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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