Near-Term Threat of Smallmouth Bass Establishment 

below Glen Canyon Dam

Smallmouth Bass Task Force

Presentation to:

The Technical Working Group

April 12, 2022
Purpose: Assess risk of SMB establishment in GC 

and identify potential responses

SMB Team Lead:

• Kirk Young – USFWS

SMB Modeling Team:

• Drew Eppehimer – USGS

• Charles Yackulic – USGS

SMB Team:

• Jeff Arnold – NPS

• Lucas Bair – USGS

• Jan Boyer – AZGFD

• Craig Ellsworth – WAPA

• Eric Frye – USGS 

• Sky Hedden – AZGFD

• Kerri Pedersen – USBR

• Pilar Rinker – USFWS

• Scott Rogers – AZGFD

• Robert Schelly – NPS

• Laura Tennant – USGS 

• Melissa Trammell – NPS

• David Ward – USGS
Outline

Background

• Why worry about SMB?

• Why aren’t SMB already here?

• What’s changing?

Risk assessment

• How reversible is a SMB invasion?

• How much entrainment required to establish population?

• Are conditions suitable?

Next steps

• What can scientists do?

• What can decision makers do?
Outline: Background

Background

• Why worry about SMB?

• Why aren’t SMB already here?

• What’s changing?

Risk assessment

• How reversible is a SMB invasion?

• How much entrainment required to establish population?

• Are conditions suitable?

Next steps

• What can scientists do?

• What can decision makers do?
Why worry about SMB?

• SMB have caused native fish decline throughout the west

• Upper Basin Cautionary Tale

• Wet Beaver Creek

• Black River

• Highly piscivorous - highest predatory threat 

in the upper basin

• Highly fecund (females have lots of eggs)

• 1 of 4 Highest Threat species in NNAS EA
Why aren’t Smallmouth Bass established in Grand Canyon?
Smallmouth bass capture locations in the 

Grand Canyon

*Spatial distribution of effort differs through time
What is changing? 

• Lower lake levels likely to increase 

entrainment; lake surface closer to 

penstocks

• Release temperatures are 

becoming warmer and suitable for 

reproduction and growth at Lees 

Ferry
Outline: Risk Assessment

Background

• Why worry about SMB?

• Why aren’t SMB already here?

• What’s changing?

Risk assessment

• How reversible is a SMB invasion?

• How much entrainment required to establish population?

• Are conditions suitable?

Next steps

• What can scientists do?

• What can decision makers do?
How reversible is a smallmouth bass invasion? 

• Scenario 1- Increased SMB population in Lees Ferry or downriver, but 

not reproducing (not established)

• Perhaps reversible if action taken quickly
How reversible is a smallmouth bass invasion? 

• Scenario 1- Increased SMB population in Lees Ferry or downriver, but 

not reproducing (not established)

• Perhaps reversible if action taken quickly

• Scenario 2- Established population in Lees Ferry

• Perhaps reversible over many years if spawning is disrupted every year
How reversible is a smallmouth bass invasion? 

• Scenario 1- Increased SMB population in Lees Ferry or downriver, but 

not reproducing (not established)

• Perhaps reversible if action taken quickly

• Scenario 2- Established population in Lees Ferry

• Perhaps reversible over many years if spawning is disrupted every year

• Scenario 3- Established population(s) downriver of Lees Ferry

• Extreme changes to dam operations required on a decadal timescale
How reversible is a smallmouth bass invasion? 

• Scenario 1- Increased SMB population in Lees Ferry or downriver, but 

not reproducing (not established)

• Perhaps reversible if action taken quickly

• Scenario 2- Established population in Lees Ferry

• Perhaps reversible over many years if spawning is disrupted every year

• Scenario 3- Established population(s) downriver of Lees Ferry

• Extreme changes to dam operations required on a decadal timescale

• Scenario 4- Established populations in tributaries 

• Perhaps reversible with extensive removal and see scenario 3
How reversible is a smallmouth bass invasion? 

• Scenario 1- Increased SMB population in Lees Ferry or downriver, but 

not reproducing (not established)

• Perhaps reversible if action taken quickly

• Scenario 2- Established population in Lees Ferry

• Perhaps reversible over many years if spawning is disrupted every year

• Scenario 3- Established population(s) downriver of Lees Ferry

• Extreme changes to dam operations required on a decadal timescale

• Scenario 4- Established populations in tributaries 

• Perhaps reversible with extensive removal and see scenario 3
Key questions related to scenarios

• How much entrainment required to establish population in Lees 

Ferry?

• Are conditions suitable for a spawning population of SMB: 

• in Lees Ferry?

• downriver?

• in the tributaries?
Conceptual Model of Entrainment Risk

Minimum population 

size required in Lees 

Ferry for establishment

SMB abundance in 

Lees Ferry at 

beginning of year

Penstock Entrainment

(numbers of fish passed through dam)

Fish Distribution by 

Depth

Reservoir elevation drives 

penstock depth

# of SMB available 

for entrainment

Lake Powell 

Elevation

SMB abundance in 

Lees Ferry at end of 

year

Survival 

Rate

Greater 

than?

Survival 

Rate
Number of SMB available for entrainment
Allee Effect
SMB abundance in 

Lees Ferry at 

beginning of year

Penstock Entrainment

(numbers of fish passed through dam)

Inflows

Outflows

(Bank Storage,

Evaporation)

SMB abundance in 

Lees Ferry at end of 

year

Survival 

Rate

Greater 

than?

