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Totaling nearly 50,000 surface acres, Colorado's large (> 1,000 surface acres), high elevation (> 6,500 feet asl) reservoirs comprise roughly 25% of the state's flatwater and about 50% of its coldwater lake and reservoir fishery resource. Many of these reservoirs undergo seasonal water fluctuations which minimize or preclude establishment of plant and invertebrate communities that produce food for fish in shallow or shoreline areas. As a result, the food webs supporting sport fish in many fluctuating reservoirs are based primarily on zooplankton, particularly Daphnia, which inhabit the surface waters of the reservoir's pelagic areas. Daphnia abundance in individual reservoirs is dependent on a combination of nutrient status, phytoplankton production, water temperatures, and the abundance of fish and invertebrates that feed on these planktonic crustaceans.

The Coldwater Reservoir Ecology project is concerned primarily with the production of sport fish from a reservoir's inherent productivity.  An important fishery component in many of these reservoirs is kokanee, which must be sustained by the annual collection of eggs and stocking of fry.  Lake trout, capable of reaching trophy proportions in some reservoirs, have the capacity to consume too much prey in the form of hatchery-reared kokanee and trout, which are important to the majority of anglers.  Recent and current research topics are briefly described below:

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Kokanee were introduced into Colorado in 1951 by importing eggs from Flathead Lake, Montana . Colorado began producing its own kokanee eggs in 1956, fulfilling its in-state demand for kokanee eggs since 1958.  Historically, Colorado has collected kokanee eggs from eight reservoirs. From 1956 until the early 1990s Lake Granby was the state's principle source of kokanee eggs.  Due to in-reservoir conditions affecting the kokanee's food supply, Daphnia, and predation on kokanee by lake trout, Granby's kokanee egg production dwindled to zero by the late-1990s.  Since the early 1990's, Blue Mesa Reservoir and its spawn run of kokanee to the Roaring Judy Hatchery have become the Colorado's primary source of kokanee eggs. Despite collecting eggs from six reservoirs in 2003, the state functionally has its egg in one basket, Blue Mesa.  Strategies to preserve Blue Mesa's kokanee population have included doubling the historic kokanee stocking rate in recent years, increasing protection of the kokanee spawn run and encouraging the harvest of lake trout to reduce predation on kokanee. Improved reservoir conditions in Granby in recent years should help kokanee numbers rebound, restoring some egg-production, improving the kokanee fishery and providing more prey for stunted lake trout. 

Figure 1. Summary of the numbers of kokanee eggs collected
in Colorado from various reservoirs, 1956-2003

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Knowing the growth rate of fishes is essential to making accurate estimates of predation demand using bioenergetics models.  Lake trout have the capacity to live a long time, thus determining their age from scales is difficult at best. The otoliths, or ear bones of a fish, often offer a superior alternative for establishing the age of fish, especially long-lived ones.  Otoliths were removed from angler caught lake trout or those that died during netting operations.  The tiny ear-bones were mounted in epoxy, sectioned with a special saw, polished, magnified and digitally enhanced using a computer. Lake trout in Colorado's largest reservoir, Blue Mesa, grow rapidly, reaching 30-32 inches in length in 10 years. This rapid growth rate, among the fastest known for the species, was confirmed by the growth rates of lake trout tagged and recaptured by CPW fishery biologist Dan Brauch. This rapid rate of growth is due to heavy predation by lake trout on calorie-rich, stocked rainbow trout and the even richer kokanee fattened on the reservoir's abundant Daphnia. Aging of lake trout from Blue Mesa, including recent state record lake trout, shows that they can reach 40 inches in about 20 years. 

Figure 2.  Length and age of lake trout in Blue Mesa Reservoir determined
from otoliths with the growth rate verified by tagged fish and sectioned,
polished otoliths from two recent state record lake trout caught in Blue Mesa.

