Personal Perspectives

4,000 Years of Marine History through the Eyes of a Seabird

A Hawaiian petrel in flight
A Hawaiian petrel flies over part of its North Pacific feeding grounds. (Photo courtesy of Jim Denny)

Most people have never heard of the Hawaiian petrel, an endangered, crow-sized seabird that spends the majority of its life searching for food over the North Pacific Ocean. Nevertheless, this bird is no stranger to human influence, and it has a stern lesson to teach us about the history of the open ocean. When it comes to what marine predators can find to eat, humans are changing things, and fast.

My colleagues and I have spent a number of years studying the history of this amazing seafarer. We’ve come to view it as a sort of indicator of food web history in the oceanic zone of the North Pacific—that is, the open ocean, past the continental shelves. This is because Hawaiian petrels are one of the most wide-ranging animals on the planet. One bird can cover foraging grounds between the Hawaiian Islands and the Aleutian Islands (some 2,000 miles) in a matter of days. As a species, they feed throughout a region of the Pacific that’s larger than the entire continental United States. What’s more, Hawaiian petrels are consummate opportunists: like many of their relatives, they feed on a wide diversity of fish, squid, and crustaceans—almost anything they can catch at the ocean surface. This means that what Hawaiian petrels have found to eat through the years can give us clues as to how ocean food webs are changing over a very large area.

So how do you study the diet of a seabird over the course of thousands of years? By using chemistry in lieu of time travel—the chemistry of ancient seabird bones, that is. More than 17,000 Hawaiian petrel bones have been found throughout the Hawaiian Islands, where the species comes to nest. Most of these bones are hundreds to thousands of years old, and they give us an unusually complete bone record for a species that only feeds in the open ocean.

Preview 400 to 1,000 year old bones from an endangered seabird, the Hawaiian petrel. Bones such as these provide a window into the lives of seabirds before and after human arrival in the open ocean environments of the North Pacific.
400 to 1,000 year old bones from an endangered seabird, the Hawaiian petrel. Bones such as these provide a window into the lives of seabirds before and after human arrival in the open ocean environments of the North Pacific. (Photo NHB13-00273 courtesy of Brittany Hance, Smithsonian Institution.)

These bones contain chemical signatures of what food the petrels ate—specifically, what level of the food chain they preyed upon. Did they eat bigger fish that ate other fish, or did they eat smaller animals that ate plankton? By studying the chemical composition of bones from every known population of Hawaiian petrel, my colleagues and I have conducted a species-wide study that traces their foraging habits back 4,000 years. That’s a 4,000-year window into North Pacific food webs.

Through that bony window, we saw a rather surprising picture of the petrel’s past. For 3,000 years, Hawaiian petrels consistently ate at the same level in the food chain. But in the past 100 years, petrels began eating smaller prey further down in the food chain. That’s a trend visible in the entire species, even for populations that feed in different regions of the ocean—and a trend that’s unprecedented in the last four millennia.

Why the change? The most likely cause is industrial fishing, which started about 60 years ago in the open ocean. One reason fishing can affect seabirds is that fisheries target bigger fish higher up in the food chain. This leaves less big food around for the birds and may force them to catch smaller prey lower in the food chain. Similar slips down the food chain have been associated with human fishing and prey declines for predators in very different kinds of environments: coastal and freshwater. For one coastal seabird, the marbled murrelet, a shift down the food chain is one possible reason the species is threatened with extinction. The fact that we see a similar trend in the Hawaiian petrel suggests that even remote and more recently fished marine environments—those in the open ocean—may be heavily influenced by fishing, such that prey availability has changed rapidly over very large areas.

This has led me to the rather uncomfortable realization that I’ve had more contact with the Hawaiian petrel than I previously thought. Yes, I’ve been rummaging through their bones and studying the chemistry of their proteins for the last 7 years. But I’ve probably also had a connection to petrels for my entire life… through my diet [*gulp*]. As a seafood-eating member of a seafood-eating culture in a seafood-eating world, I’ve played my part in changing the open ocean.

We’ve got a lot to learn about how common diet changes are among marine predators and how these diet shifts may impact species survival, but one thing is clear: showing a little restraint in our exploitation of open ocean animals couldn’t hurt. And for at least one endangered species, it could make a big difference.

May 2013