Non-native fish are main consumers of salmon in reservoirs

Date:
September 23, 2021
Source:
Oregon State University
Summary:
When warmwater fish species like bass, walleye and crappie that are
not native to the Pacific Northwest, but prized by some anglers,
overlap with baby spring chinook salmon in reservoirs in Oregon's
Willamette River they consume more baby salmon than native fish
per individual, new research found.



FULL STORY ==========================================================================
When warm water fish species like bass, walleye and crappie that are not
native to the Pacific Northwest, but prized by some anglers, overlap
with baby spring chinook salmon in reservoirs in Oregon's Willamette
River they consume more baby salmon than native fish per individual,
new research found.


==========================================================================
The research by Oregon State University, Oregon Department of Fish and
Wildlife and U.S. Forest Service Pacific Northwest Research Station
scientists, recently published in the journal Ecosphere, may have
implications for threatened salmon and future management strategies.

"Mixed stock fisheries are complicated and always changing," said
Christina Murphy, lead author of the paper who is a courtesy faculty
member at OSU and a biologist with the U.S. Geological Survey in
Maine. "We are providing the science to help managers identify tradeoffs
to make the best management decisions for each individual location."
Fisheries managers in Oregon are increasingly identifying and grappling
with threats posed by illegally introduced or invasive species overlapping
with native fish populations. In part due to this new research, management actions now include removing harvest restrictions on non-native warmwater
fish species, which are sought by some anglers because of their white,
flaky meat and sporty fighting ability, where their presence may have
negative impacts on sensitive native fish species, such as salmon.

"As the fish management agency for Oregon, it is our job to promote
and achieve goals of native fish conservation and providing angler opportunity," said Jeremy Romer, a co-author of the paper and assistant district fish biologist for ODFW in Springfield. "To do this effectively
we use adaptive management and base our decisions on the most recent,
reliable scientific research. We are always striving to pool resources and collaborate with colleagues to solve problems and inform management. This project is a great example of that." Construction of dams in the
Willamette River and elsewhere created an unnatural overlap of coldwater
fish, such as salmon and rainbow trout, and non-native warmwater fish,
such as bass and crappie, in reservoirs. Studies on the diets of fish show
that non-native species can have a large predatory impact on populations
of salmonids, including spring chinook.



==========================================================================
In the new paper, the OSU, ODFW, and U.S. Forest Service researchers
studied fish in the Hills Creek and Lookout Point reservoirs on the
upper Middle Fork Willamette River, just southeast of Eugene, Oregon. The spring chinook salmon found in that river are listed as threatened under
the Endangered Species Act.

They studied the diets of reservoir fish in two time periods: March
through June and August through September. Previously it was difficult
to identify if predators had eaten baby salmon, known as fry, because
they are quickly digested into indistinguishable goo. The researchers
solved that problem by using two methods to identify which fish consume
the spring chinook salmon in the first few months of their lives: stable isotopes and stomach contents.

Isotope analysis allows the researchers to determine what predators
ate by tracking nitrogen through the food web. Nitrogen accumulates
in salmon from the food they eat in the ocean. That nitrogen is then
passed on from the mother to the baby salmon, and when a predator eats
a salmon fry it also absorbs that nitrogen. The more fry predators eat,
the higher the marine derived nitrogen level.

This elevated nitrogen level of the fry only lasted through May in
the study.

After that, the isotope analysis only told the scientists that the primary
diet of a predator is any kind of fish. The researchers then used the
actual stomach content information to determine what fish species were
being consumed.

They found that walleye, by far, were most likely to have juvenile salmon
in their stomachs. In the early time period, 18.5% of walleye had salmon
in their stomach contents. In the later period, 15.8% did. They were the
only fish species in the later period that had salmon in their stomach.

Other predators in the early period with salmon in their stomachs were largemouth bass (5.7%), white crappie (3.1%) and then native northern pikeminnow (0.6%). The black crappie, cutthroat trout, rainbow trout and
yellow bullhead studied during both time periods had no salmon in their stomachs The isotope signature analysis confirmed the stomach content
findings. Walleye, largemouth bass and white crappie also had higher
levels of nitrogen isotopic signatures.

Ivan Arismendi, a co-author of the paper and an assistant professor
in the Department of Fisheries, Wildlife, and Conservation Sciences
in the College of Agricultural Sciences, added: "Our findings support
this strategy of integrated management, emphasizing that the capture
of popular non-native warmwater species such as walleye can be promoted
in areas that are overlapping with conservation priorities for salmon." ========================================================================== Story Source: Materials provided by Oregon_State_University. Original
written by Sean Nealon.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Christina A. Murphy, Jeremy D. Romer, Kevin Stertz, Ivan Arismendi,
Ryan
Emig, Fred Monzyk, Sherri L. Johnson. Damming salmon fry:
evidence for predation by non‐native warmwater fishes in
reservoirs. Ecosphere, 2021; 12 (9) DOI: 10.1002/ecs2.3757 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210923102059.htm

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