Characterizing the fish assemblage in the Lake Powell forebay and 

identifying the potential for nonnative fish escapement into the Lower 

Colorado River 

Barrett Friesen, Phaedra Budy, Casey Pennock 

Utah State University 

USGS Utah Cooperative Fish 

and Wildlife Research Unit 
 

Study Area 

Glen Canyon Dam 

UT 

AZ 

a) Wahweap 

b) Confluence 

c) Forebay 

50 km 

Nonnative fish distribution 

in the GCD forebay 
November 2023 status update

• Last presentation to TWG was April 2023

• Presented on Sensor Fish trials through GCD

• Fieldwork completed October 2023 (82 days on the water over 8 trips)

• Today’s update includes:

• Thermal structure of forebay

• Gillnet catch in forebay

• Fish distribution modeling

• Sampling adjacent to dam

• Hydroacoustics

• Preview of future work

• Native fish tidbit

Epilimnion

Epilimnion

Mixed Mixed





2023: 35 m depth for 82+ days

≥ 16 °C

≥ 16 °C

2022: 20 m depth for 113 days
Low DO at penstock depth 

fall 2022 & 2023
Low DO at penstock depth 

fall 2022 & 2023
Fish distribution modeling

Using data from capture methods and physical habitat metrics

1) Occupancy modeling - presence/absence only, not abundance, imperfect detection

SMB occupancy increases with:

↑ Temperature 

↓ Depth 

↓ DO 

↑ Slope

Preliminary results
≥ 16 °C

≥ 16 °C

2022

SMB occupancy: 68% (± 17%)

@ 20 m depth

2023

SMB occupancy: 34% (± 23%)

@ 35 m depth

Preliminary results
Fish distribution modeling

Using data from capture methods and physical habitat metrics

2)Catch per unit effort (CPUE) as a proxy for relative abundance

SMB CPUE increases with:

↑ Temperature 

↓ Depth 

↔ DO

↓ Slope

Preliminary results
Question from 2022: where are entrained YoY coming from?

Modification to 2023 

sampling: sample very 

close to dam
August 2023: ~300 Bluegill and Green Sunfish 

captured immediately adjacent to dam in minnow 

and larval traps down to 22 m depth
Speculation: young fish may use 

penstock gate structures as cover
Hydroacoustics

Plotting down scan targets & gillnet catch

Preliminary results

Full depth Top 30 m
Hydroacoustics

Plotting down scan targets & gillnet catch

Preliminary results

Full depth Top 30 m
Hydroacoustics

Plotting down scan targets & gillnet catch

Preliminary results

Gizzard Shad Zone

Full depth Top 30 m
Significant difference in depth selection when Powell is mixed vs stratified

Median depth = 54.6 m Median depth = 15.8 m

Preliminary results

Winter vs Summer
Hydroacoustics

Next steps

• Probabilistic species assignment (where possible)

• Calculate densities

• Model effects of covariates

• Temperature

• Depth

• DO

Preliminary results
Preliminary conclusions

↑ water level leads to:

↓ in fish density at penstock level

↓ in entrainment probability

↓ in survival of entrainment

when stratified

Note: runoff effects are seen in 

the forebay and must be included

Preliminary results
Future work:

• Rerun analyses with complete dataset

• Occupancy and CPUE models

• Hydroacoustics with catch data

• Movement data from acoustic telemetry

• Work towards publication of 2 manuscripts

• Entrainment survival (smallish paper)

• Nonnative fish distribution (main paper)

• Defend MS thesis - March 2024
Future work: 

• Rerun analyses with complete dataset 

• Occupancy and CPUE models 

• Hydroacoustics with catch data 

• Movement data from acoustic 

telemetry 

• Work towards publication of 2 manuscripts 

• Entrainment survival (smallish paper) 

• Nonnative fish distribution (main paper) 

• Defend MS thesis - March 2024 
Acoustic telemetry

30 implanted fish with transmitters

Network of 10 SURs to receive signals

Vandalized early summer 2023
Future work:

• Rerun analyses with complete dataset

• Occupancy and CPUE models

• Hydroacoustics with catch data

• Movement data from acoustic telemetry

• Work towards publication of 2 manuscripts

• Entrainment survival (smallish paper)

• Nonnative fish distribution (main paper)

• Defend MS thesis - March 2024
Sampling highlight: capturing rare native fishes

Razorback Sucker

495 mm

1800 g

Captured 10 Aug 2023 in GCD forebay

Stocked 2018 in Gunnison River, 

~500km upstream
Acknowledgments

Bureau of Reclamation

• Kato Miyagishima

• Mike Horn

• Jeremy Hammen

• Clarence Fullard

• Kerri Pedersen

• Brian Hines

• Mark McKinstry

Utah State University

• David Rosenberg

• Gary P. Thiede

• Justin Furby

• Hunter Pehrson

• Ben Miller

• Martinique Chavez

• Sarah Barnes

• Chloe Lyles

• Skylar Rousseau

National Park Service

• Jeff Arnold

• Conor Clancy

U.S. Geological Survey

USGS Utah Cooperative Fish

& Wildlife Research Unit

USGS Utah Cooperative Fish 

and Wildlife Research Unit
• Deployed March 2023, 21 trials conducted

• Acclimation pressure at depth of penstock = 237 kPa

• Median nadir pressure = 36 kPa

• Gas bladder expansion = 237/36 = 6.6 times original volume (Boyle’s Law)

• Gas bladder rupture likely

• Embolism, hemorrhage, exophthalmia possible due to gas bladder rupture

• Higher water level in Powell leads to greater acclimation pressure and 

greater gas bladder volumetric expansion

Sensor fish summary

Preliminary data