Reconstructing long-term ecological records is always a challenge, but for the open ocean the problem can be especially vexing because animal remains don’t last, and even if they did they’d be so deep as to be very difficult to access. In this week’s PNAS, a paper authored by Anne Wiley at the Smithsonian Institution in Washington, DC, (a PhD student at Michigan State University during most of the research) and colleagues presents a unique solution to the problem that offers a possible glimpse of how humans have affected open ocean waters.
The main source tapped for open ocean information is commercial fish catches. But these go back only a few decades at best, and they are influenced by countless variables such as changing fishing techniques. This has left open questions about human activities’ reach offshore.
But Wiley and her colleagues have managed to construct a long-term isotopic record that helps answer those questions by working in a rather unexpected place—on land. Conveniently, the endangered Hawaiian petrels (Pterodroma sandwichensis) nest on land but spend much of their life on foraging trips offshore, at times covering 10,000 kilometers or more in a single trip.
Since they collect prey from such a huge area, their food intake, and, hence, their bodies’ isotopic signatures, is effectively a broad average that’s not likely to be significantly influenced by small-scale changes.
So the team excavated petrel remains on various islands that they radiocarbon dated and analyzed for isotopic signatures. Their key result came from nitrogen isotope ratios from bone collagen samples.
The researchers found that for thousands of years, the petrels’ nitrogen isotope ratios stayed about the same. But relatively recently, there was an abrupt shift to a lower isotopic ratio. Past research has reliably tied lower ratios to feeding on lower trophic levels in a food web.
Initially the team’s data didn’t have high enough resolution to say more than that the shift occurred within the last 200 years. But they remembered that museums in California and Hawaii both hold samples from 1914 of petrels from the island of Molokai. It took a while, but with promises of extreme care they got permission to work with a few samples. The isotopes from these specimens were about the same as for the ancient populations, suggesting the shift observed happened after 1914—consistent with the onset of large-scale, industrial fishing in the Pacific’s oceanic waters.
There are other possibilities to explain the decline, but the authors do a thorough job in the paper of ruling out other scenarios to settle on human fishing pressure as the most likely explanation for the petrels’ dietary shift. As large fish have been removed, the petrels have had to settle for smaller stuff. One interesting factor that may have affected this is that the petrels tend to feed in association with large fish such as tuna that drive petrel prey to the surface. Less tuna would mean less prey.
Currently the petrels’ most common prey is the wonderfully named purpleback flying squid, and they also go after flying fish and deep-dwelling species such as lantern fish that come to the surface at night to feed. The isotopes don’t reveal exactly what the petrels would have eaten before the shift. In fact, they may well have fed on the very same species. But the isotopes do suggest that within the last 100 years they began eating, on average, smaller prey.
The isotope results can’t say specifically what impacts the dietary shift may have had on the petrel populations. But similar shifts among some coastal birds have been tied to population declines. And it’s reasonable to assume that if the birds are settling for smaller prey than they once did, there are going to be some negative repercussions.
The bigger picture, of course, is that the results suggest that even in the wilds of the open ocean, human activities have had dramatic impacts. That’s certainly been suspected, but difficult to document, making creative solutions like these petrel analyses a welcome contribution to the field.