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Horsetooth Reservoir

Horsetooth Reservoir

Horsetooth Reservoir

FISH SURVEY AND MANAGEMENT DATA Kyle Battige - Aquatic Biologist (Fort Collins) [email protected] / 970-472-4396

General Information: Horsetooth Reservoir (1900 acres) provides quality fishing for walleye, small- bass, and the occasional trout. Anglers may also catch splake, white bass, crappie, bluegill, and perch. The lake is open year round to fishing. For more information visit the Larimer County park website: https:// www.larimer.org/naturalresources/parks/horsetooth-reservoir Location: Larimer County. From I-25, go west on Harmony Road 7 miles to where it becomes CR 38E at Taft Hill Road. Continue west 4 more miles on CR 38E to park entrance. Recreational Management: Larimer County Fishery Management: Coldwater and coolwater angling

Amenities Previous Stocking Mercury Advisory

 4 Boat Ramps 2019 Some in Horsetooth have  Swim Beach No Stocking been found to have significant  Modern Restrooms levels of mercury. A consump-  RV hook-ups 2018 tion advisory is in effect. For  Tent Camping Rainbow Trout more information go to:  Camper Cabins https://www.colorado.gov/  Hiking Trails 2017 pacific/cdphe/wq-fish-  Marina No stocking consumption-horsetooth-  Picnic Areas reservoir  Ice Fishing 2016 Splake Sportfishing Notes Wiper Regulations Walleye

2015 Fish rock structures when water  Walleye regulation: 5 fish temps are 40-55°F. bag limit, no size re- Cutthroat Trout

striction Later, fish are scattered but tube 2014 Bluegill jigs, lindy rigs, triple ripples,  Minimum size for small- Rainbow Trout are good choices. mouth is 12” Cutthroat Trout White Bass  Statewide bag and posses- Watch for surface activity in sion limits apply for all 2013 Rainbow Trout the coves, small fish or birds on other species the surface, that indicate feed- Cutthroat Trout ing fish below.  Fishing prohibited in inlet 2012 March 15 - May 31 Smallmouth Bass Rainbow Trout Cuttbow Try rock rip-rap areas with  Entry requires a Larimer crayfish imitations. County Parks Pass: availa- ble on site 2011 Rainbow Trout

Horsetooth Reservoir

FISH SURVEY AND MANAGEMENT DATA Kyle Battige - Aquatic Biologist (Fort Collins) [email protected] / 970-472-4396

History

Construction of Horsetooth Reservior was completed in 1949. It is part of the Colorado-Big Thompson Project (CBT) trans- porting water from the west slope and storing it for east slope use. Water operations are conducted by the Bureau of Reclamation and Northern Colorado Water Conservancy District. Expect large water fluctuations in the reservoir from seasonal water use. Horsetooth has been managed for cool and coldwater fish assemblies since the 1950’s. Various proportions of rainbows, cut- throats, browns, lake trout, and splake have made up the coldwater assembly over time. Coolwater fish like walleye, smallmouth bass, white bass, and yellow perch have been stocked historically and now maintain self-sustaining populations. Horsetooth has had a very dynamic fishery. A large yellow perch population provided a great forage for walleye from the filling of the reservoir until the late 1960’s when the perch population crashed. As a result of the perch crash, walleye body condition and growth fell to unacceptable levels. So, rainbow smelt (pictured be- low) were introduced as an alternative forage. This led to a boom in walleye fishing in the 1980’s and 90’s. The smelt ended up eating themselves out of house and home, affecting the recruitment of wall- eye and other fish. The smelt population subsequently crashed and brought the whole reservoir ecosystem down with it. Smelt disap- peared in 1996 and were not seen again for over a decade Efforts in the last decade have focused on trying to re- establish a quality forage base without the potentially detrimental effects that smelt had on the fishery. Spottail shiners, emerald shin- ers, and shad have been successfully introduced and repre- sent a significant component of the fishery. Walleye maintain a self- sustaining population and their body condition and growth have con- tinued to be on the rise for the last few years. Smelt began to show up again in CPW sampling in 2010 and in the last ten years, the population has grown from undetectable to millions of smelt throughout the reservoir. This has created a very dynamic fishery that will see significant reservoir-wide changes for many years. We will be monitoring this and making changes when possible to promote optimal smallmouth and walleye fisheries.

Horsetooth Reservoir

FISH SURVEY AND MANAGEMENT DATA Kyle Battige - Aquatic Biologist (Fort Collins) [email protected] / 970-472-4396

Fishery Management

Trout Fishery: In 2012 a large number of trout were put into Horsetooth Reservoir from Watson Hatchery in response to adverse conditions caused by fires in the Cache la Poudre watershed. While there were no significant effects on fish in the river, the poor water quality coming into the hatchery with fish at high densities led to stressful conditions. To avoid a large kill, and the loss of these fish, they were evacuated to the nearest body of water that could take such high numbers. Approximately 72,750 trout weighing 23,100 pounds were moved in one day to Horsetooth Reservoir. Horsetooth is not annually stocked with rainbows because there is very poor re- turn to anglers. However, this emergency measure led to a large number of trout being caught from 2012 to 2018, but the rainbows have all but phased out of the system. Rainbow Trout are occasionally stocked in smaller numbers as fish are available. However, the walleye readily prey on these fish so they are not stocked on a consistent basis.

