Predation by Caspian terns on young steelhead means fewer return as
adults

Date:
July 14, 2020
Source:
Oregon State University
Summary:
Caspian terns feeding on young fish have a significant impact on
runs of steelhead in the Columbia River, new research suggests.



FULL STORY ========================================================================== Caspian terns feeding on young fish have a significant impact on runs
of steelhead in the Columbia River, research by Oregon State University suggests.


========================================================================== Through detailed analysis of steelhead survival and Caspian tern predation rates, the researchers found that the birds are not only preying on fish
that would perish for some other reason, but are adding to the annual
death toll by eating steelhead smolts that would have survived without
tern pressure.

In scientific terms, the findings indicate that the terns are having an "additive" effect on prey mortality rather than a "compensatory" one.

The study was published in Ecological Applications.

In the Columbia Basin, 13 of 20 populations of anadromous salmon and
steelhead are listed as threatened or endangered under the Endangered
Species Act.

Caspian terns, a protected migratory bird species native to the region,
have been the object of predator management in the Columbia Basin in an
effort to protect smolts, especially steelhead smolts, from being eaten
before they can swim downstream to the ocean.

The largest breeding colony of Caspian terns in the world was formerly
on a small island in the lower Columbia River estuary between Oregon
and Washington.

It hosted more than 10,000 breeding pairs in 2008, just prior to
implementation of nonlethal management to reduce colony size to between
3,125 and 4,375 breeding pairs.



========================================================================== "There has been little research, however, into whether reduced predation actually results in greater overall salmonid survival, either at the
smolt stage, where the predation is taking place, or across the lifetime
of the fish," said Oregon State's Dan Roby, professor emeritus in the Department of Fisheries and Wildlife of the College of Agricultural
Sciences. "Without clear evidence that reduced predation means greater
survival to adulthood, management to reduce predator impacts would
be a waste of time and resources." To tackle the question, Roby and collaborators at Real Time Research, Inc., of Bend and the University
of Washington looked at 11 years' worth of mark- recapture-recovery
data for almost 80,000 steelhead trout smolts from the Upper Columbia population that were tagged and released to continue their out- migration
to the ocean.

After release, the tagged fish were exposed to predation throughout
multiple stretches of river on their journey toward the Pacific. The tag-recovery data made possible estimates of the weekly probability of steelhead survival, mortality from being eaten by birds and death from
other causes.

"This approach allowed us to directly measure the connection between
smolt survival and tern predation," Roby said.

Estimates of tern predation on steelhead were substantial for most of the
years studied, he said. And increases in tern predation probabilities were connected with statistically significant decreases in steelhead survival
for all of the years evaluated and both of the fish life stages studied:
smolt out-migration and smolt-to-adult returns.



==========================================================================
"Our results provide the first evidence that predation by Caspian terns
may have been a super additive source of mortality during the smolt stage
and a partially additive source in the smolt-to-adult life stage," Roby
said. "A persistent pattern was clear: For each additional 10 steelhead
smolts successfully consumed by Caspian terns, about 14 fewer smolts
from each cohort survived out-migration." Another pattern: On average,
for every 10 steelhead smolts eaten by terns, one fewer individual from
each cohort returned to the Columbia Basin as an adult.

"Our model shows that mortality from tern predation was primarily additive
and therefore has a credible, significant impact on prey survival,"
Roby said.

"Predator-prey models need to consider additive effects of predation
across life stages to avoid exaggerating potential benefits from
management actions aimed at reducing predator populations to enhance
prey populations. The primary value of the study is by analyzing the
true effects of natural predators on populations of their prey, and
thereby assessing the conservation value to prey of managing predators."
Roby notes that the study by OSU, Real Time Research, and the University
of Washington contradicts recently published research by scientists with
the U.S.

Fish and Wildlife Service and the Fish Passage Center, who found that
steelhead mortality due to tern predation is compensatory.

That paper, in the Journal of Wildlife Management, suggests that
"management efforts to reduce the abundance of the [tern] colonies are
unlikely to improve the survival or conservation status of steelhead."

========================================================================== Story Source: Materials provided by Oregon_State_University. Original
written by Steve Lundeberg. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Quinn Payton, Allen F. Evans, Nathan J. Hostetter, Daniel D. Roby,
Brad
Cramer, Ken Collis. Measuring the additive effects of predation
on prey survival across spatial scales. Ecological Applications,
2020; DOI: 10.1002/eap.2193 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200714082838.htm

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