Population Dynamics of Rainbow (Oncorhynchus mykiss) 

and Brown (Salmo trutta) Trout in Glen Canyon

Josh Korman Michael Yard

Ecometric Research USGS, Southwest Biological Science Center, 

Grand Canyon Monitoring & Research Center

With contributions from:

Kim Dibble – USGS

Michael Dodrill – USGS

Charles Yackulic – USGS

Ted Kennedy – USGS

Mariah Giardina - USGS

Funded through Project H. LTEMP Goals: Rainbow Trout Fishery, Humpback Chub, Recreational Experience

Photo Credit. David Herasimtschuk, ©Freshwaters Illustrated
Trout Recruitment and Growth Dynamics (TRGD) 

Estimate vital rates (growth, survival, recruitment, % capable of spawning)

to understand causes for abundance trends

Relate vital rates to GCD operations (discharge and Lake Powell elevation)

and other factors (competition, predation, prey availability)
TRGD Sampling Reaches in Glen Canyon

1C

1B

1A

2017-2020 (TRGD)

2017-2021 (TRGD)

2011-2021 (NO+TRGD)

3 (or 2) reaches * 4 trips/yr (Sep, Nov, Jan, Apr)

1 reach (1C) 1 trip/yr (July)
Rainbow Trout Abundance (1C)

Phase 1: high 

abundance (large 2011 

cohort)

Phase 3: 

low abundance due

to poor recruitments

Phase 4: partial recovery from 

high recruitment in 2017

2012 | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 |2020| 2021 |

Phase 5: low 

abundance due to 

poor recruitments

Preliminary Data. Do not Cite or Quote. 

Phase 2: 

collapse
Recent Rainbow and Brown Trout Abundance Trends

20% BNT

22% BNT

Preliminary Data. Do not Cite or Quote. 
Brown and Rainbow Trout Condition Factor 

Both BNT and RBT condition factor scaled based on RBT length-weight relationship

CF = observed weight/ predicted weight (based on length and RBT L-W relationship)

e.g., RBT of 300 mm, CF= 1 (300 g / 300 g); BNT of 300 mm, CF = 1.2 (360 g/300 g)

2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021

Preliminary Data. Do not Cite or Quote. 
Brown Trout Grow Much Faster than Rainbow Trout, and Brown 

Trout Rarely Lose Weight

Preliminary Data. Do not Cite or Quote. 
Brown Trout Grow Faster than Rainbow Tout in all Seasons 

Except Late Winter-Spring (January-April)

summer

winter/spring

fall/early winter

Preliminary Data. Do not Cite or Quote. 
Brown and Rainbow Trout Lipid (fat) Levels

K. Dibble, unpublished data

Preliminary Data. Do not Cite or Quote. 
Differences in Brown and Rainbow trout Feeding Strategies an Important 

Cause of Differences in Growth, Condition Factor, and Lipid Levels

Digestible portion of brown diet dominated by Gammarus vs. Midges & Blackflies for rainbow

M. Dodrill and T. Kennedy, 

unpublished data

Data from ‘12-’15

Preliminary Data. Do not 

Cite or Quote.
Differences in Brown and Rainbow Diets May 

Explain Differences in Growth and Condition

Consumption of fish much more common for browns than rainbows

M. Dodrill and T. Kennedy, 

unpublished data

Preliminary Data. Do not 

Cite or Quote.
Major Patterns in Trout Dynamics in Glen Canyon

• Rainbow trout abundance is low and dominated by larger and older individuals. 

Abundance of larger fish will decline if recruitment remains low.

• Brown trout population is increasing rapidly due to high recruitment and high survival 

rates of 75-124 mm size class.

• Brown trout grow much faster than rainbow trout, and have higher condition factor and 

lipid levels, likely because they feed more effectively on benthos and fish.

• Next talk: Brown trout enter the reproductive cycle at smaller size and become spawning 

capable at younger ages. These characteristics are likely an important factor driving high 

rates of brown trout recruitment.

this talk

next talk
Acknowledgements

• Logistical, administrative, and technical support: Glen Bennett, 

Carol Fritzinger, Seth Felder, Dave Foster, Dave Ward, Ann-Marie Bringhurst, 

Tom Gushue

• Field work: Thanks to the many boatmen, biologists, and technicians!