Survival 

Rate

Statistical Model of 

Entrainment Risk

(Preliminary, do not cite)
Entrainment risk 

if elevation is 

held constant at 

a particular value

Risk increases over time as 

SMB accumulate in LF

(Preliminary, do not cite)
Results 

based on 

March 24-

month study

(Preliminary, do not cite)
Hypothetical 

scenario 

(adding ~1 maf)

(Preliminary, do not cite)
If there are enough spawners, what other 

conditions are required for SMB to establish?

• Sufficient food

• Infrequent high turbidity

• Low velocity gravel/cobble habitat

• Suitable water temperature
Lees Ferry Spawning Habitat (velocity ≤ 0.1 m/s)

Discharge (CFS) Habitat (ha)

5,000 50.8

10,000 37.0

15,000 31.3

20,000 28.4

25,000 24.0

30,000 21.8

35,000 20.5

40,000 19.3

45,000 19.8

(Preliminary, do not cite)
If there are enough spawners, what other 

conditions are required for SMB to establish? 

• Sufficient food- switch to piscivory at 40mm, unlikely to be limiting 

• Infrequent high turbidity- could limit reproduction downriver in some 

years

• Low velocity gravel/cobble habitat- may limit population size, unlikely 

to limit establishment

• Suitable water temperature- likely to be primary driver
If there are enough spawners, what other 

conditions are required for SMB to establish? 

• Sufficient food- switch to piscivory at 40mm, unlikely to be limiting 

• Infrequent high turbidity- could limit reproduction downriver in some 

years

• Low velocity gravel/cobble habitat- may limit population size, unlikely 

to limit establishment

• Suitable water temperature- likely to be primary driver
How does temperature affect population growth?

Population 

Growth (λ)

Daily 

temperature of 

releases

Hatch date “summer” water 

temperature

Lengths of 3 age-0 

cohorts

Winter starvation 

days

Overwinter 

survival

Minimum 

length

Other vital 

rates
Temperature and SMB Suitability in Lees Ferry

(Preliminary, do not cite)
Temperature and SMB Suitability at LCR Reach

(Preliminary, do not cite)
Temperature and SMB Suitability at Diamond Creek Reach

(Preliminary, do not cite)
Tributary Thermal Suitability (2000-present)

(Preliminary, do not cite)
Outline: Next steps

Background

• Why worry about SMB?

• Why aren’t SMB already here?

• What’s changing?

Risk assessment

• How reversible is a SMB invasion?

• How much entrainment required to establish population?

• Are conditions suitable?

Next steps

• What can scientists do?

• What can decision makers do?
Monitoring may provide first evidence of changes in 

SMB population(s)

0

1

2

3

4

Lees Ferry Grand Canyon Below Diamond Tributary Inflow Little Colorado

River

Havasu Creek Bright Angel

Creek

Shinumo Creek

# of trips

January February March April May June July August September October November December

may have adequate 

surveillance

Additional surveillance (citizen 

science/monitoring) may be needed 

elsewhere
0.0

3.5

7.0

10.5

14.0

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4

Cost ($M)

Lambda (population growth)

Passes

stable 25% decline 50% decline 75% decline 90% decline

50

100

150

Mechanical removal is costly and time consuming to 

maintain a stable population, let alone decrease a 

population

(Preliminary, do not cite)
Rapid Response

• Mechanical Removal

• Isolated pools: possibly effective

• Tributaries: possibly effective

• Piscicides

• Isolated pools

• Tributaries
Lees Ferry Spawning Habitat Disruption with Flow

Starting Ending Remaining 

Discharge Discharge Habitat 

(CFS) (CFS) (ha)

10,000 10,000 37.0

10,000 15,000 34.3

10,000 20,000 29.9

10,000 25,000 25.4

10,000 30,000 21.8

10,000 35,000 18.7

10,000 40,000 16.3

10,000 45,000 14.7

(Preliminary, do not cite)
Can Entrainment risk be reduced?

Short term:

Minimize time spent drawing from surface waters

DROA

Bypass

Mid term:

Fish exclusion in forebay, e.g., bubble curtain, C02 

curtain

Long term:

Changes to infrastructure
Different approaches to Manage SMB via Temperature

• Add it pilar
No Change: Baseline

(Preliminary, do not cite)
Keep Temps < 16 °C All Year

(Preliminary, do not cite)
Keep Temps < 16 °C until Sept 4

(Preliminary, do not cite)
Keep Temps < 13 °C until Sept 4, one day/week

(Preliminary, do not cite)
LCR Reach when managing 1) LF temps, 2) downriver temps

1)

Bypass

2)

Bypass

(Preliminary, do not cite)
Conclusions

• SMB are a greater threat to native fish & rainbow trout than any other invasive 

fish currently present in the system

• Reservoir conditions are changing to support increased entrainment and 

establishment below Glen Canyon Dam

• Limiting entrainment & controlling temperature are the only large-scale tools 

currently available 

• Quickly responding to small scale presence of SMB may buy us more time

• Reversing SMB establishment downriver/ in tributaries will be a drawn out and 

expensive process and may not be possible
Acknowledgements

• Models developed relied heavily on earlier work in 

collaboration with Lindsey Bruckerhoff, Kevin Bestgen, Jian 

Wang, Kim Dibble, Bryce Mihalevich, and Jack Schmidt.

• Funding for model development came from non-AMP sources 

– primarily USGS’s Southwest Climate Science Center and 

USGS’s water and ecosystem mission areas.
Questions?