Dillon Reservoir

FISH SURVEY AND MANAGEMENT REPORT

Jon Ewert - Aquatic Biologist (Hot Sulphur Springs)

General Information: Dillon Reservoir, a 3,000- acre storage reservoir, is owned and operated by the Denver

Water Board. Located adjacent to I-70, this is one of the most easily accessible mountain reservoirs in the

state. It provides good fishing for rainbow and brown trout, kokanee salmon, and arctic char. Boats need to

pass an Aquatic Nuisance Species inspection prior to launch. To expedite this process, be sure your boat is

clean, drained and dry when you arrive. In the winter, no motorized vehicles are allowed on the ice.

Location: Summit County, CO.

Recreational Management: Recreational use is managed by a combination of Denver Water, Summit

County, the US Forest Service, and the towns of Dillon and Frisco. For more information see: http://

www.denverwater.org/recreation/dillon.html

Amenities and General Info.

Boat Ramps/Marinas (2)

Picnic Areas

Camping

Regulations Standard harvest regulations

for trout (4 fish daily bag/8

fish in possession) and ko-

kanee salmon (10 fish).

All arctic char less than 20”

in length must be immedi-

ately returned to the water.

Bag and possession limit for

arctic char is one fish over

20” in length.

Previous Stocking

Sportfishing Notes

Arctic char This rare species was stocked from

2008 through 2015. Fishing for this

species has greatly improved over

the past few years as the stocked

fish have established and success-

fully reproduced. Anglers have

been most successful during ice

fishing season, using various small

jigs in less than 60 feet of water.

Some summer catches have also

been reported.

Brown trout

Dillon is home to a high density of

brown trout, which are naturally

self-sustaining and not stocked.

Fish as large as nine pounds have

been captured in CPW netting sur-

veys. Crank baits and other large

lures fished around rocky points

and other structure at ice-out, dur-

ing summer evenings, and in the

fall can be productive.

Kokanee salmon Kokanee are also self-sustaining in

Dillon, and have not been stocked

since 1978. Spawning has been docu-

mented in the Snake River inlet area,

other locations in-lake, and in the Blue

River upstream of the lake.

2016

Species Size Number

Rainbow 10” 26,158

Rainbow 4” 302,445

2015

Species Size Number

Rainbow 10” 30,353

Rainbow 4” 357,967

Arctic char 2.9” 18,153

2014

Species Size Number

Rainbow 10” 19,912

Rainbow 4” 358,586

Arctic char 3.3” 32,770

2017

Species Size Number

Rainbow 10” 44,305

Rainbow 4” 332,140

Arctic Char ID Arctic char are similar in

appearance to lake trout and

brook trout, our other Colorado

char species. However, some of

the differences are that arctic

char have a relatively small

mouth (particularly compared to

lake trout), their body shape is

slender, they do not have the

“halos” around the spots on

their body that brook trout have,

and they have no markings on

the dorsal fin.

Dillon Reservoir Gillnet Survey Information Jon Ewert, Aquatic Biologist

Gillnet catch species composition by percent

Over the past decade we have conducted two types of

gillnet surveys at Dillon Reservoir. In even-numbered

years, we set six gillnets overnight in the same locations

and as close to the same date as possible (during the third

week of June) on each occasion. The locations of over-

night gillnet sets are shown at left. The main purpose of

this survey is to observe changes and trends in the overall

fish population of the lake. In 2010, 2015, and 2017, we

conducted randomized gillnet surveys to monitor the arctic

char population. These surveys are discussed later in this

report.

Arctic char were stocked annually from 2008 through

2015. The purpose of stocking this species in Dillon is that

the reservoir is notoriously unproductive in terms of prey

availability for sport fish. However, one of the few prey

items that is in good supply is mysis shrimp. Arctic char

have been found to be successful preying on mysis in cold,

deep lakes elsewhere that resemble Dillon .

Two char were caught in the gillnets in 2008 and 2010

and none were caught in 2012. 14 were caught in 2014 and

8 were caught in 2016. Perhaps more importantly than this

increase, the 2014 survey captured char in all six nets,

which are located in widespread locations in every part of

the lake. The char caught in 2014 ranged from 8 to 18

inches in length. This information, combined with consis-

tent reports of greater angling success, suggests that 2014

may have been a “turning point” year in the development

of the arctic char population in Dillon.

CSU graduate student Devin Olsen completed a re-

search project on the arctic char fishery in Dillon in 2014.

Among other things, his study proved that a significant

percentage of the char in Dillon are the product of wild

reproduction in the lake, and that they are indeed preying

upon the abundant mysis shrimp found in the reservoir.

One of the goals of stocking char in Dillon is to estab-

lish a self-sustaining population that does not need to be

augmented with regular stocking. Because we have now

observed natural reproduction taking place, in 2015 CPW

enacted a conservative harvest regulation on char in Dil-

lon, allowing a daily bag limit 1 fish over 20” to be har-

vested. All char less than 20” must be released. This regu-

lation will ensure that these fish will have 2-3 opportuni-

ties to spawn prior to being available to harvest.

