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.