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Study: Mid-Columbia/Lower Snake Avian Predation High For Steelhead, Data Varies By Fish Species
Posted on Friday, July 01, 2016 (PST)

Colonial waterbirds foraging in the lower and mid-Columbia River and the lower Snake River eat a substantial number of salmon and steelhead, with steelhead mortality approaching 28 percent for fish that migrate through the area, according to a recent study.


The study published last month in the Transactions of the American Fisheries Society found that predation probabilities (proportion of available fish consumed by birds) for steelhead ranged from 6 percent to 28 percent, depending on where the fish are in the rivers, as well as the time of year and type of bird preying on the fish. The study says that over 50 percent of all steelhead mortality is due to avian predation.


For yearling chinook, the probability is considerably lower at 3 percent to 9 percent, and just 1 percent to 5 percent for subyearling chinook.


“Results of the study provide over-whelming evidence that predation by colonial waterbirds on juvenile steelhead was a significant source of smolt mortality and in some river reaches and years, predation by colonial waterbirds was the single greatest factor regulating steelhead survival during outmigration,” said Allen Evans, fisheries scientist at Real Time Research, Inc. in Bend, Ore. and one of 11 co-authors of the study. 


Predation impacts on chinook salmon were generally lower when compared to steelhead trout and in the case of subyearling chinook salmon, Evans said, “colonial waterbird predation was a relatively minor source of total smolt mortality; indicating that something other than colonial waterbird predation was limiting survival of subyearling chinook within the study area.”


In addition, late-migrating smolts were more susceptible to avian predation than were early migrants, the study says.


The study evaluated predation in three river reaches during 2012 and 2014, all upstream of The Dalles Dam, including:

--Rock Island Dam to the confluence of the Snake and Columbia rivers,

--Ice Harbor Dam to below John Day Dam,

--the confluence of the Snake and Columbia rivers to below John Day Dam


An evaluation of foraging locations identified few areas of concentrated predation by either double crested cormorants or white pelicans. The study did find that smolt predation by gulls was concentrated near hydroelectric dams, while predation by Caspian terns was concentrated within reservoirs. The study area did not include birds nesting in the lower Columbia River estuary.


The authors used both acoustic and passive integrated transponder tag technologies to collect data on where and when smolts were consumed by colonial waterbirds and what fraction of all smolt mortality is directly related to bird predation compared with other, non-avian mortality sources. The double-tagged smolts were released in several locations and at different times, according to the study.


With this technology, the authors were able to explain variation in weekly estimated smolt survival and assign the variation to waterbird predation or to other non-avian factors. Results demonstrated a strong relationship between juvenile fish survival and bird predation within the study area.


Avian predation has been identified as a limiting factor in the recovery of some federal Endangered Species Act-listed salmon and steelhead in the Columbia River basin.


Caspian terns, double-crested cormorants, American white pelicans, California gulls and ring-billed gulls that nest in colonies on or near the Columbia River are known to consume ESA-listed smolts, the study says.


“This information is paramount to prioritizing ESA-listed fish recovery plans,” Evans said.


The study, “Avian Predation on Juvenile Salmonids: Spatial and Temporal Analysis Based on Acoustic and Passive Integrated Transponder Tags,” was published online June 27 (


In addition to Evans, co-authors include Quinn Payton, Statistician, Aaron Turecek, GIS analyst, Brad Cramer, fish and wildlife biologist, and Ken Collis, wildlife scientist, all with Real Time Research; Daniel Roby, Professor and Wildlife Scientist, U.S. Geological Survey, Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University; Pete Loschl, wildlife biologist, OSU; Leah Sullivan, fisheries biologist, Blue Leaf Environmental, Inc. of Ellensburg, Wash.; John Skalski, professor and statistician, School of Aquatic and Fishery Sciences, University of Washington; Mark Weiland, research scientist, Pacific Northwest National Laboratory; Curtis Dotson, fisheries biologist, Public Utility District Number 2 of Grant County.


The study discovered foraging “hotspots” where predation by the birds is more intense and that some birds were capable of traveling as much as 100 kilometers (60 miles) to consume smolts, but most travel was within 40 – 50 km (24 – 31 miles).


“In general, results indicated that gulls disproportionally consumed smolts near hydroelectric dams, while Caspian terns largely foraged away from dams,” Evans said. “Colonies located within the McNary Reservoir disproportionally consumed smolts in the lower Snake River compared to the middle Columbia River.”


Predation was consistently highest between river kilometer 525 and 562 (river mile 326 and river mile 349) on the lower Snake River, which the authors attributed to large colonies of waterbirds on Foundation and Crescent islands, just downstream of the Columbia and Snake rivers confluence.


In addition, predation by nesting gulls was high in the tailraces of McNary and John Day dams, and in a section of the John Day reservoir.


While the study did not attempt to find out why steelhead are so vulnerable, Evans said that other published studies hypothesize that steelhead smolts tend swim closer to the surface where plunge-diving predators like Caspian terns can get to them. In addition, steelhead smolts tend to be larger than salmon smolts, making them more visible to avian predators and a more “energetically beneficial prey” source. 


“Predation impacts within the study likely represent minimum impacts because not all colonial waterbird nesting sites were scanned for tags in all years and not all fish-eating waterbird species (e.g., grebes, herons, mergansers, osprey, etc.) were included in the study,” Evans said.  “Despite this caveat, the most impactful colonies within the study area were included and the authors are confident that total smolt losses (those from all colonies and all predator species combined) are not grossly higher than those depicted by results.”


The study concludes that, assuming the birds are not already eating dead or injured juveniles, or that “other sources of mortality would not fill the niche created by a reduction in predation impacts from any given bird colony, a decrease in the number of piscivorous colonial waterbirds in the Columbia River basin will enhance the survival of ESA-listed juvenile salmonids.”


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