Date of survey 6/24

2008

6/23

2010

6/21

2012

6/23

2014

6/21

2016

Arctic char 0% 0% 0% 4% 3%

Kokanee 1 2 1 8 2

Brown trout 20 11 13 17 22

Rainbow trout 10 8 10 4 7

Suckers 69 78 76 66 66

Overnight gillnet locations

The largest brown trout captured in the 2012 survey, 24” 9.3 lbs.

The largest brown trout captured in the 2014 survey, 23” 4.1 lbs.

A Dillon char in spawning colors. Photo by Devin Olsen.

Brown trout size structure The size structure of brown trout captured in the gillnet

surveys is shown at right. Prior to 2012, there appeared to

be a growth barrier in Dillon for brown trout at approxi-

mately 15”. Fish larger than that size were relatively rare.

This growth barrier seems to have been broken beginning

in 2012. In 2016, the drop in catch rates from 15” to 16”

reappeared, however the total number of large fish we cap-

tured was still high compared to 2008 and 2010. Numbers

of fish captured that were larger than 16” are displayed on

the graphs.

One of the reasons for this improvement in the brown

trout fishery is the resumption of rainbow fingerling stock-

ing in 2011. Prior to 2011, no subcatchable rainbow trout

had been stocked here since 2002, and the stocking history

for the past three decades was sporadic. The 2011-2017

period has seen the most consistent annual stocking of

rainbow trout fingerlings in Dillon since the 1970’s.

There are a few reasons we resumed stocking fingerling

rainbows. First, CPW has different strains of rainbows than

those that were being stocked in the 1970’s. It is possible

that one of our recently developed strains may be more

successful in Dillon. Also, due to budget cuts it has be-

come increasingly difficult to maintain a rainbow trout

fishery with catchable stocking alone. The number of

catchables it would take to sufficiently stock Dillon is lar-

ger than what is available. We also wanted to provide

“cover” for the arctic char while they were being stocked.

Given the density of the brown trout population in the res-

ervoir, if a relatively small number of arctic char were the

only small fish to be introduced to the reservoir in many

years, their chances of survival would be relatively poor.

Trends in brown trout body condition (or “plumpness”)

by size are below. In 2008 and 2010, there was no positive

relationship between fish length and body condition. In

2008, the fish lost body condition as they got larger. This

is an indication that there is not sufficient prey available

for the fish to switch to as they become larger. However,

this relationship changed drastically from 2012 onward,

when we saw a strong positive relationship between fish

length and body condition. This suggests that prey avail-

ability now becomes better as the fish grow in size. Large

fish in good condition will continue to grow to trophy

sizes, and can now attain a size large enough to exploit the

prolific sucker population as prey.

# > 16”: 15

# > 16”: 7

# > 16”: 19

# > 16”: 13

# > 16”: 7

Arctic char use different habitat than other trout, so a

different survey approach is necessary in order to monitor

the status of this species in the reservoir. In mid-June 2010,

2015, and 2017, we conducted a survey consisting of gill-

nets set for six hours apiece in random locations (left). We

set 16 nets in 2010 and increased the number to 24 nets

over three days in 2015 and 2017.

Catch rates from these surveys are at left. The 2010 sur-

vey produced extremely low catch rates, suggesting that

the char stocked since 2008 had not yet grown to sizes vul-

nerable to capture in our gillnets. Catch rates increased in

2015 and 2017. In 2017, the catch rate increased by 12%

over the 2015 rate, but this was probably not a significant

increase considering the overlapping confidence intervals.

The size distribution of char caught in these surveys is

shown below left. Fish caught in 2015 ranged somewhat

evenly across sizes from 5” to 15”. In 2017, the fish we

captured grouped into two size groups, one averaging 8”

and the other averaging 12”. No fish larger than 15” were

caught in either survey. However, we know that larger fish

are present, due to increasing reports from anglers and a

new state record that was caught by Lindsay Regali on No-

vember 6, 2017. The fish, pictured below, measured 23.4”

and weighed 4.15 pounds.

Because the purchase of arctic char eggs was relatively

expensive, 2015 was the last year that the species was

stocked in Dillon. We hope that the consistent stocking

that the lake received from 2008-2015 will be enough for

the population to sustain itself through natural reproduction

over time.

We plan to continue to conduct this survey in odd-

numbered years.

State record arctic char caught in 2017. Photo by Randy Ford.

Randomized gillnet locations

Size distribution of arctic char, 2015 and 2017 surveys

A large brown captured in the 2015 survey. 27”, 6.9 lbs.

Arctic char stocked in Dillon Reservoir 2008-2015

Year Number Avg. size Year Number Avg. size

2008 20,478 3.5” 2012 16,683 4.2

2009 17,237 3.6 2013 21,321 4.1

2010 24,932 3.5 2014 32,770 3.3

2011 16,857 3.5 2015 18,153 2.9