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 pro- vide recreational opportunity, a prey base to produce tro- phy 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 suck- er 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 shore- line 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 loca- tions 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 sur- vey 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. enough harvest pressure to drive down the lake trout pop- ulation at a slow or moderate pace. However, other factors 8 may be having a stronger effect than harvest on this de- 7.6 7.6 cline, such as declining overall productivity of the reser- 7 7.1 voir or water level fluctuations impacting spawn success. 6.4 It is also worth noting that a single year of high catch rates 6.3 6.3 6.3 y = -0.23x + 6.42, P = 0.07 6 would have the potential to change our perspective on this 5.7 5.5 5.5 trend entirely. 5.3 5.4 5.1 5 5.1 5 4.9 4.8 4.7 4.4 4.3 4.3

Avg. Lake Trout Catch Net Per Trout Lake Avg. 4 4 4

3.5

3 2011 2012 2013 2014 2015 2016 2017 2018 Figure 7. Average lake trout catch per 6-hour net set, 2011- 2018. N is same as above. Least-squares trend line also dis- played with equation. Error bars represent 80% confidence in- tervals.

Figure 5. A 2018 lake trout in excellent body condition.

2011 2012 2013 2014 2015 2016 2017 2018 5/25, 27, 5/28, 29, 5/18, 20, 5/19, 23, 5/22, 23, 5/21,22, 23, Dates 5/21—24 5/27—30 31, 6/1 6/3, 4 22, 26 24, 25 24, 26 29, 30 Number of nets 30 30 32 32 32 32 32 32 Avg. Lake elevation 8253 8263 8237 8254 8274 8262 8264 8271 Average surface temp 43.3 50.1 50.2 50.4 46.9 45.5 47.7 51.6 Total lake trout caught 190 164 199 152 201 162 136 150 Lake trout >24” caught 11 (6%) 10 (6%) 33 (17%) 12(8%) 5(2%) 10(6%) 5(4%) 13(9%) Range of catch per net 0-19 0-16 0-24 0-12 0-13 0-11 0-11 0-11 Avg. lake trout size 16.2 16.3 17.7 16.4 16.4 17.3 15.9 17.1 Avg. condition for<24” 83.3 79.9 81.8 83.5 84.7 87.2 85.7 86.3 Avg. condition for >24” 79.6 80.8 73.5 79.4 81.8 78.1 81.3 79.9 Table 2. Various parameters of the standard gillnet survey. The 8 new shoreline nets from 2018 are not included. 3

When considering the results of this type of index- In order to test the hypothesis stated above regarding based survey, it’s important to assess whether or not other the effect of habitat associations on catch rates, we exam- variables are affecting catch rates in a way that has the ined the relationship between net location and catch rates potential to lead to erroneous conclusions about the popu- over time (Figure 8). We found that net location account- lation trend. We tested for correlations between lake trout ed for 27% of the variability in catch rate, and the rela- catch and lake elevation, lake volume, and water tempera- tionship was highly significant. In other words, locations ture (Table 3). We found a significant relationship be- that have historically produced higher catch rates are more tween our catches of large (>24”) lake trout and lake sur- likely to continue to produce high catch rates, and vise- face elevation and lake volume. The correlation with ele- versa. This relationship is presumably a manifestation of vation was slightly stronger. Most reservoirs in the area habitat in the immediate area of each net location. fill or come close to filling in normal water years, but Granby undergoes multi-year high– and low-water cycles based on water availability and demand. Therefore, the elevation and volume of the reservoir has varied widely during our surveys even though the dates correspond closely from year to year (Table 2). While this does not appear to have an effect on our overall average catch rates, it does affect catch rates of large fish. This makes sense when considering a scenario in which smaller fish are more likely to stay in close association with certain habitat features, while large predatory fish likely have to cover more area in order to make a living. While the reservoir is drawn down, it would follow that the rate of encounter with large fish would increase due to the smaller volume of the lake. If this pattern holds, in future years we will consider using a correction factor to account for differ- ences in lake elevation or volume when assessing trend in the density of lake trout >24” in the lake.

All lake trout avg. >24” average Figure 9. The largest lake trout from 2014. 39.7” long, 27.8 lbs.

R2 = 0.12 R2 = 0.70 Lake elevation P = 0.39 P = 0.01 R2 = 0.10 R2 = 0.66 Lake volume P = 0.44 P = 0.01 R2 = 0.15 R2 = 0.12 Water temperature P = 0.34 P = 0.39 Table 3. Regression analysis to test the relationships between lake trout catch rates and physical conditions of the reservoir. Values in bold indicate significant correlations.

25

20 y = 0.24x + 1.69 R² = 0.27, P<0.00 15

10

5 # Lake Trout Caught in Individual Nets Trout #Lake 0 0 5 10 15 20 25 30 Rank of net Figure 8. Relationship between net site and catch. Locations that had been netted at least 7 times were ranked based on average catch rate from lowest to highest. Figure 10. This lake trout in 2017 had recently eaten a kokanee.

4

Lake trout size structure move into the intermediate size range, filling in the gap 18 that has occurred in recent years.

16 Figure 12 presents a possible explanation for the gap in 2011-2015 sizes that we are currently observing. Because lake trout 14 N=911 are long-lived and slow-growing, it is difficult to parse 12 2016 individual age-groups from length data such as this. How- N=161 10 ever, from past aging work we do know that it may take approximately 3 years for Granby lake trout to reach 10” 8 in length. The only significant drought and drawdown of 6 Granby that has occurred during the history of these sur- 4 veys was in fall of 2012. It is possible that the drawdown

2 of 21 feet that fall and winter was enough to kill some or all of the lake trout eggs that had been laid that year, creat- 0 18 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 ing this gap in the size distribution.

