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Green Mountain Reservoir
FISH SURVEY AND MANAGEMENT REPORT
Jon Ewert - Aquatic Biologist (Hot Sulphur Springs)
General Information: Green Mountain, a 2,125- acre storage reservoir, is operated by the U.S. Bureau of Reclamation
as part of the Colorado-Big Thompson project. It provides good fishing for lake trout, rainbow, cutbow, brown trout,
and kokanee salmon. Boats need to pass an Aquatic Nuisance Species inspection prior to launch. To expedite this proc-
ess, be sure your boat is clean, drained and dry when you arrive.
Location: Approximately 13 miles south of Kremmling, CO. Colorado Highway 9 runs along the east side of the reser-
voir.
Recreational Management: U.S. Forest Service, Dillon Ranger District. Forest Service user fees do apply at Green
Mountain. For more information, visit: http://www.dillonrangerdistrict.com/index.htm
Check with Heeney Marina at 970-724-9441 or Master Bait and Tackle in Heeney at 970-724-3677 for current informa-
tion.
Continuing in 2018: $20 NORTHERN PIKE HARVEST INCENTIVE. If you catch a northern pike, bring it to
Heeney Marina and receive $20. Program is only in effect during operating season of Heeney Marina.
Amenities and General Info.
Boat Ramps (1)
Picnic Areas
Camping
Restrooms
Marina
Regulations
Standard harvest regulations
for trout (4 fish daily bag/8
fish in possession) and ko-
kanee salmon (10 fish).
The bag an possession limit
for lake trout is 8 fish. This is
in addition to the normal 4-
fish bag limit for other trout
species.
Snagging of kokanee salmon
is permitted September 1
through December 31 in the
reservoir and in the Blue
River from the point it enters
the reservoir going upstream
to the Highway 9 bridge
crossing.
Previous Stocking
Sportfishing Notes Lake trout
Green Mountain is currently home
to a large number of small lake
trout, as well as a healthy popula-
tion of fish larger than 24”. The
small lake trout (12-16”) are over-
abundant and in relatively poor
condition. These fish need to be
thinned in order to restore a better
balance of populations in the reser-
voir. To encourage this, the bag
limit on lake trout was increased in
2011 to 8 fish, which is in addition
to the normal daily bag of 4 trout.
Rainbow trout
Rainbow trout have not been
stocked since 2015 for reasons dis-
cussed on the following pages.
Kokanee salmon
Kokanee have not been stocked
since 2014 for reasons discussed
on the following pages. Brown trout
Despite the lack of stocking of
other species, Green Mountain
continues to have a productive
brown trout population that is sus-
tained naturally without stocking.
Hot spots include the Blue River,
Black Creek, Cataract Creek and
Elliott Canal inlets.
2013
Species Size Number
Rainbow 10” 37,307
Rainbow 4” 50,000
Snake River cutthroat 6” 20,000
Kokanee 1.5” 100,000
2014
Species Size Number
Rainbow 10” 20,000
Rainbow 4” 100,000
Snake River cutthroat 6” 20,000
Kokanee 1.5” 100,000
2015
Species Size Number
Rainbow 10” 17,520
Rainbow 3” 100,000
Green Mountain Reservoir Fish survey information
Jon Ewert, Aquatic Biologist
Gillnets were set for six hours apiece in 40 randomly selected loca-
tions (above) on May 30, 31, and June 1, 5, & 6, 2017 to assess fish popu-
lations. This was a repeat of the survey that we conducted in 2015. The
lake was not netted in 2016. Depth of net sets ranged from 0 (shoreline)
to 118 feet, with an average of 41 feet. Average surface water tempera-
ture at netting locations was 54.6 F.
The composition of fish species by percent is shown Table 1 (left). In
2012, a northern pike was captured in gillnet sampling for the first time at
Green Mountain Reservoir. We captured individual pike in 2013 and
2014 also. In 2015, we captured 17 northern pike, representing 5% of the
total catch. This species was likely illegally introduced to the reservoir. If a population of pike proliferates and persists at
Green Mountain, it will drastically alter the stocking regime and management of the fishery there. Illegal introductions
of fish are a major criminal offense and we are seeking any information regarding individuals responsible for this act.
