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Led By

Lake and Reservoir Researchers

Study Area

Select reservoirs

Project Status

Ongoing

Research Objectives

• Characterize and quantify interactions among key fish predators and prey in select reservoirs when required for addressing existing or emerging management uncertainties.

Project Description

Understanding how different species interact is central to sport fisheries management. For example, if predation on stocked fish intended for anglers (e.g., rainbow trout) limits development of a recreational fishery, modifying stocking practices (e.g., size of fish or timing) or other actions may be needed to improve the survival of stocked fish.

Alternatively, the recruitment success, growth and survival of sport fish sustained through natural reproduction rather than stocking can depend on numerous factors such as fluctuations in water levels or primary forage fish populations. Understanding relationships among factors helps identify potential management actions for improving sport fish growth and survival when necessary.

Lake and Reservoir Researchers use a suite of sampling methods and analytical tools to characterize reservoir food webs and predator-prey interactions when detailed assessments are needed to address management uncertainties. Sampling methods include various types of nets to catch different life-stages of fish occupying near- and offshore habitats to characterize spatial-temporal interactions among species. Analytical tools such as stable isotopes (chemical composition of fish), bioenergetics models, and population dynamics models are used to map food webs and quantify the influence of different species on others.

Current research involves investigating interactions between rainbow smelt and walleye in Horsetooth Reservoir (northeast Colorado). Rainbow smelt, a small-bodied forage fish, were introduced in 1983 to provide a prey base for walleye and smallmouth bass. The population of smelt can fluctuate dramatically. When highly abundant in the reservoir, growth of sport fish greatly improves, but they also limit natural reproduction by walleye through competition with and/or predation on larval/juvenile walleye. This research aims to identify management strategies for balancing the advantages of smelt with their disadvantages as the primary forage fish in the Horsetooth Reservoir food web.

Associated Publications

Hansen, A.G., J.S. Thompson, L.N. Hargis, D. Brauch, and B.M. Johnson. 2019. Predatory threat of introduced yellow perch in a salmonid dominated reservoir food web. North American Journal of Fisheries Management 39:172-190.

Johnson, B.M., W.M. Pate, and A.G. Hansen. 2017. Energy density and dry matter content in fish: new observations and an evaluation of some empirical models. Transactions of the American Fisheries Society 146:1262-1278.

Johnson, B.M., J.M. Lepak, and B.A. Wolff. 2015. Effects of prey assemblage on mercury bioaccumulation in a piscivorous sport fish. Science of the Total Environment 506-507:330-337.

Lepak, J.M., C.N. Cathcart, and W.L. Stacy. 2014. Tiger muskellunge predation upon stocked sport fish intended for recreational fisheries. Lake and Reservoir Management 30:250-257.

Lepak, J.M., A.G. Hansen, E.T. Cristan, and D. Williams. 2023. Rainbow smelt (Osmerus mordax) influence on walleye (Sander vitreus) recruitment failure: mitochondrial DNA evidence supporting the predation hypothesis. Journal of Fish Biology 103:1543-1548.

Lepak, J.M., A.G. Hansen, B.M. Johnson, K. Battige, E.T. Cristan, C.J. Farrell, W.M. Pate, K.B. Rogers, A.J. Treble, and T.W. Walsworth. In press. Cyclical multi-trophic-level responses to a volatile, introduced forage fish: learning from four decades of food web observation to inform management. Fisheries.

Pate, W.M., B.M. Johnson, J.M. Lepak, and D. Brauch. 2014. Managing for coexistence of kokanee and trophy lake trout in a montane reservoir. North American Journal of Fisheries Management 34:908-922.

Wolff, B.A., B.M. Johnson, and J.M. Lepak. 2017. Changes in sport fish mercury concentrations from food web shifts suggest partial decoupling from mercury loading in two Colorado reservoirs. Archives of Environmental Contamination and Toxicology 72:167-177.Predation on Stocked Fish fact sheetFish Bioenergetics Research fact sheet

Responses of waterbirds to experimental disturbances were studied from April 1984 to August 1985 at Russell Lakes State Wildlife Area in the San Luis Valley, Colorado. Black-crowned night-herons (Nycticorax nycticorax), snowy egrets (Egretta thula) and white-faced ibis (Plegadis chihi) were approached directly by a person on foot or in a motor vehicle. Mean (±SD) flushing distances for these species were 153±66, 93±43, and 95±48 m when approached on foot and 47±26, 57±27, and 54±27 m when approached in a motor vehicle. Reactions of American coots (Fulica americana), redheads (Aythya americana), ruddy ducks (O ura jamaicensis), mallards (Anas platyrhynchos), gadwalls (A. strepera) and American avocets (Recurviroslra americana) to disturbance were also quantified. Intensity of response was independent of agent of disturbances (i.e., person on foot or in vehicle) for all species. Disturbance reduced species diversity and abundance for up to one hour. American coots and avocets were only minimally impacted by the disturbance agents used.

Rio Grande cutthroat trout, Oncorhynchus clarkii virginalis, ​can be found in high elevation streams and lakes of the Rio Grande, Canadian, and Pecos River drainages in Colorado and New Mexico, giving it the southern-most distribution of any form of Cutthroat Trout. It now only occupies just 12 percent of its historic habitat in approximately 800 miles of streams. Biologists estimate that 127 conservation populations now exist in the two states, and 57 of those populations are considered to be secure.  The historic range of Rio Grande cutthroat trout has been reduced over the last 150 years due to many changes on the landscape, including: drought, water infrastructure, habitat changes, hybridization with nonnative Rainbow and Cutthroat Trout, and competition with Brook and Brown Trout.  As a result, pure populations of Rio Grande Cutthroat Trout are restricted primarily to headwater streams.​ The first conservation agreement for Rio Grande Cutthroat Trout ​between state, federal, and tribal resource agencies was signed in 2003, and gave rise to the Rio Grande Cutthroat Trout Conservation Team. The stated goal of the agreement is to assure the "long-term viability of Rio Grande Cutthroat Trout throughout its historic range by minimizing or removing threats to the species and promoting conservation." This collaborative framework was updated in the 2013 Conservation Agreement(12MB) ​and the ​2013 Conservation Strategy which outline long-term conservation objectives for this subspecies.

Conservation Team D​ocuments

Status Assessments

• ​2016 Status Assessment​​​
• 2008 Status Assessment (27MB)​

Conservation Strategies

• ​2013 Conservation Strategy
• 2013 Conservation Agreement​

Rangewide Accomplishments

• 2022 Rangewide Accomplishments​​
• 2021 Rangewide Accomplishments​
• 2020 Rangewide Accomplishments
• 2019 Rangewide Accomplishments
• 2018 Rangewide Accomplishments​​
• 2017 Rangewide Accomplishments
• 2016 Rangewide Accomplishments
• 2015 Rangewide Accomplishments​
• 2014 Rangewide Accomplishments
• 2013 ​Rangewide Accomplishments
• 2012 Rangewide Accomplishments
• 2011 Rangewide Accomplishments
• 2010 Rangewide Accomplishments
• 2009 Rangewide Accomplishments
• 2008 Rangewide Accomplishments

Summary