Research / Sea Grant News

Trash-loving birds take bite out of wild salmon

Dozens of Western gulls congregate at the mouth of Scott Creek in Santa Cruz County to bathe, drink and snack on protected salmonids such as steelhead. Credit: Anne-Marie Osterback

A gauntlet of Western gulls congregate at the mouth of Scott Creek in Santa Cruz County to bathe, drink and prey on passing fish, including threatened steelhead. Credit: Anne-Marie Osterback

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.


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

A Bioenergetics Approach to Assessing Potential Impacts of Avian Predation on Juvenile Steelhead during Freshwater Rearing

Assessing Avian Predation on Juvenile Salmonids using Passive Integrated Transponder Tag Recoveries and Mark–Recapture Methods

For more information on the research, contact:

Scott A. Shaffer, San Jose State University,

Sean A. Hayes, NOAA/SWFSC,

Ann-Marie K. Osterback, UCSC, (831) 420-3986,

Written by Christina S. Johnson, California Sea Grant,

2 thoughts on “Trash-loving birds take bite out of wild salmon

  1. Reblogged this on Literarysurfer and commented:
    Can people and salmon co-exist? I really wonder sometimes.

    According to a new study, Central California steelhead have about a 30-percent chance of being eaten by Western gulls as they exit creeks into the sea. Endangered coho salmon may be on the menu, too. In some years, in some creeks, mortality rates were estimated to exceed 80 percent.

    It is thought that our trash and a local landfill are artificially boosting gull numbers, incidentally increasing predation on young migrating salmon and steelhead.

    I wrote this story but there are a lot of things that I didn’t mention that I think are interesting and worth noting or asking, maybe for a follow-up post.

    1) It is not clear that heavy predation on young migrating salmon is currently reducing adult salmon returns. That remains to be seen and then rigorously investigated. The birds, for example, could be feasting on weak fish that would have died in the ocean anyway. Nonetheless, if young outmigrating salmon mortality rates were to keep climbing, it is conceivable that the gulls could push coho to the brink and then some.

    2) The gulls are visual predators. They hunt during the day. I wonder if heavy gull predation is now shaping salmon and steelhead behavior by selectively favoring survival of night-migrating fish? This shows (at least theoretically) how man-made changes in food-web dynamics can affect animal behaviors. We really are changing our world in ways we can’t even begin to fathom.

    3) The title of the study, “High Predation on small populations: avian predation and imperiled salmonids,” hints at what I think is the main message of the study: There can’t be more cats than mice, unless the cats are being feed somewhere else. This is basically what this study is suggesting and it means that we are the problem not the gulls.

    That is all for now. Must go.

  2. Pingback: Science and Conservation In the News, Around the World | Conservation Science News December 20, 2013

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s