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Birds are taking a bite out of young salmon populations in Central California, and researchers suspect that our trash is the likely root of the avian-predation problem.
A new study by California Sea Grant-funded researchers shows that a young steelhead has about a 30-percent chance of being eaten by Western gulls during its transit to sea through creek mouths in San Mateo and Santa Cruz counties.
Depending on the specific creek and year, gauntlets of gulls lining narrow streams may consume anywhere from 7-83 percent of young steelhead in the Waddell, Scott and Gazos watershed mouths, according to the same study, published in the journal Ecosphere.
Intriguingly, the gulls appear to prefer wild over hatchery-born fish. (Who would guess the raucous dumpster divers would have a gourmet streak?)
Central California watersheds support both steelhead, which are listed as threatened under the Endangered Species Act, and coho salmon, which have gone locally extinct in 12 of their 14 historic streams south of San Francisco and are considered in danger of extinction.
Steelhead were the focus of the Sea Grant project for the practical reason that they can still be found in multiple watersheds in the region. There is no reason to believe, however, that gulls are not also impacting young coho salmon numbers.
“We are at a loss of what to do,” said Jon Ambrose, a NOAA Fisheries biologist who is involved with Central California coho salmon recovery and familiar with the Sea Grant project. “Scott Creek is ground zero for coho salmon recovery efforts. It’s the only creek left that supports all three cohorts of coho.”
“We have thought of the ocean as this big dangerous place,” said Sean Hayes, a co-investigator on the Sea Grant project and a salmon ecologist at NOAA Southwest Fisheries Science Center. “But, it may be that the last 200 or 300 meters of a river and estuary are the most dangerous. These fish are literally being scooped out right before they enter the ocean.”
Ironically, the gulls may truly only be snacking on salmon, and feasting on our trash. Tagging and tracking studies show the birds make frequent trips to the Santa Cruz landfill. This virtually endless supply of easily accessible human-waste food may be artificially increasing both gull populations and, by extension, opportunistic predation on young steelhead and salmon.
“We see thousands of gulls at the landfill,” said Ann-Marie Osterback, the California Sea Grant graduate student trainee on the project and the lead author of the 2013 study.
“‘Mystery meat’ is a gull staple,” said Scott Shaffer, a bird biologist at San Jose State University and a co-investigator on the California Sea Grant project. “I don’t know if I would go out on a limb and say that the dump subsidizes gull populations, but that is what some people speculate, and if it is true, it creates an indirect effect on salmon.”
While salmon populations are struggling, numbers of Western gulls have roughly doubled in the last 30 years. There are now about 1,000 breeding pairs on Año Nuevo Island, located off the coast of San Mateo County not far from the Scott Creek watershed.
“It would be easy to say let’s get rid of some gulls, but the bigger issue is that there are way too few fish,” Hayes said.
In light of their findings, biologists tried but failed to build a bird-exclusion device over the mouth of Scott Creek. They are now talking about gull-proof trash lids, landfill practices and “pick up your trash” public education.
“It is not sexy but trash management could be good be a good thing for salmon,” Ambrose said.
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California Sea Grant provided funding for the project R/FISH-205 “Exploring the Impact of Avian Predators on Central California Salmonids” in 2008-2011. The grant recipients were: Scott A. Shaffer, now at San Jose State University; Jonathan W. Moore, now at Simon Fraser University in British Columbia, and Sean A. Hayes at NOAA Southwest Fisheries Science Center.
Publications referenced in this article and/or produced with support from this grant include:
High Predation on Small Populations: Avian Predation on Imperiled Salmonids
For more information on the research, contact:
Scott A. Shaffer, San Jose State University, 408.924.4871, Scott.Shaffer@sjsu.edu
Sean A. Hayes, NOAA/SWFSC, 831.420.3937, sean.hayes@noaa.gov
Ann-Marie K. Osterback, UCSC, (831) 420-3986, osterbac@biology.ucsc.edu
Written by Christina S. Johnson