Aug. 4, 2009 -- Rising seagull populations are laying siege to endangered salmon in central California, according to a new study.
Salmon fisheries along the Pacific coast of the United States have witnessed an unprecedented collapse over the last two years, forcing state and federal officials to issue a ban on commercial and sport fishing. Grounded, commercial fishermen have been forced to subsist off of federal disaster aid.
Everything from dams to overfishing to climate change has been blamed for the collapse, but a definitive answer has proven elusive.
Now, a new study suggests exploding seagull populations are contributing to the decline, their numbers growing along with garbage piles in cities and along coastal areas.
Anne-Marie Osterback of the University of California, Santa Cruz, and a team of researchers found 213 transponder tags strewn throughout a colony of western gulls (Larus occidentalis) on Ano Neuvo Island, off the California coast between 2002 and 2008. The tracking devices are surgically implanted in young salmon before they swim out to sea, but gulls were feasting on the salmon, eating as much as 2.5 percent of each year's population.
"Gull populations have been increasing for at least the last five years," Osterback said. "And they are opportunistic feeders. They'll go into a river to bathe or drink, and when the young salmon come along, they'll snack on them."
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Osterback will present the research this week at the annual meeting of the Ecological Society of America.
The team focused on endangered populations of Steelhead (Oncorhynchus mykiss) and Coho salmon (Oncorhynchus kisutch) in Scott Creek, a small river in central California that empties into the Pacific Ocean. Only a few hundred salmon return to spawn each year here, a far cry from the hundreds of thousands of fish that fishermen and wildlife managers expect in the San Joaquin/Sacramento River system to the north.
Still, Osterback noted that the transponders the team found represent a minimum estimate for the number of fish birds eat. Many fish aren't tagged, and many more transponders may never be recovered. Increasing gull populations will likely impact salmon stocks up and down the West Coast.
"People talk about canary in the coal mine situations, where you put the cage down in the mine, then pull it up and the canary is dead," David Craig of Willamette University said. "Well, if you pull the cage up and there are 10 canaries in it, that's also a problem. And that's what's happening here."
Gulls probably won't be the deciding factor in whether salmon populations thrive or die off, he added. However, they are adding stress to species already beset by a myriad of environmental pressures, including habitat loss, warming ocean waters and pesticide runoff.
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