CSU Fisheries Ecology Lab
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
lake trout 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
Simple Food Webs = Delicate
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)
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 Coldwater
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.
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 generation.
It takes a lot of groceries to feed big
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.
lake trout have overrun their kokanee food supply in several lakes and
reservoirs across the West (Martinez et al. 2009)
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.
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maintained by: Brett Johnson
Colorado State University