Save a third for the birds

Successful seabird breeding depends on access to one third of the world's fish

Results from the most comprehensive study ever undertaken shows that many seabird species decline strongly when the food available for them in the ocean declines below a third of the maximum amount recorded. Photo: Baltic seabird project
Seabirds are some of the best and most easily measured indicators when it comes to diagnosing the health of the world's marine ecosystems.

In the most comprehensive study ever undertaken, an international group of scientists, including centre researcher Henrik Österblom, has shown that many seabird species decline strongly when the food available for them in the ocean declines below a third of the maximum amount recorded.

A global pattern
The study, which was recently published in Science, covers birds from the Arctic to the Antarctic and from the Pacific to the Atlantic. Österblom and his colleagues used data collected from 14 species including guillemots, gannets, puffins and penguins.

Their breeding capacity was measured over periods of 15 to 47 years related to how much of their main fish food source was available around their breeding colonies.

What they found was a remarkable global consistency between access to fish and seabird breeding success.

Wherever they occurred in the world the effect of low fish was similar. Österblom and his colleagues found that breeding reaches a plateau and does not change even as food abundance increases. When the amount of fish in the sea was greater than one-third of maximum levels of fish, the number of chicks produced remained pretty much unaffected. But if the fish abundance fell below this one-threshold, the number of chicks produced declined.

"The global pattern shows a threshold below which the numerical breeding response declines strongly as food abundance decreases," says Henrik Österblom.

Österblom and his colleagues also found that breeding reaches a plateau and does not change even as food abundance increases.

"We were amazed by the consistency of the relationship around the globe. This suggests that we have found an important benchmark that can be used as a guide to limit the amount of fish taken from the sea in order to maintain seabird populations in the long term", says Dr. Philippe Cury of the French Research Institute for Development and leader of the research group.

Sufficient birds means healthy ecosystems
The study is not only interesting for its findings about the relations between seabirds and access to fish. It can also provide an overall indication of what state the world´s marine ecosystems are in.

"Determining the relationships between predator and prey is critical for our understanding of marine ecosystem dynamics," says Henrik Österblom.

He argues that the thresholds defined in the study can be implemented in management objectives to maintain forage fish biomass above the necessary one-third. Though there might be some variations between different fish populations, most of the economically important ones have sufficient data to define thresholds.

"Our study demonstrates a sufficient consistency to argue the one-third-for-the-bird principle could be applied widely," Österblom concludes.

See whiteboard seminar with Henrik Österblom explaining more in detail the one-third-for-the-bird principle:

Time:
00:05:58

References

Global seabird response to forage fish depletion — one-third for the birds". Philippe M. Cury, Ian L. Boyd, Sylvain Bonhommeau, Tycho Anker-Nilssen, Robert J.M. Crawford, Robert W. Furness, James A. Mills, Eugene J. Murphy, Henrik Österblom, Michelle Paleczny, John F. Piatt, Jean-Paul Roux, Lynne Shannon and William J. Sydeman. Science 23 Dec 2011, Vol. 334, Issue 6063.
Henrik Österblom is a researcher at Baltic Nest Institute and Stockholm Resilience Centre and is also science-policy co-ordinator at the centre. His research is focused on food-web interactions, trophic cascades and ecosystem regime shifts, as well as governance of marine resources.

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