Birdwatch News Archive


Hawaiian Petrel in flight. Photo: Peggy Ostrom.
Hawaiian Petrel in flight. Photo: Peggy Ostrom.Enlarge image

Petrels shift diet due to commercial fishing


Posted on: 16 May 2013

The remains of the Endangered Hawaiian Petrel have shown to researchers how drastically wild fish populations have changed over time.

A research team from Michigan State University and the Smithsonian Institution in the USA analysed the bones of Hawaiian Petrel, which spends the majority of its life foraging the open waters of the Pacific, for chemical indications of the species diet. In their stable isotope analysis, they found that a substantial change in the seabirds' eating habits to prey lower in the food chain, coincides with the growth of industrialised fishing.

The birds’ dramatic shift in diet, shown in the current Proceedings of the National Academy of Sciences, has left 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 non-target species and potentially, entire food webs in the open ocean.”


Anne Wiley examines the sub-fossil skull of an Hawaiian Petrel. Photo: Peggy Ostrom.


The diet of Hawaiian Petrels is recorded in their bone chemistry. By studying the ratio of nitrogen-15 and nitrogen-14 isotopes, researchers are able to tell at what level in the food chain the birds' prey is found; generally, the larger the isotope ratio, the bigger the fish, squid or crustacean.

Between 4,000 and 100 years ago, petrels had high isotope ratios, indicating that they ate bigger prey. After the onset of industrial fishing, which began extending past the continental shelves at around 1950, 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, but open ocean covers nearly half of the Earth’s surface. Due to a lack of historical records, fishing’s impact on most open-ocean animal populations is completely unknown, said lead author Anne Wiley. “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.”


Anne Wiley examines Hawaiian Petrel bones in a cave on Hawaii. Photo: Peggy Ostrom.


Addressing fishery impact through a chronology of bones is remarkable, as most marine animals die at sea where their bones are buried on the ocean bottom. After three decades of fossil collection in the Hawaiian Islands – the breeding grounds of Hawaiian Petrel – co-author Helen James of the Smithsonian Institution and her colleagues amassed a collection of more than 17,000 ancient Hawaiian Petrel bones.
“The petrels breed in burrows and caves where 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 the species. For coastal seabirds, similar shifts in diet have been associated with decreases in population – bad news for a federally protected bird.

Since petrels exploit fishing grounds from the equator to the Aleutian Islands – an area larger than the continental United States – their foraging habits are quite telling. If petrels 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.


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