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 
Historic conditions: withdrawal from hypolimnion when stratified 

Epilimnion 

Hypolimnion 

flow 
Current conditions: withdrawal from epilimnion when stratified 

Current level: 3520 ft 

Epilimnion 

Hypolimnion 

flow 
Study Area 

a) Wahweap 

b) Confluence 

c) Forebay 

Glen Canyon Dam 

UT 

AZ 

50 km 

1. Nonnative fish distribution 

2. Risk of entrainment 

3. Survival of entrainment 
Updates to TWG 

• Thermal structure of forebay 

• Sampling near dam 

• Sensor Fish preliminary data 

Interpolated Measured 
Epilimnion 
Epilimnion 

April 

November 

December 

July 
Top of penstock 


WAE 

SMB, GZS, WAE 

SMB, GZS, WAE 

GZS 

WAE 

WAE 
Modification to sampling program: sample within chains close to dam. 

Gillnets, minnow traps, larval traps throughout 2023 
Question from 2022: where are entrained YoY coming from? 

Shallow water habitat. 

Spawning? 
 

Possible answer: right next to the dam 

BGL and GSF captured immediately adjacent to dam 

Will we find spawning SMB and YoY there this summer? 

Juvenile BGL & GSF 
 

 

Sensor fish deployed: March 2023 

Deployed Sensor Fish through dam to measure conditions of passage 

• Pressure 

• Acceleration 

• 17 trials completed 

Turbine 

Sensor Fish 
Sensor fish deployed: March 2023 

Sources of immediate and delayed mortality 

• Turbine blade strike 

• Injury due to abrasion 

• Gas bladder rupture 

• Gas bubble disease 

• Embolism 

• Hemorrhage 

• Exophthalmia 
Sensor fish deployed: March 2023 

Typical pressure curve 

Preliminary data 

Descending 

through 

penstock 

Traveling to 

and through 

turbine 

Exiting turbine 

Nadir pressure 

Tailrace 
Sensor fish deployed: March 2023 

Typical acceleration curve 

Exiting turbine 

Preliminary data 
 

Sensor fish deployed: March 2023 

One second of action 

Exiting turbine 

Preliminary data 
 

 

 

 

 

Sensor fish deployed: March 2023 

Initial findings: 

• No evidence of turbine strikes 

• Acclimation pressure dependent upon depth of penstock (water level) 

• Gas bladder rupture likely 

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

• Gas bubble disease unlikely (short time period, low dissolved gas) 

Preliminary data 
 

 

 

 

 

 

Sensor fish deployed: March 2023 

Initial findings: 

• No evidence of turbine strikes 

• Acclimation pressure dependent upon depth of penstock 

• Gas bladder rupture likely 

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

• Gas bubble disease unlikely (short time period, low dissolved gas in forebay) 

Injury/mortality appear linked to change in pressures – no significant effects 

from turbine passage per se 

Preliminary data 
Sensor fish deployed: March 2023 

Boyle’s Law 

• P1V1=P2V2 

• Assume V1 = 1 

• Acclimation pressure at penstock intake = P1 = 237 kPa 

• Median nadir pressure = P2 = 36 kPa (atmospheric = 98 kPa) 

• V2 = 237(1)/36 = 6.6 (gas bladder expands 6.6x) 

• From literature: 

• Gas bladder rupture at 2-4x expansion 

• 84% mortality (for Chinook Salmon) 

• Physostomous vs physoclistous 

• Life stage implication 

Preliminary data 
 

Acoustic telemetry 

• Movement 

• Depth 

• 30 fish implanted with ultrasonic transmitters October 2022 

• 20 Smallmouth Bass 

• 10 Channel Catfish 
24

12

9

8

5

1

6

0

4

3

 

 

Acoustic telemetry 

Where have the 30 fish traveled? 

Unique fish detections higher 

near Wahweap, zero detected at 

dam 

Expectation: fish will move into 

forebay as water warms and 

epilimnion forms 

Acoustic telemetry 

Smallmouth Bass (20 fish implanted) What depths? 

• Date range: Nov 1 to 

n = 230,000 Mar 5 

• Most detections <10 m 
Acoustic telemetry 

Smallmouth Bass (20 fish implanted) What depths? 

• Date range: Nov 1 to 

Depth: 0 – 10 m Mar 5 

• Most detections <10 m 
Acoustic telemetry 

Channel Catfish (10 fish implanted) What depths? 

• Date range: Nov 1 to 

n = 160,000 Mar 5 

• Most detections <50 m 
Acoustic telemetry 

Channel Catfish (10 fish implanted) What depths? 

• Date range: Nov 1 to 

Depth: 0 – 50 m Mar 5 

• Most detections <50 m 
Relative Probability of Entrainment (PoE) 

Probability of presence at depth given: 

• Season (stratification) 

• Water temp 

• Location 

• Species 

Work on model database setup has begun, exact form of model TBD 
Bureau of Reclamation

• Kato Miyagishima

• Clarence Fullard

• Mike Horn

• Kerri Pedersen

• Mark McKinstry

Utah State University

• David Rosenberg

• Gary P. Thiede

• Justin Furby

• Hunter Pehrson

• Ben Miller

• Martinique Chavez

• Sarah Barnes

National Park Service

• Jeff Arnold

U.S. Geological Survey

USGS Utah Cooperative Fish 

& Wildlife Research Unit

Acknowledgments
Bureau of Reclamation

• Kato Miyagishima

• Clarence Fullard

• Mike Horn

• Kerri Pedersen

• Mark McKinstry

Utah State University

• David Rosenberg

• Gary P. Thiede

• Justin Furby

• Hunter Pehrson

• Ben Miller

• Martinique Chavez

• Sarah Barnes

N

•

U.

U

& 

ational Park Service

Jeff Arnold

S. Geological Survey

SGS Utah Cooperative Fish

Wildlife Research Unit

Acknowledgments