Dr. Brett Johnson's Fisheries Ecology Lab at CSU
Food Web Dynamics in
Colorado's Coldwater Reservoirs
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 =
coldwater reservoirs have relatively simple food
species has important roles to play and changes in the abundance of one species may have consequences for others in the
- 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)
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
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).
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
kokanee have a short life cycle
and therefore they are vulnerable to population crashes when stressed.
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).
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.
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, March 2005.
B. M. and J. P. Goettl, Jr. 1999.
Food web changes over fourteen years following introduction of rainbow
smelt into a Colorado reservoir. North American Journal of Fisheries Management
- Martinez, P.J., P.E.
Bigelow, M.A. Deleray, W. A. Fredenberg,
B.S. Hansen, N.J. Horner, S.K. Lehr, R.W. Schneidervin,
S.A. Tolentino and A.E. Viola. 2009. Western
Lake Trout Woes. Fisheries 34:424-442.
CSU Fisheries Ecology Lab
Colorado Division of
Wildlife - Aquatic Research
| Equal Opportunity
maintained by: Brett Johnson