Lake Granby

FISHERY MANAGEMENT REPORT

Jon Ewert - Aquatic Biologist (Hot Sulphur Springs)

General Information: Lake Granby, approximately 7,000 acres when full, is one of the largest coldwater res-

ervoirs in the state. It is a focal point of the Grand County tourism economy and offers many amenities.

Location: Approximately 6 miles northeast of Granby, Colorado, off US Highway 34.

Recreational Management: Lake Granby is the centerpiece of the Arapaho National Recreation Area, ad-

ministered by Arapaho National Forest. The ANRA borders Rocky Mountain National Park on its southwest

corner. The ANRA charges daily and/or annual use fees. For more information, call the US Forest Service of-

fice in Granby at 970-887-4100, or visit the ANRA official website here: ANRA

All boats must undergo an ANS inspection prior to launch. In order to expedite this process, boats

should arrive at the lake clean, drained and dry.

Amenities and General Info.

Boat Ramps (3)

Picnic Areas

Camping

Restrooms

Marinas (3)

Regulations

See CPW regulations here,

#196, “Lake Granby.”

Previous Stocking

Sportfishing Notes

Lake trout

Conditions for reproduction of lake

trout have been good in recent

years, producing large numbers of

small fish. In order to maintain the

predator-prey balance in Granby,

anglers are encouraged to harvest a

daily bag limit of four lake trout

(especially fish smaller than 19”)

as often as possible.

Kokanee salmon

Kokanee populations have experi-

enced a downward trend in recent

years. Anglers have found it diffi-

cult to locate the fish during the

summer months.

Other species

Granby also features good fishing

for rainbow and brown trout.

Shoreline areas with a large

amount of rocky structure such as

Arapaho Bay are generally the

most productive.

2017

Species Size Number

Rainbow 10” 59,124

Rainbow 3” 503,045

Kokanee 1.5” 1,037,731

2016

Species Size Number

Rainbow 10” 58,423

Rainbow 3” 530,334

Kokanee 1.5” 1,004,967

2015

Species Size Number

Rainbow 10” 65,000

Rainbow 4” 300,000

Kokanee 1.5” 1,037,731

Lake trout are the main draw for recreational angling at

Granby. The population there is self-sustaining and no

lake trout have been stocked since 1995. The only period

of consistent annual lake trout stocking at Granby was

during the years from 1973 through 1976. In 2011, we

adopted a gillnet survey strategy with the goal of produc-

ing a relatively thorough picture of lake trout population

dynamics in Granby. We set nets for six hours apiece at

randomly located sites throughout the lake (see map at

right). In 2011 and 2012 we netted 30 locations and begin-

ning in 2013 we increased the number to 32. The survey

has occurred 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 temperatures are

cool and the lake is not stratified, incidental mortality from

gillnet capture is lower. This approach will provide satis-

factory statistical power to detect changes in the lake trout

population over time.

Information from the survey efforts is below. The sta-

tistic that will indicate population changes is the average

catch per net. One challenge at Granby is that the reservoir

volume responds to dry and wet weather cycles over mul-

tiple-year periods. It does not follow a “normal” annual fill

-and-spill pattern in the way that most Colorado reservoirs

do. The lake level during our survey has varied over a

range of 26 feet. The differences in volume among years

does not appear to be correlated with lake trout catch rates.

The highest water year, 2015, did not produce the lowest

catch rates, and the lowest-water year, 2013, did not pro-

duce the highest catch rates. The low reservoir volume in

2013, however, may have played a role in the high catch

rate of large fish that year.

Granby Fishery Survey Information Jon Ewert, Aquatic Biologist

The largest lake trout from the 2012 survey. 42”, 26.8 lbs.

2011 2012 2013 2014 2015 2016 2017

Dates 5/25, 27, 31, 6/1 5/21—24 5/28, 29,

6/3, 4 5/27—30

5/18, 20,

22, 26

5/19, 23,

24, 25 5/22, 23, 24, 26

Number of nets 30 30 32 32 32 32 32

Lake elevation 8253 8263 8237 8254 8274 8262 8264

Average surface temp 43.3 50.1 50.2 50.4 46.9 45.5 47.7

Total lake trout caught 190 164 199 152 201 162 136

Lake trout >24” caught 11 (6%) 10 (6%) 33 (17%) 12(8%) 5(2%) 10(6%) 5(4%)

Range of catch per net 0-19 0-16 0-24 0-12 0-13 0-11 0-11

Avg. catch per net 6.4 5.5 6.3 4.8 6.3 5.1 4.3

Avg. lake trout size 16.2 16.3 17.7 16.4 16.4 17.3 15.9

Avg. condition for<24” 83.3 79.9 81.8 83.5 84.7 87.2 85.7

Avg. condition for >24” 79.6 80.8 73.5 79.4 81.8 78.1 81.3

Lake trout size distribution

The graph above displays the size distribution of all lake

trout captured in the annual surveys since 2011 by percent.

