Dr. Brett Johnson's Fisheries Ecology Lab at CSU

Food Web Dynamics in Colorado's Coldwater Reservoirs
Frequently Asked Questions (updated November 2010)

fishing on Blue Mesa Reservoir. NPS photo, Lisa Lynch.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. 

Mysis relicta, opossum shrimp.  NOAA GLERL photo.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.

CDOW's Bellvue fish hatcheryWhy 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.

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

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Colorado State University, Fort Collins, CO 80523 USA
Brett Johnson