Anonymous calls can be made to Operation Game Thief at 1-877-265-6648. CPW has offered a $20 northern pike har-
vest incentive at Green Mountain in 2016 and 2017. Any pike caught in the lake can be turned in at Heeney Marina and
the angler will receive $20 per fish. The program is in place only during the months when Heeney Marina is open for
business.
For the first time in the history of the reservoir, CPW ceased all stocking of fish into the lake in 2016 and 2017. There
are two main reasons for this. The first is that there has been a severe outbreak of the gill lice parasite in both kokanee
salmon and rainbow trout in the lake, and the only known way to attempt to control the density of this parasite is to re-
duce the density of susceptible hosts — in this case, kokanee and rainbow trout. The other reason is the appearance of
northern pike in the reservoir. Continued stocking of trout and kokanee salmon would simply fuel the expansion of the
species by providing them with a plentiful prey base. There are no plans to stock any fish in 2018, and future stocking
plans will be based on whether or not we are successful in controlling these two biological challenges at Green Moun-
tain.
One other potential benefit of the lack of stocking in recent years is that large predatory lake trout are forced to prey
on the prolific sucker population in Green Mountain. The decrease in sucker composition in the gillnet catch displayed in
Table 1 may be the first indication of increased predation by lake trout.
2012 2013 2014 2015 2017
Northern Pike 1% 2% 1% 5% 2%
Rainbow trout 3 5 3 3 2
Brown trout 7 4 6 5 13
Lake trout 25 33 21 26 36
Suckers 65 57 70 62 48
Table 1. Gillnet catch percent composition
Figure 1. Locations of gillnet sets on Green Mountain Reservoir
The purpose for the netting survey is to monitor any de-
tectable changes in species composition (see Table 1 previ-
ous page), lake trout abundance and other population pa-
rameters following various management changes over time
in the reservoir. Prior to 2015, the survey consisted of 24 net
sets annually. Beginning in 2015, we added 16 more net sets
for a total of 40, and switched to a biannual schedule. This
change was made because increasing the number of sets to
40 increased the statistical power of the survey.
In 2011, CPW increased the bag limit for lake trout at
Green Mountain to 8 fish of any size, separate from the ag-
gregate trout bag limit of 4. The reason for the change was
that the population of kokanee salmon, the main prey base
for lake trout in Green Mountain, had crashed due to a com-
bination of factors, including a heavy infestation of the gill lice parasite as well as high lake trout predation. One of our
management goals at Green Mountain is to recover the kokanee population sufficiently so that it can again offer quality
recreational opportunity for kokanee as well as provide a prey base which consistently produces trophy lake trout. In or-
der to accomplish this goal, a reduction in lake trout density was sought.
Aside from a single-year increase in lake trout catch in 2013 (Table 2 and Figure 2, above), these surveys have not
detected any trend in the density of lake trout in the lake since 2011. However, the 2015 and 2017 surveys yielded the
highest catch rates to date of lake trout greater than 24”. This is the size at which lake trout need to switch to mostly ver-
tebrate prey in order to continue growing to trophy size. There are three possible reasons for this increase in catch rates:
1.) There were more large lake trout in Green Mountain in these years; 2.)The 16 new net locations that were added hap-
pen to be in locations frequented by large lake trout and thus caught them at a higher rate; or 3.) Due to the cessation of
stocking, large lake trout have to move more frequently over longer distances to find prey in the lake, making them more
susceptible to capture in our nets.
Conditions in the lake (decline in prey base, liberalized regulation with no size protection) do not provide any reason
to believe that the overall number of large lake trout should have increased. In fact, if the increased bag limit was having
its intended effect, this is the opposite result of what would be expected. The second possibility from the list above is
easily tested by examining the catch rates of large lake trout in only the 24 previously established net locations, and ex-
cluding the 16 new ones. In 2017, 16 lake trout >24” were captured in the original 24 net locations, while 11 were in
2015. These are still the highest catch rates of large lake trout with the new net locations excluded, ruling out this possi-
bility. The third possibility listed above is the most likely, and is also supported by body condition data, discussed on the
following page.
