April 2024 e-edition - 27

WALLEYE CONSUMPTION OF STOCKED
TROUT IN NORTH PLATTE RESERVOIRS
Nathan Jaksha, a graduate student in the lab of
William Fetzer at the University of Wyoming, is
working with Game and Fish to use stable isotopes
to understand the impact walleye have on stocked
trout in Alcova and Pathfinder reservoirs. Predatory
interactions between walleye and trout can make
managing these two species in a single lake or reservoir
challenging. Jaksha, working with a team of
Game and Fish fisheries biologists in Casper, was
tasked with understanding these interactions through
the use of stable isotopes.
Jaksha completed fieldwork in 2021-23 and collected
walleye and stocked trout from the reservoirs.
He analyzed samples for their sulfur stable isotope
ratios. Sulfur provides insights into the consumption
of stocked fish because fish reared in a hatchery are
typically fed food with ocean-sourced protein that
is rich in a particular stable isotope of sulfur with a
marine signature. This food chemically tags a fish as
being raised in a hatchery since marine sulfur isotopes
cannot have originated in Wyoming. This tag,
which manifests as a high-sulfur ratio value, persists
for the first several weeks or months after stocking
until eating natural food in the environment begins
to dilute and eventually erases the unique signal of
the hatchery.
Jaksha submitted muscle and egg samples from
walleye to the UW Stable Isotope Facility for analysis,
and the data he got back was fascinating.
" When we looked at the results from muscle tissues,
walleye collected in the summer had high sulfur
values in their muscle, indicating they were eating
the trout that were stocked during the spring and
kokanee salmon that were stocked in the summer, "
Jaksha said. " But when we looked at walleye collected
in late fall and winter, their muscle tissue showed
almost no trace of the elevated sulfur levels of stocked
fish, despite additional trout stocking events that
took place in the fall. However, when we analyzed
the eggs collected from those same winter-collected
walleye, they had very high sulfur values. "
Reasoning through these trends points to a clear
cause of the pattern. During spring and summer,
walleye dedicate most of the energy they consume
to growing muscle and body fat, so that is where the
elevated sulfur values of stocked trout went. However,
in the fall and winter, walleye are preparing to spawn,
and most of the energy they consume goes toward
reproduction - growing eggs. This indicated that
walleye were eating stocked trout and gave insights
into how these fish allocate their energy resources
toward growth throughout the year.
These results tell Game and Fish that walleye likely
have a large impact on stocked trout in Alcova and
Walleye tend to focus more of its energy on reproduction in the fall and winter and consuming food in the
spring and summer. (Photo by Nathan Jaksha)
Nathan Jaksha, a graduate student at the University of Wyoming, holds a walleye captured as part of a study
looking into the impact of walleye on stocked trout in Alcova and Pathfinder reservoirs.
(Photo courtesy of Nathan Jaksha)
Wyoming Wildlife | 27

April 2024 e-edition

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