Lake Granby Fishery Management Report Jon Ewert, Aquatic Biologist, Colorado Parks and Wildlife February 2019

Lake Granby Fishery Management Report Jon Ewert, Aquatic Biologist, Colorado Parks and Wildlife February 2019 Introduction Lake Granby, approximately 7,250 surface acres when full, is one of the largest coldwater reservoirs in the state. It is the main storage reservoir in the west slope portion of the Colorado-Big Thompson Project which supplies water to the northern Front Range through the Adams Tunnel at Grand Lake. It is a focal point of the Grand County tour- ism economy and offers many amenities. Recreational access is managed by the U.S. Forest Service as part of the Arapaho National Recreation Area. The recreational fishery of Granby is dominated by lake trout (aka mackinaw) and hosts the highest density of the species that has been documented in Colorado. Kokan- ee salmon have been stocked in Granby since 1951 to provide recreational opportunity, a prey base to produce trophy lake trout, and spawning adults are captured annually to provide eggs for restocking. Rainbow trout of various sizes are also stocked and there is a moderate density of self-sustaining brown trout. Suckers and mottled sculpin Figure 1. Lake Granby are also present. Lake Granby also contains a dense population of mysis separate from the aggregate bag limit of other species. shrimp, which are an excellent prey source for smaller Due to reasons described above, the lake trout fishery in (<24”) lake trout. The high density of lake trout is a result Lake Granby is healthiest when a generous amount of har- of the availability of this prey base. However, the mysis vest is occurring. compete with Kokanee salmon and small trout for their The purpose of regulations (b) - (e) is to allow for a prey base of zooplankton, making the lake less productive kokanee snagging season and to increase the bag limit for for those species. Therefore, the dominant challenge to kokanee during that time. In recent years those regulations managing the recreational fishery at Granby is to maintain have had limited usefulness due to the poor kokanee popu- enough prey to produce quality- and trophy-sized lake lation (discussed below). The purpose of regulation (h) is trout while at the same time maintaining angling oppor- to protect spawning kokanee that run up the Colorado Riv- tunity for other species. er inlet toward Shadow Mountain dam, where we harvest eggs annually. Angling Regulations a. The bag and possession limit for lake trout is four Stocking fish. Catchable Fingerling Kokanee b. From January 1 through August 31, the bag and pos- Rainbow (10”) Rainbow (3-5”) session limit for trout (except lake trout) and kokanee salmon is four fish, singly or in aggregate. 2014 55,100 300,000 1,000,000 c. From September 1 through December 31, the bag and 2015 65,000 300,000 1,000,000 possession limit for trout (except lake trout) is four fish, singly or in aggregate. 2016 58,400 530,000 1,000,000 d. From September 1 through December 31, the bag and 2017 59,100 503,000 1,000,000 possession limit for kokanee salmon is 10 fish. e. Snagging of kokanee salmon is permitted in Lake 2018 58,400 506,500 1,100,000 Granby only from September 1 through December 31 except snagging is prohibited in Columbine Bay from Table 1. Five-year stocking history the inlet of Twin Creek upstream. Rainbow trout and kokanee are stocked annually f. Gaffs and tail snares are prohibited. (Table 1). Both catchable-sized and fingerling rainbows g. Ice fishing shelters must be portable. are stocked. Catchable rainbows are stocked off of the h. In Columbine Bay from the inlet of Twin Creek up- boat ramps several times throughout the season. Prior to stream, fishing is prohibited from October 1 through 2016, fingerling rainbows were also stocked from the boat December 31. ramps, but beginning in 2016, the stocking location moved to the Colorado River upstream of the reservoir. The purpose of regulation (a) above is to encourage The reason for this change is that there appeared to be harvest of lake trout, allowing for four to be harvested very little recruitment of fingerling rainbows stocked by 1 100% 2.9 the traditional method at the boat ramps. We hope that by 7.5 7.8 5.0 5.6 90% 16.2 18.7 stocking rainbows in the river above the reservoir, they 22.1 26.9 80% will have more time in the river to become acclimated to 28.2 their surroundings, work their way downstream into the 70% 14.1 lake at a slower rate, possibly avoid predation in the main 60% Rainbow trout body of the lake, and have a better chance at recruiting Brown trout 50% 95.7 into the recreational fishery. Because this is a relatively 90.9 92.2 92.6 40% 81.8 79.5 Suckers 75.1 recent change, we have not yet been able to assess wheth- 48.7 er or not this is successful. 