All fish caught from 2011-2016 are pooled (black bars)

while 2017 is displayed separately in the gray bars. View-

ing the data this way enables comparisons to see if the 2017

sample yielded differences in the size distribution.

In 2017 we caught greater numbers of fish in the 9-12”

range, fewer fish in the 13-17” range, and greater numbers

again in the 18-21” range. We also caught relatively fewer

fish greater than 24” (see table on previous page).

Lake trout body condition by size is displayed at left.

24” is the size at which lake trout typically switch to verte-

brate prey in order to continue growing. In many ways, lake

trout populations actually behave as if they’re two separate

populations—smaller than 24” and larger. In Granby, fish

>24” have consistently had poorer body condition than

smaller fish. Granby has a dense population of mysis

shrimp, and lake trout <24” prey heavily on them. Fish

>24” continue to consume mysis, but they do not appear to

provide enough nutrition for the fish to grow to trophy size.

Kokanee salmon are the most valuable prey base to accom-

plish this, and the kokanee population in Granby has strug-

gled in recent history. In order for Granby to continue pro-

ducing trophy-sized lake trout, it is critical for the kokanee

population to recover.

N = 131

The largest lake trout from 2014. 39.7” long, 27.8 lbs.

The figure above displays the catch rates of lake trout

in average fish per net set over the past seven years. The

vertical red bars display the 80% confidence intervals

(CI). It is interesting to note that over the past four years,

the average catch per net has not fallen within the previous

year’s 80% CI. We use the exact same netting protocol at

Williams Fork and Green Mountain reservoirs to monitor

lake trout, and the catch rates at those waters have been far

less variable from year to year.

The black line on the figure depicts the overall trend in

catch rate. On average, our catch rates have declined dur-

ing this period. Because of the high density of mysis

shrimp in the lake, Granby is capable of producing ex-

tremely high densities of lake trout—probably the highest

in Colorado. Therefore, it can sustain a high amount of

harvest pressure. A slow-but-steady downward trend in

lake trout density is a positive sign and could play a posi-

tive role in recovering the kokanee population (see discus-

sion on following page).

This lake trout in 2017 had recently eaten a kokanee.

Lake Granby is home to Colorado’s longest-running

kokanee egg harvesting operation. Kokanee were first in-

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

hatchery system. The following spring, 1 million kokanee

fry are stocked into the Colorado River above the lake.

Historically, Granby produced enough kokanee eggs to not

only restock Granby itself, but also to provide kokanee to

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

Granby has failed to provide enough eggs to support itself.

CPW research crews conduct scientific sonar surveys

every year on Granby to estimate the number of kokanee

in the lake. This has provided a strong predictor of the

number of fish in the kokanee spawning run the following

year (see graph at right). The downward trend in egg take

from 2008-2014 was predicted by a general downward

trend in sonar estimates from 2007-2013. The sonar sur-

vey was not conducted in 2014 due to budgetary issues.

We are hopeful that kokanee numbers are rebounding,

with higher sonar estimates in 2015 and 2017, and an in-

creased egg take in 2016. The low egg take in 2017 was

not entirely attributable to poor kokanee numbers. Blue

Mesa Reservoir near Gunnison produced enough kokanee

to supply the entire state, and therefore there was little de-

mand for Granby eggs in 2017.

Research crews also monitor the density of mysis

shrimp in the reservoir (right). Because mysis eat the same

zooplankton that kokanee do, they are a highly efficient

competitor and can create difficult conditions for kokanee

growth and survival. Mysis densities in Granby have

proven to be highly variable over the years. They follow

the general pattern of declines during drought periods fol-

lowed by population explosions to extremely high densi-

ties when the reservoir refills. These high densities do not

appear to be sustainable over time, and so the population

seems to undergo a boom-and-bust oscillation.

Kokanee abundance estimates from sonar surveys

Current status of kokanee in Granby