What Determines the Abundance of Rainbow Trout near 

the Little Colorado River Confluence?

Josh Korman

Ecometric Research

Michael Yard

GCMRC – USGS

Data and analytical 

contributions from: 

Maria Dzul

Charles Yackulic

Michael Dodrill

Bridget Deemer

Theodore Kennedy

GCMRC-USGS

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

• 5 reaches sampled quarterly, 2012-2016 

(Natal Origins)

• Glen Canyon (reach I) and LCR inflow 

reach (IVb), 2017-2019 (TRGD, JCM)

• No sampling in Marble Canyon (II-IVa) 

after 2016 except for one night in reach II 

on July and September trips

• Mark-recapture used to estimate:

• abundance

• survival rate

• recruitment (births and immigration)

• growth rate

• movement

• Drift measured in each reach on each trip, 

2012-2016

I (I)

(IVa)

(II)

(IVb)

(III)

Little Colorado River
Trends in Abundance Depend on Survival and Recruitment

N2014 = N2013 * S2013 + R2013

Dispersal from 

upstream 

populations

(immigration)

Local reproduction

(births)

Persistence

Survival rate Recruitment
Trend in Rainbow Trout Abundance Downstream of the 

Little Colorado River (IVb, inflow reach)

Phase 1:

expansion from 

immigration

Phase 2: 

collapse due 

to poor 

condition

Phase 3: 

low abundance due to 

limited recruitment

Phase 4: moderate 

abundance from local 

reproduction & immigration

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

Preliminary data, do not cite
Phase 1: Expansion from Immigration

Phase 1:

expansion from 

immigration

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

Preliminary data, do not cite
Phase 1: Expansion from Immigration

(dispersal from Lees Ferry to Marble Canyon, slide 1)

• Immigration of trout to LCR reaches is a two-step process: 

1) initial short-duration dispersal of young-of-year from Lees Ferry to upper-middle 

Marble Canyon in their first summer;

2) Longer-duration downstream dispersal from Marble Canyon to the LCR in following 

years

• Higher recruitment in Lees Ferry leads to higher numbers of young rainbow trout 

dispersing downstream into Marble Canyon

2011

Preliminary data, do not cite
Phase 1: Expansion from Immigration

(dispersal from Lees Ferry to Marble Canyon, slide 2)

• Large recruitment event in Lees Ferry in 2011 resulting from high inflows to 

Powell  equalization flows  high densities of small rainbow trout in Glen 

Canyon and upper half of Marble Canyon by April 2012 trip

2012 | 2013 | 2014 | 2015 | 2016

2012 | 2013 | 2014 | 2015 | 2016

Preliminary data, do not cite
Phase 1: Expansion from Immigration

(dispersal from Marble Canyon to LCR)

A very small proportion of 

Glen and Marble Canyon 

populations moved 

downstream to the LCR 

between 2012 and 2014

# of recaptures

I II III IVa IVb

I 3,133 12 1 0 2

II 16 3,517 12 8 6

III 1 15 3,162 10 8

IVa 1 0 2 1,382 82

IVb 1 0 1 13 560

Release Reach

Recovery Reach

Abundance * dispersal distribution = immigration to LCR reaches 

upstream downstream

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

Phase 2: 

collapse due 

to poor 

condition

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

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(what is condition factor (CF)?)

CF=1

Fork Length (mm)

Weight (g)

Fatter (CF>1)

Skinnier (CF<1)

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(condition factor influences survival rate, slide 1)

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(condition factor influences survival rate, slide 2)

2012 | 2013 | 2014 | 2015 | 2016

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(variation in growth rate drives variation in condition)

2012 | 2013 | 2014 | 2015 | 2016

LCR Inflow Reach (266 g (300 mm) rainbow trout)

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(cause of variation in growth at LCR, slide 1)

above average conditions for growth 

(high food, low competition, clear water)

below average conditions for growth 

(high water temperature, unexplained)

LCR Inflow Reach (266 g 

(300 mm) rainbow trout)

net growth is positive (+22 – 10 > 0), gained weight & condition

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(cause of variation in growth at LCR, slide 2)

LCR Inflow Reach (266 g 

(300 mm) rainbow trout)

net growth is negative, lost weight and condition

warm water temperature and low water clarity (turbidity) 

are causes of lost weight and condition

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(2012-2014: a bumpy ride to the bottom)

2012 | 2013 | 2014 | 2015 | 2016

LCR Inflow Reach (266 g 

(300 mm) rainbow trout)

high turbidity (and warmer water in ’14)

low food

Even lower 

competition after 

population collapse

condition factor

high food

Preliminary data, do not cite
Phase 2: Collapse Due To Poor Condition

(potential explanation for food base changes 2012-2016)

Changes in food base between 2012-2016 likely driven by low 

concentrations of phosphorous in releases from Glen Canyon Dam

Preliminary data, do not cite
Trend in Rainbow Trout Abundance Downstream of the 

Little Colorado River (LCR)

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

Phase 3: 

low abundance due to 

limited recruitment

Preliminary data, do not cite
Phase 3: Low Abundance Due To Limited Recruitment

1. Low recruitment in Lees Ferry in 

2015, limiting dispersal to Marble 

Canyon

2. Small trout populations in 

Marble Canyon after collapse, 

limiting immigration to LCR reach

3. Few potential spawners near LCR 

after collapse

4. Potential spawnersin poor 

condition and smaller in 2015, 

leading to low rates of 

maturation and fecundity in 

2016, and limiting recruitment

in 2017

low condition-less spawning

immigration

Local

reproduction

smaller trout – fewer eggs

Brian Healey, NPS

unpublished data

Preliminary data, do not cite
Phase 4: Moderate Abundance From 

Local Reproduction and Immigration • Good growth and condition starting in 2016 likely led to local reproduction

• Average size of rainbow trout <125 mm in July 2018 was 80 mm

- too small to have originated from Glen Canyon from large 2017 recruitment-dispersal event

- too big and too far downstream to have originated from Glen Canyon form 2018 dispersal event

- therefore likely from spawning in reach IVb or Marble Canyon in winter 2018

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

Phase 4

Preliminary data, do not cite
Conclusions

Factors controlling trout abundance at the LCR

• Dispersal of young rainbow trout from 

Lees Ferry to Marble Canyon

• Prey availability (food base)

• Water clarity (turbidity)

• Water temperature

Conditions in Lees Ferry

(immigration)

Conditions in Marble Canyon

& below LCR (persistence)

Trout Control Management Options

Trout Management Flows 

(limit immigration) *

Mechanical Removal near 

LCR (local reductions

in immigration & 

persistence) Turbidity Management -

limit Fall HFEs (limit 

persistence) *

* = more study needed

Do Nothing -

(2007-2019)

No active trout control

Did we increase trout 

persistence in Marble 

Canyon through 

frequent fall HFE’s?

Preliminary data, do not cite
Acknowledgements

• Administrative and logistical support: Ted Melis, Scott Vanderkooi, Carol Fritzinger, Seth 

Felder, Dave Foster, Dave Ward

• Sampling design and modelling: Carl Walters

• Supporting data: Dave Topping, Nick Voichick

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