Dr. Brett Johnson's Fisheries Ecology Lab at CSU
Web Dynamics in Colorado's Coldwater Reservoirs
Colorado's large coldwater reservoirs have
supported some of the finest kokanee and mackinaw fisheries anywhere. However, keeping a sustainable balance between these two sport fish has been a challenge. At Lake Granby, the home of the state record mackinaw for
years, the kokanee population crashed and the big macks got skinny.
The Granby kokanee population was restarted with eggs from Blue Mesa Reservoir, but now Blue Mesa's kokanee population has declined to dangerously low levels. A crash of Blue Mesa's kokanee population, the state's largest egg source, could spell disaster for the entire state's kokanee fishery, and could even lead to the end of kokanee fishing in Colorado.
CSU's FIsheries Ecology Lab has collaborated with CDOW for over 15 years seeking to unravel the complex factors driving dynamics of fisheries and food webs in Colorado's coldwater reservoirs. The interplay between fisheries management (fish stocking, harvest regulations), water management (dam operations, water level fluctuation) and climate (precipitation, inflow and temperature) provide a fascinating backdrop within which we try to understand why fish populations, and the fisheries they support, vary the way they do.
This website provides some background information to help interested anglers and others learn more about Colorado's coldwater reservoirs, their foodwebs and fisheries, and some of the scientific fundamentals that underlie sound management of these ecosystems.
Simple Food Webs = Delicate Balance
- Colorado's coldwater reservoirs have relatively
simple food webs.
- Each species has important roles to play and
changes in the abundance of one species may have consequences for
others in the web.
- When the smelt population crashed in Horsetooth
Reservoir, zooplankton recovered but walleye body condition plummeted (Johnson
and Goettl 1999). Now (2010), without an adequate prey fish supply, walleyes grow slowly and bioaccumulate high levels of mercury.
- When new species invade one of these food
webs, for example by illegal introduction, these species usually cause
large disruptions and have detrimental impacts on existing fisheries (Johnson et al. 2009a).
- If walleye are illegally introduced into Blue Mesa Reservoir the impacts are predicted to be disastrous for kokanee and lake trout with only a mediocre walleye fishery to replace them (Johnson et al. 2009b)
- Reservoir fishery managers have a challenging
job to find and maintain the right balance between predators and prey in
these manmade ecosystems.
Kokanee - the Key to Good Fishing in
- In Colorado, kokanee salmon have proven to
be a highly cost-effective fish for making great fishing in coldwater reservoirs.
With skill, they are easy to raise in the hatchery, resistant
to whirling disease, and until recently have survived well when stocked
at a small size (2 inches).
- Kokanee have also been the preferred prey
of lake trout in many western reservoirs and are no doubt essential for
producing trophy lake trout in these waters.
- Unfortunately, kokanee have a short
life cycle and therefore they are vulnerable to population crashes when
- Since kokanee don't reproduce in the wild
in Colorado, sport fishing in coldwater reservoirs depends on maintaining
healthy populations for egg sources so the hatcheries can produce the next
It takes a lot of groceries to feed
- Because lake
trout feed heavily on other salmonids, managing for trophy lake trout
requires tradeoffs from kokanee and rainbow trout sport fisheries
(Johnson and Martinez 2000).
- To maintain diverse fishing opportunities,
fishery managers need to seek a balance between predators and prey.
If the balance is lost, kokanee and rainbows decline and macks get skinnier.
- Unfortunately, lake trout have overrun their kokanee food supply in several lakes and reservoirs across the West (Martinez et al. 2009)
FOR MORE INFORMATION
Presentation "Kokanee & Lake Trout in Blue Mesa & Granby
Reservoirs, Colorado: Implications for Ongoing Kokanee Egg Production"
from Colorado-Wyoming Chapter of the American Fisheries Society Meeting,
- Fisheries magazine article: Martinez, P.J., P.E. Bigelow,
M.A. Deleray, W. A. Fredenberg,
S.A. Tolentino and
A.E. Viola. 2009. Western Lake Trout Woes. Fisheries 34:424-442.
Fisheries Ecology Lab
Division of Wildlife - Aquatic Research
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maintained by: Brett Johnson