Dr. Brett Johnson's Fisheries Ecology Lab at CSU
Food
Web Dynamics in Colorado's Coldwater Reservoirs
Frequently Asked Questions
(updated November 2010)
Why is it tricky to
manage kokanee and lake trout in the same reservoir?
Kokanee and lake trout did
not evolve together and experts consider this predator-prey combination
to be unstable. As a result, routine and intensive sampling of a
reservoir's
food web and fish population characteristics are needed to monitor and
manage these fisheries. Charlie Meyers (Denver outdoors columnist, deceased) offered
the
analogy of trying to maintain domestic sheep as a prey base for
mountain
lions - sheep can thrive on Colorado's rangelands, but they are not
well
adapted to resist lion predation.
Does the CDOW wish
to
eradicate lake trout in the state?
No. What is
sought is a better balance between lake trout and their food
supplies.
Maximizing lake trout catch rates AND lake trout trophy potential are
mutually
exclusive goals. There is a limited food supply in reservoirs so
it is biologically impossible to manage for large numbers of big lake
trout.
Over-protecting lake trout from harvest can become self-defeating as
lake
trout numbers overshoot their prey and eliminate or jeopardize other
fisheries. In order to have fast-growing, trophy sized lake trout it is necessary to keep lake trout numbers lower than if the management goal was to produce lots of lake trout of smaller size.
Why can't Granby and
Blue
Mesa be set aside for trophy lake trout?
Not only are these the
state's
two largest coldwater reservoirs, they historically were among the best
kokanee producers.
The
combination of water expanse and kokanee prey made it no
coincidence that lake trout numbers and growth topped that of other
reservoirs in
Colorado.
However, these two lakes have historically supplied the vast majority
of
the eggs needed to supply the hatcheries and perpetuate kokanee
populations
in every water in the state. These two reservoirs must be managed to maintain healthy kokanee populations to supply eggs for the kokanee stocking program and simultaneously support lake trout growth.
If lake trout are so
abundant,
why don't I see more lake trout on my fishfinder?
It is incorrect to assume
that a region of a reservoir is devoid of lake trout just
because they can't be seen on a fishfinder (sonar). Netting in
Colorado
and in other
states
demonstrates the presence of high lake trout numbers even when deepwater
targets cannot be detected by sonar. Another "error-of-ego"
is to assume that a fish
target
that won't strike a properly presented lure is not
a lake trout. Both of these sonar interpretations contribute to
angler underestimation
of lake trout abundance and to a disbelief that lake trout
numbers could be at
problematic
levels when in fact, unbiased scientific data are clear.
Why must lake trout
numbers
need to be reduced at Blue Mesa?
In Blue Mesa Reservoir lake trout numbers need to be reduced because 1)
kokanee and their egg supplies must be restored and protected, and
2) lake
trout
growth, relative weight and trophy potential have declined.
The bottom line is that predator and prey populations need to be brought back into balance.
Won't the
predator-prey
balance be restored once lake trout die off due to reduced prey?
No. Lake trout, in
comparison to other North American
freshwater predatory fishes, have the capacity to develop the
greatest "predator
inertia". Lake trout can maintain a high demand for
prey during lean times
because
they do not readily die of starvation.
Because they evolved under extremely cold and often
unproductive conditions,
they are long-lived and programmed to withstand long periods of
time, even years, with
poor
food availability. Once food becomes available again they spring into action, preying heavily on the newly available prey resources. This allows lake trout to delay or
prevent reestablishment
of prey fishes by stocking and makes them a particularly troublesome predator to deal with.
What's the prey base
for
lake trout in Colorado reservoirs?
Lake trout diet
studies in Colorado show that lake trout consume mainly kokanee and rainbow trout (Diet figure). Work in Colorado and in other states show that despite apparently
high
numbers of suckers, lake trout typically do not feed
heavily
on these shallow water, nearshore fishes in the presence
of preferred open-water prey species. It is also evident that
when lake trout
do not have a highly abundant, energy-rich prey
fish (e.g., kokanee and rainbow trout) they cannot acheive or
maintain
high body condition despite consuming some suckers.
Why did the kokanee
crash
at Granby?
A "perfect storm" of factors, but primarily lake trout predation. Many years of research
show
that kokanee at Granby are strongly influenced by a productivity cycle
driven by climate and dam operations. In wet years, when the
reservoir
is full in summer, it tends to be cooler, allowing the cold-adapted
mysis
shrimp to reach the surface and decimate the zooplankton that kokanee
rely
on for food. In dry years the reservoir is not so full and it
becomes
too warm on the surface for the mysis shrimp, and the kokanee have all
the zooplankton they need. Several high water years at Granby
created very poor food conditions for kokanee. This
created
slower growing kokanee which were in turn more susceptible to lake trout
predation. So, the combination of high water at Granby,
competition
for food with mysis shrimp, and predation from lake trout appear to be
the causes for the kokanee's near demise at Granby.
What was done to
restore
kokanee at Granby?
