Photograph by Paul Souders, Corbis

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Pink salmon (shown above spawning in Alaska) have increased since the 1970s, with an estimated 640 million returning to their breeding rivers in Asia and North America in 2009 alone.

Photograph by Paul Souders, Corbis

Too Many Salmon in the Sea, Pacific Study Hints

Burgeoning numbers of pink salmon may threaten the food supply of young seabirds.

Too many fish in the sea? Surging pink salmon stocks in the Pacific Ocean pose a risk to other wildlife, suggests a seabird study released on Monday that points to climate change as a culprit. (Related video: "Alaskan Salmon Adventure.")

Along with other salmon, pink salmon (Oncorhynchus gorbuscha) numbers have grown since the 1970s, with an estimated 640 million returning to their breeding rivers in Asia and North America in 2009 alone. (Read "The Long Journey of the Pacific Salmon" in National Geographic magazine.)

Tied to rising ocean temperatures in the Bering Sea and North Pacific that spurred the growth of the prey of salmon and seabirds alike, the "much larger than previously known" impact of pink salmon is reported in a new Proceedings of the National Academy of Sciences report.

It's "an uncommon case of too many fish in the sea," says the report. The study, led by Alan Springer of the University of Alaska Fairbanks, found that salmon eating the food of seabirds appears to be cutting the birds' numbers.

"Very little is known about how open ocean ecosystems work, and the apparent effect on them by salmon, wild and hatchery produced, really must be considered," Springer said by email.

The finding points to unanticipated side effects on wildlife from climate change, with unexpected winners and losers. The just-released Intergovernmental Panel on Climate Change report (unrelated to the new salmon study) warns, for example, of warming oceans threatening Atlantic cod and tuna species. (Related: "New Climate Change Report Warns of Dire Consequences.")

Seabird Colonies

To investigate potential food competition between pink salmon and other marine life, the team focused on seabird colonies in the Bering Sea and Aleutian Islands. Monitored by scientists since 1984, these colonies changed as the number of pink salmon increased.

Specifically, the study team tied the breeding fortunes of seabirds to the two-year life cycle of pink salmon. Each year, the salmon naturally alternate between high and low levels of abundance in the sea.

In the salmon-rich years, the team found, the breeding success of birds such as kittiwakes and puffins was significantly less than in the alternate years. Some species laid fewer eggs, up to half as many as they did previously; the eggs also hatched much later, and fewer of the young survived.

The affected seabirds are species that, like pink salmon, have an omnivorous diet, with prey ranging from zooplankton to squid and Atka mackerel.

Evidence that pink salmon are "a major influence" on these seabirds is "compelling," the team concludes.

"Phenomenal Increase"

The findings come as no surprise to Pacific salmon scientist Greg Ruggerone of Natural Resources Consultants, a marine fisheries consultancy based in Seattle, Washington.

Ruggerone, who wasn't involved in the study, has previously raised the issue of the booming pink's ravages, finding that the species cut the Alaskan populations of another Pacific salmon, the sockeye, through prey competition.

Explaining how rising sea temperatures have fueled the pink's "phenomenal increase" since the 1970s is a complex issue, Ruggerone said. But their springtime departure from the river to the sea as tiny juveniles could be key.

"Early marine survival is critical to overall abundance," he said, and higher sea temperatures in spring would mean more prey are available and lead to a higher metabolic rate in the salmon, allowing them to eat more and grow faster.

In their second year, the new study notes, pinks consume enough food in a four-month period—one which overlaps with the seabirds' breeding season—to increase their body mass by 500 percent.

This drain on the North Pacific's food resources could well have direct and indirect effects on other marine animals, such as recovering populations of great whales, the authors write.

Rethinking the Ocean

Salmon-biased conservation measures, such as commercial fish catch limits and large releases of hatchery-reared salmon, may have to be rethought in order to conserve other marine creatures, the study argues.

While recognizing the importance of Pacific salmon both commercially and as a food, the authors caution against "using the oceans as unattended feedlots." (Related: "Sea Lice From Fish Farms May Wipe Out Wild Salmon.")

This idea of scaling back on pink salmon doesn't unduly faze Rich Lincoln, director of State of the Salmon in Portland, Oregon, a program set up by the Wild Salmon Center to guide the conservation and sustainable management of Pacific salmon.

"The idea of at least 'freezing the footprint' of existing hatchery programs has been increasingly heard as a logical precautionary response to some of the uncertainty surrounding the potential impacts that pink salmon abundance may have on other species," Lincoln said by email.

Ruggerone's own research puts the hatchery-bred proportion of the total pink salmon stock at about 13 percent, although the hatchery-to-wild fish ratio is significantly higher for regions such as Alaska. (Related: "Salmon Farming Gets Leaner and Greener.")

As for increasing the commercial catch of pink salmon to deliberately reduce numbers, "this would be a novel idea," Lincoln said.

"I've only really heard this kind of discussion to date surrounding introduced species that are having a negative ecosystem effect," he added. "It might be logical first to address the magnitude of artificial inputs of hatchery fish."

Even so, "it's an intellectually interesting idea," Lincoln conceded.

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