In May 2013, American scientists announced startling findings about the Pacific environment: Seabird bones reveal changes in the open-ocean food chain. Michigan State University zoologist Peggy Ostrom, who participated in the study, says that “our bone record is alarming because it suggests that open-ocean webs are changing on a large scale due to human influence.”
Open-ocean ecology is surprisingly poorly known outside of large species like sharks and whales, and economically important species, like fish stocks. However, in recent years, there have been widespread concerns that increasing exploitation of open-ocean fish stocks and other food sources by humans was having widespread environmental consequences. Closer to shore, many fish stocks are in serious decline and large animal species, like marine mammals, may be suffering a range of difficulties, too. Until now, though, scientists could only speculate that there would be similar effects further out in the open ocean, which is already nutrient-poor compared to coastal zones at the best of times.
The biologists who put together this latest study, which was published in the Proceedings of the National Academy of Sciences, are trying to test this theory. To do so, they performed radiochemical analyses of a wide range of bone remains of the Hawaiian petrel, a large seabird which was once widespread throughout the Hawaiian islands. Today, the Hawaiian petrel is considered a vulnerable species, and most of the surviving members are found only on Maui. The petrel nests in Hawaii but spends most of its time foraging in the open Pacific, ranging from equatorial waters as far north as the Aleutian Islands off Alaska.
The Hawaiian petrel is already known to have suffered considerably because of habitat loss to humans on the islands, but these scientists, who included Ostrom, Anne E. Wiley, of Michigan State, and nine other co-authors, wondered whether its diet was suffering from serious disruption to the open-ocean food chain, too. They put together a collection of bones ranging from the present to approximately 3000 years ago. Then they analyzed the carbon isotope ratios in those bones. Higher carbon isotope ratios indicate that an animal is getting its food by eating other species high on the food chain, like fish.
By analyzing those bones, the scientists were able to confirm that the diet of the Hawaiian petrel was basically the same for almost all of that time, and that they had high carbon isotope ratios, suggesting they were eating other food species relatively high on the food chain. Until, that is, the previous century. Modern fishing in the open ocean really only began in the 1950s, but, since that time, the isotope ratios indicate that the Hawaiian petrels have been forced to shift down the food chain in search of smaller, poorer food sources. That suggests that large food sources, like fish, are being over-fished to the point where seabirds like the Hawaiian petrel have serious trouble finding their normal food sources.
The decline in the Hawaiian petrel population doesn’t directly impact the security of the human food chain. However, if the petrels are having difficulty finding food because of a rapid collapse in the open-ocean food chain, that in turn suggests that the day is coming when the fish stocks are low enough that the fishing industry will have difficulty bringing food to human plates.
The new research can be found in the May 28, 2013, issue of the Proceedings of the National Academy of Sciences under the somewhat intimidating title “Millennial-Scale Isotope Records From a Wide-Ranging Predator Show Evidence of Recent Human Impact to Oceanic Food Webs.” However, Michigan State has also prepared a simple summary of the research for non-experts.
