Smallmouth Bass Depletion 

Effort in Glen Canyon NRA

Laura Tennant, U.S. Geological Survey (USGS ), Southwest Biological Science Center (SBSC), Grand Canyon Monitoring & Research Center (GCMRC)

Charles Yackulic, USGS, SBSC, GCMRC

Andrew Morin, National Park Service (NPS)

Jeff Arnold, NPS

Melissa Trammell, NPS

Pilar Rinker, U.S. Fish & Wildlife Service (USFWS)

J. S. Tennant

U.S. Department of the Interior

U.S. Geological Survey 
Project Title:

Smallmouth Bass Science Proposal

Project Elements

Project I.2 (Not funded in workplan)

Project Objectives

 Determine if multiple pass electrofishing depletions are effective at removing smallmouth bass 

 Determine smallmouth bass distribution in GLCA (Glen Canyon)

 Determine hatch date and possible natal origin of smallmouth bass

Funding amount and Source

FY 23 - $137,000 – IA with BOR through already existing AMP agreement

Cooperators

National Park Service, AZ Game and Fish, 

U.S. Fish and Wildlife Service, Bureau of Reclamation

LTEMP Resource Goals Addressed

Conserve and protect native fishes
Glen Canyon Dam

3470 ft: Penstocks

3370 ft: Bypass Jet Tubes

Warmer Water

Colder Water

Lake Powell

Lees Ferry

Colder

Slide Credit: Drew Eppehimer
Warmer

3470 ft: Penstocks

3370 ft: Bypass Jet Tubes

Slide Credit: Drew Eppehimer

Glen Canyon Dam

Warmer Water

Colder Water

Lake Powell

Lees Ferry
Lees Ferry

Slide Credit: Drew Eppehimer
20 mm, spawned fish

Slide Credit: 

Drew Eppehimer

Smallmouth Bass Discovered in Slough at Lees Ferry 
Science Plan

• Hypothesis 1 – Mechanical removal via 

electrofishing can cause significant depletion in 

catch of young of year (YOY) smallmouth bass 

(SMB)

• Hypothesis 2 – SMB YOY are primarily distributed 

in the upper portions of GLCA (assumes nests 

present below dam) -Detections from Trout Recruitment & Growth Dynamics 

(TRGD; 2 sites)

-Adults obs. near dam (RM -15.6)

-Detection of juvenile SMB at slough (~RM -12.0)

• Hypothesis 3 – YOY SMB present in GLCA (2022) 

were hatched from ≥1 nest(s) in June

J. S. Tennant
Field Plan

• Depletion electrofishing in 8 

designated reaches (dam -> LF)

-EF pre-determined 250-m sample sites (both sides of river) w/in a reach -10 to 15 sites/reach

• If capture 

≤ 4 SMB, then move onto the next 

reach or 

≥ 5 SMB, then complete another pass 

(identical sampling) until ≤ 4 SMB 

captured or 5 passes have occurred in that reach

• Potential Off-Ramp

-If depletions were deemed ineffective 

(H1 rejected)

• Fish handling -Smallmouth Bass collected for otolith 

(H3) & diet analysis -Other warm water non

-natives = 

preserved or beneficial use -Minimize impact to trout – aerators in 

buckets used; YOY and adults counted 

(DR vs. RA)

J. S. Tennant
Preliminary data, subject to review, do not cite. 

N = 93

Pass 1 for Each Reach: 

SMB Captures ↑ Closest to Dam & 

SMB Distributed Throughout GLCA B

A

C

D

E

F*

F*

H

G

46

6

12

9

11

1

1 2

5
A

B

C

D

E

F*

F*

H

G

155

74

55

29

17

1

7 2

5

N = 345 SMB Captured from 

Sept. - Mid Dec. 2022

SMB trips (6 total)

Oct. 25th – Dec. 15th

N = 251

TRGD trips (2 total)

Sept. 10th – 13th

Nov. 3rd – 6th

N = 69

AZGFD trips (1 total)

Oct. 17th-20th

N = 25

Preliminary data, subject to review, do not cite. 
# of EF Passes Varied by Reach & # of SMB Captured Varied by Pass

Preliminary data, subject to review, do not cite. 
(Sept.)

(Nov.)

Preliminary data, subject to review, do not cite. 

Capture of SMB Generally Decreased Over Time
Variable Probability of Capture for SMB via Electrofishing

Preliminary data, subject to review, do not cite. 

(3 passes)

(3* passes)

(4 passes)

(4* passes)

(2 passes)

(2 passes e/reach)

(2 passes e/reach)

*equipment malfunction on 

one pass

Note: Only single passes 

occurred in B, E, F, G, H by 

SMB trips
Uncertain Relationship between Capture Probability and Temperature

Highest temp: 

20.2°C

Lowest temp:

10.0°C

Preliminary data, subject to review, do not cite. 
N=318, YOY only

Mean Size of SMB Captured = 80 mm FL

YOY Size Range = 58 – 122 mm FL

Two adults captured in Reach A 

(not pictured in graph):

TL = 358, FL = 349

TL = 294, FL = 283

Note: This graph includes TRGD 

data.

Preliminary data, subject to review, do not cite. 
Largest SMB & Most SMB Captured Near Dam

Downstream

DamLees Ferry

Slough

N=320, adults and YOY

Note: This graph includes 

TRGD data.

Preliminary data, subject to review, do not cite. 
A snapshot look at Smallmouth Bass 

diets in Lees Ferry Reach

• Subset of fish collected during TRGD trip in Sept. (n=14), and depletion trip in Oct. (n=29)

• Examined contents of true stomach only.

• High percentage of fish had identifiable food items in the stomach, >74%. 

Courtesy of Kate Behn, GCMRC 
Percentage of recently feeding SMB with different prey items

SMB prey by % biomass in recently feeding fish

Courtesy of Kate Behn, GCMRC; preliminary data, subject to review and change, do not cite.
Observations:

• Young-of-the-year Smallmouth frequently consumed tailwater-loving 

invertebrates (Gammarus & zooplankton).

• There was also evidence of piscivory in these young-of-the-year fish, 

starting at 64 mm FL of the consuming individual. Fish was the 2nd

highest prey item by biomass. 

• Gammarus tend to decline in abundance & biomass moving 

downstream. Gammarus prod.= 26% of total invert prod. @ Ferry vs. 

1.6% of production @30 mile (Cross et al. 2013)

Courtesy of Kate Behn, GCMRC; preliminary data, subject to review and change, do not cite.
Other Species of Concern

* Slough in this reach

*

Preliminary data, subject to review, do not cite. 
J. S. Tennant

Thoughts & Preliminary Conclusions

• We need to test multiple gear types to determine effectiveness of each in 

capturing SMB

• We currently have a rare situation when compared to other areas of the CRe

(Colorado River ecosystem), with ↑ numbers of native fish and ↓ numbers of 

non-native in GRCA (Grand Canyon) – how do we preserve that? 

• SMB not the only threat – other warmwater, piscivorous, non-native fish 

species are being entrained

• Consider strategies (thinking ahead/out of the box) and come to a consensus on 

what is feasible to implement because SMB entrainment will remain high so 

long as reservoir elevations are low, and SMB will reproduce and establish 

under current thermal conditions.