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
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
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 (http://www.tandfonline.com/doi/abs/10.1080/00028487.2016.1150881).
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
In addition, predation by nesting gulls was
high in the tailraces of McNary and John Day dams, and in a section of the John
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