2011 2012 2013 2014 2015 2017
Dates 6/17, 20, 21 5/29, 30, 31 6/6, 10, 11 6/9, 10, 11 6/1, 2, 3, 4 5/30, 31, 6/1, 5, 6
Number of nets 24 24 24 24 40 40
Lake elevation 7930 7922 7926 7935 7930 7917
Average surface temp 53.4 52.5 54.9 52.0 51.5 54.6
Total lake trout caught 41 49 79 42 86 93
Average lake trout catch per net 1.7 2.0 3.3 1.8 2.2 2.3
Range of lake trout catch per net 0-9 0-7 0-18 0-7 0-14 0-12
Average lake trout size 15.7 16.7 16.2 16.5 19.8 18.1
Lake trout >24” caught 4 (10%) 8 (16%) 7 (9%) 6 (14%) 24 (28%) 22 (24%)
Average body condition for <24” 73.2 75.3 80.7 74.1 72.1 77.5
Average body condition for >24” 99.2 88.7 102.4 90.4 95.7 78.3
Table 2. Gillnet survey information
Figure 2. Average lake trout catch per net with 80%confidence
intervals
Figure 5. The largest lake trout captured in 2013, measuring 38”
and 24.5 lbs. Relative weight = 87.1
Figure 3. Relationship of lake trout size and body condition,
2011-2015 combined
Figure 4. Relationship of lake trout size and body condition,
2017
Figure 3 (right) shows body condition (relative weight -
a measure of the fish’s plumpness on a scale of 100; a fish
with a relative weight of 100 is considered to be in excel-
lent condition) by size of lake trout for all fish captured and
weighed in the 2011-2015 surveys (287 fish). This informa-
tion if helpful when we consider the predator-prey balance
in the lake and how much food is available to fish of vari-
ous sizes. This plot shows a pattern that we have consis-
tently seen at Green Mountain and did not differ from 2011
-2015: relatively poor body condition of smaller (less than
24”) fish, followed by a significant jump in body condition
in fish larger than 24” (see Table 2 previous page also).
This difference in body condition is highly significant (P =
<0.01) in all of these years.
Figure 4 (below right) displays the same information for
lake trout captured and weighed in the 2017 survey. Fish
>24” in 2017 were no longer in better body condition than
small fish. For the first time in the history of these surveys,
there was no significant difference in body condition be-
tween small fish and large fish. Also, the average body con-
dition in large fish in 2017 declined by 16.9 points from the
previous years, a highly significant drop. This is the most
biologically significant finding of the 2017 survey.
Figure 6. The largest lake trout captured in 2017, measuring 37”
and 17.4 lbs. Relative weight = 74.8
Fish
length 2011-2015 2017 change
30” 10.2 lbs. 8.8 lbs. -1.4 lbs.
36” 19.1 15.9 -3.2
40” 27.5 22.3 -5.2
Table 3. Typical weights for large lake trout in Green Mountain
Reservoir based on the observed length-weight relationships in
2011-2015 (pooled) and 2017 alone.
Figure 5 (right) displays the size
distribution of all lake trout captured
since 2011 by percent, with 2011-
2015 samples pooled together and
2017 displayed alone. The purpose of
viewing the information this way is to
determine if the size distribution of
lake trout in 2017 differed from previ-
ous samples, and if so by how much.
In 2017 we captured fewer 14– and 15
-inch lake trout than in past years, but
more 7, 8, 16, and 28—38-inch fish.
Figures 6 and 7 (below) are photos
of a large lake trout that had recently
eaten a white sucker. This provides
anecdotal evidence that the lake trout
are indeed preying on the suckers in
the lake, but the body condition infor-
mation previously discussed supports
the observation that suckers provide a poorer quality prey
base than other species such as kokanee. It is also known
that white suckers contain fewer calories than kokanee or
rainbow trout.
Northern pike harvest incentive program
In order to enlist the help of anglers in controlling the
proliferation of northern pike, in 2016 and 2017 CPW of-
fered a $20 per fish harvest incentive program. The pro-
gram was administered through Heeney Marina. In 2016
anglers turned in 49 pike, and in 2017 they turned in 65.
The program will continue in 2018, and probably beyond as
budget allows.
Figure 5. Size distribution of lake trout captured in 2011-2015 (pooled) and 2017 alone.
Figure 6. This 36” lake trout had recently eaten a 15” sucker. Figure 7. Another view of same fish in Figure 6.