Part I: 

Update on Rainbow and Brown Trout Growth, 

Abundance, and Recruitment in Glen Canyon 

(TRGD)

1

Josh Korman

Ecometric Research, Inc

Mariah Giardina

US Geological Survey, 

Southwest Biological Science Center,

Grand Canyon Monitoring and Research Center

Charles Yackulic

US Geological Survey, Southwest Biological Science Center, Grand Canyon 

Monitoring and Research Center

This information is preliminary and is subject to revision. It is being provided to meet the need for 

timely best science. The information is provided on the condition that neither the U.S.G.S. nor the 

U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized 

use of the information. 

U.S. Department of the Interior

U.S. Geological Survey
 

 

 

 

TRGD Objectives 

Project H, Element 2 

1. Provide reliable estimates of abundance of rainbow and brown trout in Glen 

Canyon 

2. Provide estimates of key vital rates (survival, spawning, recruitment, growth) to

understand the causes of fluctuations in abundance to link to GCD operations and

other factors. 

2 

Prey, Competition, Predation Risk, Flow, 

Water Temperature, Phosphorous, Light 
RBT vs BNT Condition Factor

3

Condition Factor = observed weight/predicted weight

Predict weight from length using RBT length-weight data (for both species, ‘12-’22)

brown

rainbow

Preliminary data, do not cite
RBT vs BNT Growth Rates (Weight)

4

Preliminary data, do not cite
RBT vs BNT Growth Rates (Weight)

5

Summer

Fall

Late Fall

Winter

Spring

2019-20 2020-21 2021-22 2022

Preliminary data, do not cite
RBT and BNT Abundance by TRGD Reach

6

Preliminary data, do not cite
Summary

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Growth

1. Growth of larger RBT have been lower since spring 2022, leading to 

reduced condition.

2. Growth rates of larger BNT during spring, summer, and fall intervals of 

2022 were considerably lower compared to earlier years, leading to a 

steep decline in condition of larger fish.

Abundance

3. Abundance of RBT is still relatively stable, but recruitment is low so 

population is dominated by older/larger fish (vulnerable to collapse).. 

4. Collapse of RBT in winter 2023 not likely because larger fish went into 

2022-low growth period (fall) in good condition (unlike 2014 when 

condition was low prior to high temperatures/low DO leading to collapse) 

5. BNT abundance may have stabilized – recruitment in 2022 was lower than 

in 2020 and 2021, and lower condition could indicate that lower 

recruitment rates will continue.
Part II: 

Modelling Brown Trout Population Dynamics

in Glen Canyon 

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Objectives 

Project H: Salmonid Research and Monitoring

• H.4. Brown trout population modeling 

Resource goals: 

• Monitor trout population dynamics in Glen Canyon 

• Possible impacts on other resources

• Reduce uncertainty of population response to experimental flows and other 

drivers

This talk: 

• Brown trout abundance 

• Runge Model

• Downstream catch summary

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Background 

• 2014-2016 observed increase in BNT - primarily in adult size classes 

• 2017 – AMWG asked for an investigation of causes and implications of brown trout 

expansion in Glen Canyon 

• Impact on HBC downstream

• Evaluate management options

• Open File Report (2018)

• Runge, M.C., Yackulic, C.B., Bair, L.S., Kennedy, T.A., Valdez, R.A., Ellsworth, C., Kershner J.L., Rogers, R.S., Trammell, M.A., and 

Young, K.L., 2018, Brown trout in the Lees Ferry reach of the Colorado River—Evaluation of causal hypotheses and potential 

interventions: U.S. Geological Survey Open-File Report 2018–1069, 83 p., https://doi.org/10.3133/ofr20181069. 

• Ongoing monitoring in Glen Canyon

• Experimental management

• Incentivized harvest 

• TMF lit review and hypsometric analysis 

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Model from Runge Report 

• Goes back to year 

2000

• Utilizes TRGD data, 

supplemented by 

AZGFD

• Includes all of Glen 

Canyon

Runge, M.C., Yackulic, C.B., Bair, L.S., Kennedy, T.A., Valdez, R.A., Ellsworth, C., Kershner J.L., Rogers, R.S., Trammell, M.A., and Young, 

K.L., 2018, Brown trout in the Lees Ferry reach of the Colorado River—Evaluation of causal hypotheses and potential interventions: U.S. 

Geological Survey Open-File Report 2018–1069, 83 p., https://doi.org/10.3133/ofr20181069. 11
BNT Abundance Estimate for Size Class 1

Preliminary data, do not cite 12
BNT Abundance Estimate for Adult Size Class

Preliminary data, do not cite 13
Houserock

• Gateway

• Residual from Natal Origins study

• 1 pass - CPUE 

• Pulse on downstream movement

June/July Sept/Oct

BNT TOTAL BNT TOTAL

2021 1% (6) 718 15% (22) 1507

2022 1% (9) 797 17% (20) 1192

Preliminary data, do not cite 14
15

0

50

100

150

200

250

300

350

400

Apr-12

Jul-12

Sep-12

Jan-13

Apr-13

Jul-13

Sep-13

Jan-14

Apr-14

Jul-14

Sep-14

Jan-15

Apr-15

Jul-15

Sep-15

Jan-16

Apr-16

Jul-16

Sep-16

Apr-17

Jul-17

Sep-17

Apr-18

Jul-18

Sep-18

May-19

Jul-19

Sep-19

Jul-20

Oct-20

Apr-21

Jun-21

Sep-21

May-22

Jun-22

Sep-22

TOTAL CATCH 

(CPUE – 6KM/3PASSES)

TRIP DATE 

RBT Catch by Size Class (mm) at LCR 75-124

125-174

175-224

225-274

>=275

Preliminary, do not cite

0

2

4

6

8

10

12

14

Apr-12

Jul-12

Sep-12

Jan-13

Apr-13

Jul-13

Sep-13

Jan-14

Apr-14

Jul-14

Sep-14

Jan-15

Apr-15

Jul-15

Sep-15

Jan-16

Apr-16

Jul-16

Sep-16

Apr-17

Jul-17

Sep-17

Apr-18

Jul-18

Sep-18

May-19

Jul-19

Sep-19

Jul-20

Oct-20

Apr-21

Jun-21

Sep-21

May-22

Jun-22

Sep-22

TOTAL CATCH

(CPUE

– 6 KM/3 PASSES)

TRIP DATE 

BNT Catch By Size Class (mm) at LCR 75-124

125-174

175-224

225-274

>=275
Summary 

• BNT adult abundance is increasing

• TRGD shows decreasing condition factor 

• Abundance of smaller size class decreasing

• Low catch numbers at LCR and Houserock

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Questions?