16 2010 2011 2012 2013 2014 2015 2016 2017 2012-2016 14 N=882 12 2017 -5 N=136 10 -6 -8 -8 8 -10 -10

6 -15 -14 -14

4 Percent of lake captured of lake trout Percent

2

-21 Change in Lake Elevation in Feet ElevationLake in Change 0 -25 18 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 Figure 12. Change in Lake Granby surface elevation from Octo- 16 ber 1 of listed year to March 1 of following year. This is the 2013-2017 14 n=849 portion of the year in which lake trout spawning (October), egg 12 2018 incubation and hatching occur. N=158 10

8

6

4

2

0 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 Length of fish in inches Figure 11. Lake trout size structure. In each graph, all lake trout sampled in the previous five years (black bars) are pooled to represent the “normal” or “expected” size distribution, and the current year’s catch is overlaid in order to assess differences or trends in size distribution. Throughout the history of the survey, the most com- mon size of lake trout that we have captured is 15”. How- Figure 13. A 2018 lake trout in moderate body condition. ever, simply tracking average (Table 2) or median sizes 1400 over the years can miss key information regarding the size 1365 distribution of Lake Granby lake trout. Figure 11 exam- 1200 1186 ines size distribution in more detail. Each year's catch is 1000 946 overlaid against the background of the previous five 892 years’ catch. In 2016, the size range of 10-15” was un- 800 678 derrepresented, while there was an abundance of 16-19” 630 fish. In 2017, the gap in smaller-sized fish had advanced 600 554 to the 13-17” size range, while 18-21” were especially 462 400 422 abundant. These trends continued in 2018, with the gap 373 Mysis per square meter Mysissquare per 314 280 295 now appearing at 14-18”, and high catch rates of 19-24” 200 30 fish. The past two years have also seen sizeable groups of 0 smaller fish, measuring 10-11” in 2017 and 11-13” in 2018. This larger group of juvenile fish appears poised to Figure 14. Annual mysis density estimates. 5

Lake trout body condition Status of kokanee 110 Lake Granby is home to Colorado’s longest-running N = 1,188 kokanee egg harvesting operation. Kokanee were first in- y = -0.55x + 92.28 100 troduced to Granby in 1951. There is not enough success- ful natural reproduction of kokanee to sustain the popula- tion, so eggs are manually harvested and raised in our 90 hatchery system. The following spring, 1 million kokanee fry are stocked into the Colorado River above the lake. 80 Historically, Granby produced enough kokanee eggs to not Relative Weight Relative only restock Granby itself, but also to provide kokanee to 70 be stocked in many other reservoirs throughout the state. Due to unfavorable conditions in the lake in recent years kokanee numbers have dwindled to the point where Gran- 60 6 11 16 21 26 31 36 41 by has failed to provide enough eggs to support itself. Length of fish in inches CPW research crews conduct scientific sonar surveys Figure 15. Lake trout body condition by size, 2011-2017 every year on Granby to estimate the number of kokanee pooled. in the lake. This has provided a strong predictor of the 110 number of fish in the kokanee spawning run the following N = 155 year (Figure 17). The downward trend in egg take from y = -0.79x + 99.46 2008-2014 was predicted by a general downward trend in 100 sonar estimates from 2007-2013. The sonar survey was not conducted in 2014 due to budgetary issues. We are 90 hopeful that kokanee numbers are rebounding, with higher sonar estimates in 2015 and 2017, and an increased egg 80 take in 2016. The low egg take in 2017 was not entirely

Relative Weight Relative attributable to poor kokanee numbers. Blue Mesa Reser- voir near Gunnison produced enough kokanee to supply 70 the entire state, and therefore there was little demand for Granby eggs in 2017. The 2018 run was a disappointment. 60 The poor egg takes of recent years are a serious concern, 6.0 11.0 16.0 21.0 26.0 31.0 36.0 41.0 Length of fish in inches because Granby cannot be counted on as a reliable egg Figure 16. Lake trout body condition by size, 2018. source in the event that other egg sources decline. Research crews also monitor the density of mysis Lake trout body condition by size is displayed above. shrimp in the reservoir (Figure 14, previous page). Be- All fish weighed in 2011-2017 are pooled in Figure 14, cause mysis eat the same zooplankton that kokanee do, while 2018 is presented alone in Figure 15. they are a highly efficient competitor and can create diffi- 24” is the size at which lake trout typically switch to cult conditions for kokanee growth and survival. Mysis vertebrate prey in order to continue growing. In many densities in Granby have proven to be highly variable over ways, lake trout populations actually behave as if they’re the years. They follow the general pattern of declines dur- two separate populations—smaller than 24” and larger. In ing drought periods followed by population explosions to Granby, fish larger than 24” have consistently had poorer extremely high densities when the reservoir refills. These body condition than smaller fish (see also Table 2). Lake high densities do not appear to be sustainable over time, trout smaller than 24” prey heavily on the dense mysis and so the population seems to undergo a boom-and-bust population. Larger fish continue to consume mysis, but oscillation. they do not appear to provide enough nutrition for the fish to grow to trophy size. Kokanee salmon are the most valu- able prey base to accomplish this, and the kokanee popu- lation in Granby has struggled in recent history. In order 3,992,255 for Granby to continue producing trophy-sized lake trout, it is critical for the kokanee population to recover. 3,211,462 2,839,197

2,224,809

2,047,427 1,721,407

1,900,336 Egg Take Egg 1,273,776

829,663 947,399 72,276 285,477 454,652 105,248 216,630 487,951 357,425 106,862 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Figure 18. Kokanee egg take at Granby, 2001-2018.

Figure 17. Kokanee abundance estimates from sonar surveys 6