30% 63.1 Lake trout Kokanee are stocked in late spring annually into the 20% river below Shadow Mountain Reservoir, typically during 10% 10.3 runoff stage. We stock a smaller number in the Dike 3 0% area of the lake in hopes that some spawning adults will 2011 2012 2013 2014 2015 2016 2017 2018 2018* return there, contributing to the recreational fishery in the Figure 3. Species composition in gillnet surveys. (2018*) de- fall. notes species composition in only the 8 new shoreline nets that we set in 2018, and separate from the 32 traditional sets repre- sented in the other 2018 column. and Shadow Mountain reservoirs for comparison), and the difference in species composition for the eight nets that we set on shore (the 2018* column) is striking. Regarding the variation in sucker catch, the majority of this variation is explained by catch rates in a single net (R20, near the Stillwater Creek inlet) which has varied widely in its sucker catch between 4 and 30 fish. If this net were excluded, there would be far less variation in these percentages. The difference in species composition in the 8 shoreline nets in 2018 suggests that brown trout, rainbow trout, and suckers are confined to a narrow band of habitat close to the shorelines at the time these surveys are conducted. This is typical of these species and expected, but appears to be more pronounced in Granby. This is probably due to Figure 2. Gillnet locations. the high density of lake trout in the majority of the lake. Kokanee are notoriously difficult to capture in gillnets, Fish Population Gillnet Surveys and are incidentally captured in these surveys. We conduct In 2011, we adopted a gillnet survey strategy with the annual SONAR surveys (Figure 17, page 6) to monitor goal of producing a relatively thorough picture of lake trends in the kokanee population. trout population dynamics in Granby. We set nets for six hours apiece at randomly located sites throughout the lake (Figure 2). In 2011 and 2012 we netted 30 locations and beginning in 2013 we increased the number to 32. The survey has occurred annually around the third or fourth week in May. The reason for this timing is that the period after ice-off but before thermal stratification seems to be the part of the year in which lake trout are most evenly distributed throughout the lake. Also, when water temper- atures are cool and the lake is not stratified, incidental mortality from gillnet capture is lower. This approach will provide satisfactory statistical power to detect changes in the lake trout population over time. In 2018, we added 8 net sets along the shoreline, roughly evenly distributed around the lake, to attempt to monitor the success or fail- ure of fingerling rainbow trout stocking and to better ob- serve trends in the brown trout and sucker populations. Figure 3 contains species composition by percent of fish captured in each survey. For the traditional net locations through 2018, lake trout are the overwhelmingly Figure 4. The largest lake trout from the 2012 survey. dominant species in the catch. The variation in sucker 42”, 26.8 lbs. catch is unusual for this netting protocol (see Fishery Management Reports for Williams Fork, Green Mountain, 2 Figures 6 & 7 contain catch rate information for lake 25 trout, analyzed in two different ways. Figure 6 plots the catch for each individual net in each year without calculat- 20 ing any averages, with the trend line overlaid. Figure 6 displays the average lake trout catch per net each year, 15 with 80% confidence intervals and trend. The first method y = -0.22x + 457.35, P =.04 gives a result that is more statistically significant, but both methods are in agreement that lake trout catch has moder- 10 ately declined over the period from 2011-2018. This survey is carefully designed to yield an index of lake trout 5 density in the lake, meaning that if lake trout catch de- # Lake Trout Caught in Individual Nets Trout #Lake clines, it is because their density in the lake has declined. 0 These analyses provide evidence of a slow decline in the 2011 2012 2013 2014 2015 2016 2017 2018 lake trout population, and no evidence of an increase. Figure 6. Lake trout catch per 6-hour net set, 2011-2018. For From a management perspective, this is a positive sign 2011 and 2012, N=30; For 2013-18, N=32. Least-squares trend and suggests that the 4-fish bag limit for lake trout is ac- line also displayed with equation and P–value for significance complishing what is intended, which is to maintain of the slope.

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