Rainbow Smelt: CPW sonar data from 2016 to 2019 suggests a dynamic and variable smelt population. Esti- mates in 2019 suggested roughly 6 million smelt in Horsetooth Reservoir. This number was down 37% from 2018, but 2018 numbers were an increase of 35% compared to 2017. It is un- clear whether these trends repre- sent natural variation in the pop- ulation or the start of a popula- tion decline, similar to the late 1990’s. Smelt impact the reser- voir and fishery in a number of ways. A large smelt population provides a readily available prey base for adult walleye; however, adult smelt can prey on juvenile walleye and also directly compete for zooplankton, in particular large-bodied Daphnia which are an important prey for juvenile walleye. Adult smelt up to 200mm (8 inches) have been sampled during vertical netting targeting smelt. Sonar results from 2020 will provide valuable insight into smelt population dynamics and hopefully provide some clarity into potential trends in this important prey species.

Other prey species: There are a number of other prey species present in Horsetooth Reservoir, including (pictured from right to left): bluegill, gizzard shad, rainbow smelt, smallmouth bass, and yellow perch. Having a diverse prey base helps maintain balance in the fishery. However, the smelt population is what is sustaining the current fishery in Horsetooth Reservoir. Horsetooth Reservoir

FISH SURVEY AND MANAGEMENT DATA Kyle Battige - Aquatic Biologist (Fort Collins) [email protected] / 970-472-4396

Fishery Management: Walleye

Body condition (relative weight) of wall- eye in Horsetooth increased from 2010 to 2018 as shown in the figure to the right. In- creasing body condition corresponded direct- ly with when CPW started seeing the resur- gence of smelt in Horsetooth. In 2019, wall- eye were in above average condition, but there was a slight drop from 2018 to 2019. Anglers should expect healthy fish in 2020 as smelt continue to persist in the reservoir and provide an abundant prey base. The figure below shows the number of walleye captured per net and average length from standardized gill netting data. CPW sets the same 20 gill nets in Horsetooth every year at the end of May to reduce environmental variables impacting catch rates. Catch rates have been decreasing since 2016 with fewer walleye captured. Catch rates for larger individuals (greater than 18 inches) has been increasing over the last few years as fish grow into those upper size classes. However, we are not cap- turing smaller individuals, in par- ticular 10-15 inch fish are not re- cruiting and replenishing those smaller sizes. As a result, average length has been increasing over the years as well. The lack of recruitment of smaller sized walleye is likely a combination of a number of fac- tors. It appears that there are still plenty of adult fish in the reservoir to sustain the popula- tion, but recruitment of young walleye is not happening as readily. This could be due to environmental conditions and water levels, as well as preda- tion and competition impacts from the smelt population on walleye recruitment. Horsetooth Reservoir has not been stocked with walleye since 1997, but it is being considered. Horsetooth Reservoir

FISH SURVEY AND MANAGEMENT DATA Kyle Battige - Aquatic Biologist (Fort Collins) [email protected] / 970-472-4396

Fishery Management: Smallmouth Bass

The average temperature in Horsetooth is colder than most comparable lakes in the area be- cause the water comes through the Colorado-Big Thompson Project pipeline from a very cold Grand Lake. Relatively slow growth rates makes Horsetooth a poor candidate for trophy smallmouth manage- ment. It takes several years for a smallmouth to reach harvestable size at the protected 12 inch mini- mum size limit. This limit allows sufficient age and growth for successful re- production. With this regulation, smallmouth bass currently do not have recruitment limi- tations in Horsetooth and the population is completely sustained through natural repro- duction. Length frequency data (figure above) shows a diverse size range of smallmouth bass, with fish up to 19 inches being sam- pled. The number of quality fish sampled continues to increase each year, as small- mouth have benefited from the diversified prey base with the resurgence of smelt to the system. The relative weight figure shows a condi- tion factor which is a relationship between an individual fish length and it’s weight compared to a standardized weight. Fish smaller than six inches are not included in the analysis. As you can see there is a range of values for relative weights with no direct relationship. There are small and large bass in good and poor condition. Overall, small- mouth bass are in average condition. The length vs weight figure to the right shows the relationship between length and weight. Bass grow relatively slow in - tooth, but quality bass are present in the reservoir. Several white bass and wiper were also handled during CPW sampling in 2019. White bass maintain rela- tively low population numbers through natural reproduction and are caught each year. In 2016, 5,000 one inch wiper were stocked in Horsetooth to provide a unique angling opportunity and several of those individuals were sampled in 2019, ranging from 16 to 18 inches.