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 nontarget species and potentially, entire food webs in the open ocean.”

Hawaiian petrels’ diet is recorded in the chemistry 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 (fish, squid and crustaceans).

Between 4,000 and 100 years ago, petrels had high isotope ratios, indicating they ate bigger prey. After the onset of industrial fishing, which began extending past the continental shelves around 1950, the isotope ratios declined, indicating a species-wide shift to a diet of smaller fish and other prey.

Much research has focused on the impact of fishing near the coasts. In contrast, the open ocean covers nearly half of the Earth’s surface. But due to a lack of historical records, fishing’s impact on most open-ocean animal populations is completely unknown, said lead author Anne Wiley, formerly an MSU doctoral student and now a Smithsonian postdoctoral researcher.

“Hawaiian petrels spend the majority of their lives foraging over vast expanses of open ocean,” she said. “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.”

Addressing fishery impact through a chronology of bones is remarkable. Most marine animals die at sea, where their bones are buried on the ocean bottom. But after three decades of fossil collection in the Hawaiian Islands – the breeding grounds of the Hawaiian petrel – co-author Helen James of the Smithsonian Institution and her colleagues have amassed a collection of more than 17,000 ancient Hawaiian petrel bones.

“The petrels breed in burrows and caves where, if they die, their bones are likely to be preserved for a long time,” James said. “It’s fortuitous to find such a rich bone record for a rare oceanic predator.”

Further studies are needed to explore how the shift down the food chain is affecting Hawaiian petrels. For a coastal seabird, however, a similar shift in diet has been associated with decreases in population – bad news for a federally protected bird.

Since petrels exploit fishing grounds from the equator to near the Aleutian Islands – an area larger than the continental United States – their foraging habits are quite telling. If petrels, signal flares for open-ocean food webs, have had a species-wide change in feeding habits, how many other predators around the world has fishing impacted? And what role do consumers play?

“What you choose to put on your dinner plate – that’s your connection with the endangered Hawaiian petrel, and with many other marine species,” Wiley said.

The research was funded by the National Science Foundation, MSU and the Smithsonian Institution.