Mysis compete with kokanee for zooplankton prey, particularly Daphnia, and Mysis also harm kokanee by facilitating lake trout reproduction by providing an important food supply for juvenile lake trout. Unfortunately, a method to eliminate Mysis from the reservoir does not presently exist. Without the ability to control water levels, biologists were left with stocking and fishing regulations as their primary tools for restoring kokanee at Granby. Fortunately, kokanee eggs were still available from Blue Mesa to perpetuate the kokanee population at Granby. While the kokanee population was rebuilding the bag limit on kokanee was reduced to four and limits on lake trout were liberalized to encourage anglers to harvest more lake trout. The protective lenght limits on lake trout were removed and the bag limit was changed to four fish per day. It is imperative that anglers contribute to controlling lake trout abundance in Granby to help maintain abundant kokanee which contribute to the sport fishery, supply eggs, and provide the energy-rich prey needed to sustain lake trout growth and produce lake trout of trophy size.
Will anglers
cooperate
by taking home more lake trout?
That remains to be seen. Some avid lake trout
anglers
will no doubt continue to practice catch and release, despite the
danger
this presents for kokanee and the lake trout's food supply. Even
with an increase in lake trout angling activity
at Blue Mesa, the number of anglers willing
to harvest lake trout during the short spring period of maximum lake
trout
susceptibility to angling
hasn't been that high and over half the lake trout caught are released. Anglers need to understand that some
reduction
in lake trout abundance is needed for the good of the kokanee and
therefore,
in the long run, for the good of the lake trout populations.
Why can't CDOW use
Elevenmile
Reservoir as a kokanee egg supply?
Kokanee eggs have been
collected
from Elevenmile in the past, from 1978-1981, but it did not become an
annual
source of eggs. Although the reservoir has large kokanee,
populations
containing the largest kokanee are not the best sources for eggs.
Years of study at Lake Granby showed more steady annual egg production
from mid-size kokanee spawners over the years than from either the
largest
or smallest sized spawners. This is because mid-sized fish are
larger
than small ones and more numerous than large ones. Also, recent efforts to build up the Elevenmile kokanee population were stymied by a severe gill lice outbreak.
Can kokanee be held
in
the hatcheries as "captive broodstock" to supply the State's egg needs?
No, Colorado does not have the facilities to maintain a sufficient kokanee broodstock to supply the State's egg needs, nor would such a plan work. In a last ditch effort to
restore kokanee at Flathead Lake, MT, the Creston National Fish
Hatchery
obtained eggs from Colorado, initially from Granby until those eggs
became
too few to share, and then from Blue Mesa. Creston was successful
in producing several million eggs annually from their effort. But
the restoration project failed due
to intense lake trout predation on stocked fry and
subcatchable
kokanee. This outcome occurred despite efforts to stock the
kokanee
in a manner to optimize their survival. This captive broodstock
effort
for Flathead was abandoned a couple of years ago.
Could Blue Mesa
kokanee
succumb to lake trout predation resulting in a crash similar to that at
Granby?
Absolutely, and it appears that a kokanee crash is underway in Blue Mesa already.
Why can't we just
stock
more rainbows to feed lake trout?
It is too costly. Kokanee is the fish
species
in Colorado best able to make use of the natural productive capacity of our
fluctuating reservoirs. Kokanee stocked as fry provide the biggest
"bang for the buck" because they are relatively inexpensive to produce
and are able to grow large on the reservoir's natural food
supply.
Thus, kokanee anglers and lake trout benefit from the kokanee's
efficient
feeding habits. Too many lake trout in a system "short-circuit"
this
production cycle, and jeopardize the kokanee egg supply for future
kokanee
generations. If the state loses the ability to maintain kokanee
populations,
not only will kokanee enthusiasts suffer,
but other fishery components may feel the economic void left by the loss
of this species. It is much more costly to stock additional
rainbow
trout as replacement fisheries or to supply prey for piscivores.
How much does it
cost
to supply hatchery rainbows as lake trout prey?
If kokanee were lost from
Granby, the cost to
stock enough hatchery rainbows to sustain lake trout growth and body condition
could exceed $500,000 per year. The cost to stock enough rainbows to sustain lake trout growth and condition in the absence of kokanee at Blue Mesa would be much higher than $500K per year.
Can increased
kokanee
stocking be used to offset increasing lake trout predation?
No, this is not
the answer. The number of kokanee stocked in Blue Mesa was increased several times since 1990 and each time predation increased to compensate. Unfortunately, lake trout predation demand has grown faster than the CDOW's ability to make more kokanee. Now the Roaring Judy Hatchery, which stocks Blue Mesa, is running at maximum capacity and kokanee numbers in the reservoir are still falling. Increased stocking is rarely the answer to predation problems because adding more food simply grows more predators which means you need to add more food to satisfy them and that grows more predators which means you need to add more food... It's a vicious cycle, and you can't win.
DO YOU HAVE A QUESTION
ABOUT LAKE TROUT, KOKANEE AND RAINBOW IN COLORADO RESERVOIRS?
Check out the CDOW Aquatic
Research Section's Coldwater Reservoir Ecology homepage at http://wildlife.state.co.us/Research/Aquatic/ColdwaterResEcology/
Or contact:
Brett Johnson , Professor, CSU Department of Fish, Wildlife and
Conservation Biology, Fort Collins, CO
Dan Brauch , Aquatic Biologist, Colorado Division of
Wildlife, West Region Fishery Biologist, Gunnison,
CO
Pat Martinez,
Aquatic Researcher, Colorado Division of Wildlife, Aquatic Research, 711 Independent Drive,
Grand Junction, CO 81505
Disclaimer | Equal Opportunity
Colorado State University, Fort Collins, CO 80523 USA
Brett Johnson
