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.