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Using the growth rate of lake trout in Blue Mesa Reservoir, determined by otolith analysis, and an estimate of lake trout abundance from CSU graduate student Harry Crockett, bioenergetics modeling was performed in cooperation with Dr. Brett Johnson, also of CSU.  Stomach analyses of lake trout in Blue Mesa showed that the diet of lake trout 17 to 24 inches is about 60% fish by weight. The diet of lake trout 24 to 35 inches in length is 90% fish by weight.  Nearly all of the fish eaten by lake trout in these size categories were rainbow trout and kokanee, with very few suckers, which are not as calorically-rich as salmonids, being eaten.  Based on these findings, it is assumed that the largest lake trout, over 35 inches, eat exclusively rainbow trout and kokanee to maintain their incredible mass.  Estimates of the pounds of rainbow trout and kokanee eaten annually by lake trout are about 6 pounds by a 20 incher, about 22 pounds by a 30 incher and about 51 pounds by a 40 incher.  Expanding these estimates of the amount of rainbow trout and kokanee eaten by lake trout of different sizes by their estimated abundance, and summing this weight of fish eaten by the entire lake trout population over 5 years old (20 inches) in the reservoir, illustrates the threat that current numbers of lake trout pose to the reservoir's kokanee and rainbow trout.  Lake trout consume 33,000, and possibly up to 55,000 pounds of kokanee annually in Blue Mesa. This lower value exceeds the poundage of fish needed to sustain a spawn run, and represents millions of kokanee eggs, and the higher value rivals or exceeds the poundage of kokanee harvested by anglers in a year.  A similar poundage of rainbow trout is also eaten annually by lake trout, threatening that popular fishery as well.  While current levels of lake trout predation pose a serious threat to the Blue Mesa's kokanee and rainbow trout, any increase in predation by lake trout could prove devastating for the reservoir's entire fishery as well as the kokanee egg supply for the entire state.

Numerous Western States Face the Same Issue
Lake trout predation on kokanee salmon is not a problem exclusive to Colorado. Wildlife agencies in Wyoming, Idaho, Montana, California, Utah and Washington are also working to maintain their kokanee populations.
 
You can learn about how this issue is affecting all of these western states by reading Introduction Species: Western Lake Trout Woes (pdf), published in "Fisheries," the journal of the American Fisheries Society. The paper explains the extensive research on the kokanee-lake trout issue and possible solutions.

Figure 3. Photo of kokanee in stomach of a lake trout from Blue Mesa Reservoir
and estimates of the amount of fish eaten by various sizes of large lake trout
in the reservoir based on current growth rates and body condition of lake trout.

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The effects of the water level in Lake Granby on the reservoir's food web are well known.  During years in which water levels are high the water column remains cooler Mysis shrimp proliferate and invade surface waters where they can eliminate or reduce the Daphnia population resulting in reduced survival of kokanee fry and growth of older kokanee, and predatory lake have greater access to kokanee and rainbow trout in surface waters.  During years when water levels are lower, the water column warms more quickly limiting the survival and access of Mysis to surface waters resulting in Daphnia appearing sooner in the spring and reaching higher densities resulting in improved survival of kokanee fry and growth of all kokanee and restricting the access of lake trout to their rainbow trout and kokanee prey from late-spring to early-fall.  Slot-limit regulations at Lake Granby, intended to produce and preserve a trophy lake trout fishery, contributed to the demise of the reservoir's kokanee fishery and egg production in the 1990s. Overprotecting lake trout while kokanee numbers plummeted resulted in the stunting of lake trout, which was reflected in the dramatic loss of body condition of larger lake trout since the mid-1980, and in the stalling of growth of all lake trout over 20 inches since the mid-1990s.  A large lake trout from Blue Mesa Reservoir will weigh from 12 to 20 pounds more than a lake trout of the same length from Granby, attesting to the presently poor body condition of lake trout in Granby and the effects of too many predators and too little prey. Recent removal of the slot limit protection for lake trout, and lower water levels improving Daphnia production and kokanee survival while reducing Mysis abundance, has contributed to more kokanee in annual sonar surveys and improved kokanee egg numbers. 

Figure 4.  Comparison of Lake Granby's water level and kokanee spawner
average length (note cyclic pattern in water level and inverse response of
kokanee), and a map of the reservoir showing sonar transects and
estimates of pelagic fish abundance from annual sonar surveys.

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Cooperative research led by graduate student Harry Crockett of CSU sought to improve the use of scientific sonar for the rapid assessment of the numbers of predator-sized lake trout relative to their pelagic prey, kokanee. While scientific sonar provides digital data on the length of fish it detects in a way that recreational sonars cannot, this estimate of a fish's length is not as accurate as directly measuring a fish's length. The error associated with sonar estimates of fish length may cause large kokanee to be "acoustically" counted as lake trout, inflating estimates of predator abundance and the number of fish eaten by them.  Conversely, this error may cause small, but predatory, lake trout to be "acoustically" counted as kokanee, thus underestimating the potential threat to a fishery's overall stability posed by a burgeoning lake trout population and the amount of fish they will eat as the lake trout grow. By applying the sources of error in estimating fish lengths from sonar determined from CSU's modeling of targets strengths obtained from sonar surveys conducted at Blue Mesa reservoir, our understanding of the capacity for sonar to provide rapid assessments of pelagic kokanee and lake trout in other reservoirs was greatly improved. Additional work is needed in this area, but our scientific sonar can provide data on fish abundance and size in many cases that can be used to assess and forecast the impact of predatory lake trout on kokanee and other pelagic fishes.

Figure 5. Diagram comparing average "error" between measured
fish length and the length of that fish determined by sonar.