Dr. Brett Johnson's Fisheries Ecology Lab at CSU

Food Web Dynamics in Colorado's Coldwater Reservoirs

Brett Johnson (P.I., CSU), Patrick Martinez (P.I., CDOW Aquatic Research)

INTRODUCTION

Colorado's large coldwater reservoirs have supported some of the finest kokanee and mackinaw fisheries anywhere.  But today, the state's top kokanee populations (and egg supplies) are in peril.  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.

BASIC CONCEPTS
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 (2009), 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 introductions, these species usually cause large disruptions and have detrimental impacts on existing fisheries.
• 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. 2009)
• 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 Coldwater Reservoirs

• 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 stressed.

• 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 generation.

It takes a lot of groceries to feed big mackinaw

• 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

• The scientific journal article entitled "Trophic economics of lake trout management in reservoirs of differing productivity" by Brett Johnson and Patrick Martinez (Johnson, B. M. and P.J. Martinez.  2000.   North American Journal of Fisheries Management 20:115 131) was honored as the Most Significant Paper Published in the North American Journal of Fisheries Management in 2000.  A committee of international fisheries experts reviewed all papers published in Volume 20 of the journal and ranked them on the basis of technical merit, integration with existing knowledge, and significance of the findings.

• Poster 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.
  

• Literature

• Frequently Asked Questions

CSU Fisheries Ecology Lab
Colorado Division of Wildlife - Aquatic Research

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