Climate Change May Be Cause of Seabird Deaths

April 24, 2007

Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334

For the third year in a row, large numbers of seabirds have washed up dead on beaches in California and Oregon, apparent casualties of shifts in the California Current’s primary productivity.

Bill Sydeman, director of marine ecology at PRBO Conservation Science in Petaluma, believes that changes in productivity, which have translated into less food for seabirds, may in part be the result of climate change, a sort of regional footprint of the global warming trend.

“I think the bird deaths relate to long-term climate-related issues,” Sydeman said. “We are seeing that it doesn’t take much warming, at the wrong time of year, to push the California Current system into a less productive state. This may be the consequence of global warming. The system is primed to be warm and somewhat unproductive.”

Tufted Puffin. Credit: Roy W. Lowe/USFWS

Sydeman has California Sea Grant funding to study patterns of krill abundance in areas of Northern California supporting large colonies of seabirds. Ultimately, he would like to map the sequence of steps linking changes in the marine physical environment to the breeding success of Cassin’s auklets, a species whose diet is comprised primarily of krill, hence the emphasis on understanding krill production in the early stages of his research.

California Sea Grant Extension Program is also participating in a West Coast “citizen science” seabird survey, in which volunteers are trained to look for and identify seabird carcasses that have washed ashore. About 200 beaches in Northern California, Oregon, Washington and Alaska are currently monitored monthly.

“We have seen an unusual composition of seabird carcasses,” said Sea Grant marine advisor Pete Nelson, who helps organize the California component of the survey. There have been more rhinoceros auklets and horned puffins – pelagic species usually found further offshore. Presumably the birds are coming inshore to look for food they cannot find in their normal foraging grounds.

What could cause a redistribution, or worse, decline in food for seabirds? For one, changes in wind-driven upwelling, the process in which colder, nutrient-rich waters from depth are brought to sunlit surface waters where plankton grow. Researchers consider upwelling to be the single most important factor controlling primary productivity along the coast, which is why California is sometimes called an upwelling coast.

Global warming may be making it harder to vertically mix coastal waters, an idea put forth by Dean Roemmich and John McGowan, both at Scripps Institution of Oceanography in La Jolla, in an article “Climatic Warming and the Decline of Zooplankton in the California Current” published in Science in 1995. Their analysis showed that waters in Southern California had become more stratified since 1951 and that enhanced stratification had translated into a shallower source of “upwelled” waters to support zooplankton production. They hypothesized that continued warming could lead to further declines in zooplankton productivity.

A newer idea is that the location at which the eastward-flowing North Pacific Current splits may also play a role in redistributing zooplankton. The North Pacific Current is a large current that originates off Japan, travels across the entire Pacific Ocean and then, in the southeastern Gulf of Alaska, splits into the northward-flowing Alaska Current and southward- flowing California Current.

Howard Freeland of the Institute of Ocean Sciences in Sydney, British Columbia has found that there is a strong correlation between where this split occurs and the distribution of warm- and cold-water plankton species off the coast.

Cassin's Auklet. Credit: Ulrich Wilson/USFWS

“What we have seen is that when the bifurcation point heads north, to say 53 degrees north latitude, effectively the whole California Current system moves north too,” Freeland said. The phytoplankton off British Columbia become characteristic of warmer-water species typically found off California. “The biology responds very, very quickly.”

“We don’t know yet if there is any relationship between the bifurcation point and global warming,” Freeland said, because the Argo array that is monitoring water temperatures has been in place in the northeastern Pacific only since 2002.

There does, however, seem to be a connection between the splitting point and the health of seabird populations, at least for one species, Cassin’s auklets. “We have seen high breeding success of Cassin’s auklets on the Farallones Islands when the bifurcation of the North Pacific Current is roughly at the Queen Charlotte Islands in British Columbia, as happened in 2002,” Sydeman said. “In 2005-06, when the North Pacific Current bifurcated approximately in central Oregon, the auklets suffered complete reproductive failures, unprecedented in the 35 plus years of studying seabird ecology at the Farallones.”

The breeding success of Cassin’s auklets is also itself a one-year leading indicator of chinook salmon landings and escapement rates in Central California, a fact that may be of use to salmon fishers and fisheries managers. About 60 percent of the variability in chinook salmon landings and escapement rates can be explained by the birds’ breeding success, Sydeman said, likely because both auklets and salmon feed extensively on krill. “We call it the trophic equivalency hypothesis. Animals that exist on the same trophic level may be used as indictors for each other. ... Statistically, the results are promising, though we still need to do more work to understand the mechanisms.”