The Angler’s Crystal Ball

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Josh Nehring shows a catfish from a recent survey. All photos by © Wayne Lewis/CPW.

Have you ever wished that you could see beneath the surface of a new fishing spot – just to get a quick glimpse – a small clue – of the variety and size of fish? Often, what lies beneath the surface of Colorado’s fishable waters would shock the average angler and, at times, even shocks CPW biologists. Colorado Parks and Wildlife (CPW) is the lead agency responsible for fisheries management of public waters in the state of Colorado. And while fishing pressure, weather changes and a number of other factors can impact fishing locations from year to year, CPW aquatic biologists spend a great deal of time in the field making sure that they have their fingers on the pulse of the underwater world.

Josh Nehring, a Senior Aquatic Biologist with Colorado Parks and Wildlife, explains the process and importance of the Fishery Survey, the primary tool that guides fish management. “The surveys that CPW aquatic biologists perform provide a lot of useful information to assist in making management decisions. During each survey, every fish is identified to species, weighed and measured. This data is then analyzed and compared to previous sampling efforts to look for different trends. Aquatic biologists can look at the predator to prey ratios, relative weights (which is an indicator of fish health), species composition, and year-class strength among others. This information along with creel surveys help biologists determine stocking rates, potential regulation changes or needs for habitat improvement.”

During a recent survey at two lakes in Lathrop state parkHorseshoe Reservoir and Martin Lake – Nehring and his team set a series of gillnets to sample fish populations. To minimize mortality, gillnets are set in the late evening and pulled early the following morning. As a survey method, gillnets efficiently target adult walleye, wiper, lake trout, rainbow trout, suckers, common carp and gizzard shad. The number of gillnet sets during a survey is dependent on the particular lake acreage.

Gillnets consist of panels of net held vertically in the water, entangling fish that attempt to swim through the net. The mesh size of the net determines the size range of fish caught. The standard gillnet is 6 feet tall by 150 feet long, with 6 different mesh sizes to gauge all sizes of fish in particular water. Because fish inhabiting colder waters grow at slower rates (e.g. Dillon Reservoir), gillnets deployed at higher altitudes have slightly smaller mesh compared with the mesh sizes used in warmer lakes (e.g. Pueblo Reservoir).

Cold, Cool, Warmwater and Illegal Fish

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Gizzard Shad

CPW stocks a variety of cold, cool and warmwater fish in Horseshoe and Martin. Horseshoe receives trout during the cooler months, and black crappie, channel catfish, bluegill and tiger muskie throughout the year. Several years ago, biologists collected several northern pike in Horseshoe due to an illicit stocking. Anglers are encouraged to remove any northern pike they may catch from Horseshoe Reservoir. However, Horseshoe reservoir also contains tiger muskie, which look very similar to northern pike.  We encourage anglers to do their research on how to identify the two species and tell them apart. Martin receives seasonal trout, walleye, saugeye, wiper, channel catfish and black crappie. The aquatic biologist for that area, Carrie Tucker, also moves forage fish such as gizzard shad into the lake on occasion to bolster the forage base.

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Northern pike caught during a recent survey.


Mt. Shavano HatcheryCPW Hatcheries – Visitors are welcome! CPW manages 18 different fish hatcheries that raise around 35 species of coldwater and warmwater fish. All hatcheries are open to the public 7 days a week, 365 days a year (including holidays). To learn more about fish hatcheries, please visit the CPW website.


Survey Summaries Created with the Angler in Mind

“Our goal with the fishery survey summaries is to inform anglers about some of our more popular destinations,” says Nehring. “These reports give them information about the species we catch during our surveys, but also information about what the biologist is seeing and what goals they have for those waters. The surveys educate anglers on not only the type of fish they might come across but the overall health of the fishery. Aquatic Biologists statewide will be making a big effort to update all of the survey summaries over the next 1-2 years as part of our strategic plan.”

The fisheries section of CPW conducts hundreds of lake and stream surveys each year. High priority and brood waters, such as Chatfield, Pueblo, Horsetooth and the Gunnison River, are surveyed annually. Smaller, more remote, or lightly used lakes or streams may only be surveyed once every 5-10 years. Most of the survey fieldwork takes place from early June through late September; however, many areas also conduct more specialized sampling beginning right after ice-out and again in the fall prior to freeze-up.


CPW Fishing AppTech Tip: CPW Fishing App
CPW Fishing lets you discover and search over 1,300 fishing destinations across Colorado. View a location’s popularity, ease of access, which fish species are present, stream gauges and more. Record your days on the water and journal your catches. Use the fish identification guide to learn about Colorado’s many fish species. Download CPW Fishing today!


Conservation Supported by Hunters & Anglers!

Did you know that sportsmen pay for all fishery management in Colorado? Because CPW does not receive general tax dollars to fund its fish hatcheries and conservation programs, these efforts are paid for, almost exclusively, by hunters and anglers through Habitat Stamp purchases and license sales​. Proceeds from Great Outdoors Colorado (GOCO) also help to support Colorado’s fishing resources.


Written by Doug Skinner. Skinner is an editor for Colorado Outdoors Online and is a media specialist for Colorado Parks and Wildlife. All photos by Wayne D. Lewis. Lewis is the editor and art director of Colorado Outdoors magazine.

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