TRGD: Trout Recruitment, Growth 

and Population Dynamics

Project Element: H.1

Michael Yard, U.S. Geological Survey, Grand Canyon 

Monitoring and Research Center

Josh Korman, Ecometric Research, Vancouver, BC
LTEMP Resource Goal: 

• Achieve a healthy high-quality recreational rainbow trout fishery in GCNRA and reduce or eliminate 

downstream trout migration consistent with NPS fish management and ESA compliance.

Study Objectives:

• The data collection and analyses are intended to determine the effects of LTEMP ROD flows on the 

recruitment of young-of-year (YOY) rainbow and brown trout in Glen Canyon, the growth rate of juveniles and 

adults, and dispersal of YOY trout from Glen Canyon. 

• The effects of higher and potentially more stable flows in spring and summer during equalization events on 

trout recruitment, growth, and dispersal. 

• The effect of fall High Flow Experiments (HFEs) on recruitment of trout in Glen Canyon, measured either 

through direct effects on juvenile survival or through reduced egg deposition in later years driven by reduced 

growth of trout (which reduces fecundity and rates of sexual maturation).

• The effect of spring HFEs on trout recruitment, growth, and dispersal.

• The effect of Trout Management Flows (TMFs) on rainbow and brown trout recruitment and dispersal. 
RAINBOW TROUT ABUNDANCE AND CONDITION

Overview: 2012 – 2019 

• High abundance of juveniles in 2012 and 

2013 due to large recruitment in 2011 from 

equalization flow.

• Trout grew into catchable sizes (>=225 mm), 

which lead to very high catch rates in 2013 

and early-2014.

• Prolonged and reduced growth and 

condition between 2013 & 2014 led to the 

collapse in rainbow trout fishery.

• Limited recruitment.

• Post population collapse resulted in 

increased annual recruitment and growth.

• Highest recruitment in 2017.

• Abundance has increased and appears 

stable.

• Condition is declining.

1C - Subreach

(2017)

Preliminary data, do not cite
INVERTEBRATE PRODUCTION

(Prey: chironomids, gammarus, and simuliids)

2012 2013

2014

2015

2016

R2 = 0.94

0

5

10

15

20

25

30

35

0.004 0.006 0.008 0.01

Annual Invertebrate Production - (Prey Items)

(g m-2 yr-1

)

Soluble Reactive Phosphorus

(g m-3)

Preliminary data, do not cite
RAINBOW TROUT ABUNDANCE

Overview: 2017 – 2019 

Spatial variation in abundance:

• Age-0 Recruits – higher abundance upstream (1A1B)

• Catchable sized trout (> 9”) – higher abundance 

downstream (1C)

• Condition Factor higher upstream

1A

(Upstream)

1B

(Middle)

1C

(Lower)

Preliminary data, do not cite
Crash Post-Crash

Spring - Summer

Fall - Winter

1C - Subreach

SMALL RAINBOW TROUT

Preliminary data, 

do not cite
Crash Post-Crash

LARGE RAINBOW TROUT

Preliminary data, 

do not cite
RAINBOW TROUT

BROWN TROUT

COMPARISON BETWEEN RAINBOW TROUT AND BROWN TROUT

Preliminary data, 

do not cite
Length-weight Relationship

Sample Size:

• RBT (n) = 101,989

• BNT (n) = 5,197

Growth Differences:

• BNT are fatter than RBT. 

• RBT display negative-allometric 

growth (b < 3) 

• This indicates a decrease in growth 

or elongation in length without the 

commensurate increase in weight. 

WW = aaLLbb

BNT b = 3.0

RBT b = 2.88

Brown Trout Rainbow Trout

Preliminary data, do not cite
2019 – RAINBOW TROUT MONTHLY GROWTH RATES

Rainbow Trout Mean 

Monthly Growth 

Rate (g/mo)

• RBT growth higher in lateWinter/Spring and Summer.

• RBT growth is more reduced 

in late-Summer/Fall and 

early-Winter.

Preliminary data, do not cite.
2019 – BROWN TROUT MONTHLY GROWTH RATES

Brown Trout Mean Monthly 

Growth Rate (g/mo)

• BNT Growth Rates Overlaid 

(black) onto RBT figure.

• BNT growth is quite a bit 

higher than RBT. 

• Increased BNT growth in 

weight occurs in Apr-Jul, 

Jul-Sep, and Sep-Nov 2019.

Preliminary data, do not cite
BROWN TROUT – CPUE

Brown Trout (BNT) spawn between NovemberJanuary, whereas, Rainbow Trout (RBT) around midMarch. 

• We should expect to observe BNT in our 

electrofishing catch sooner than RBT.

• Based on relative catch in the fall, BNT appear 

to be growing and surviving better than age-0 

RBT.

Why are BNT not as susceptible to capture by our 

sampling gear earlier in the year (April-July effort)?

• Catch difference between the two trout species 

suggest that BNT are not occupying the near 

shoreline (wetted edge) when smaller in size.

• If Age-0 BNT are utilizing different habitat at 

sizes <75 mm, then they are not as likely to be 

affected by TMFs in late Spring and Summer.

2015

2016

2017

2018

2019

Preliminary data, do not cite
FALL HIGH-FLOW EXPERIMENT

Mean growth for 266 g 

RBT (typical weight for 

a 300 mm fish).

Rainbow Trout 2015

NO-HFE

2017

NO-HFE

2018

HFE

Population Crash

2016

HFE

2019

NO-HFE

?

Preliminary data, 

do not cite.
STABLE FLOWS – “BUG FLOWS”

Mean growth for 266 g 

RBT (typical weight for 

a 300 mm fish)

Rainbow Trout No Bug Flows

2015 2016 2017

Bug Flows

2018 2019

Preliminary data, 

do not cite
STABLE “BUG-FLOWS”

Mean growth for 266 g RBT 

(typical weight for a 300 

mm fish).

• Growth differences between 

flow periods are 

inconclusive.

• Other extrinsic factors like 

fish density and increasing 

biomass are likely influencing 

growth.

• Flow effect is inconclusive.

Rainbow Trout No Bug Flows

2015 2016 2017

Bug Flows

2018 2019

Preliminary data, do not cite.
RAINBOW TROUT – ANNUAL RECRUITMENT

1C 

- Subreach

2012 2015 2016 2017 2018 2019

Preliminary data, 

do not cite.
TROUT MANAGEMENT FLOWS

The purpose of TMFs is to regulate large 

recruitment events (excess age-0 trout) 

to avoid downstream dispersal.

Problems are:

• Uncertainty about the overall 

efficacy of TMF’s.

• Negative effects to other resources.

• Unable to forecast recruitment 

reliably until July. 

• TMFs flow implementation is 

scheduled between May-July. 

Preliminary data, do not cite
TROUT MANAGEMENT FLOWS, cont.

TMF’s Assumptions:

• We assume that recruits (Age-0, <75 mm

FL) are most vulnerable to stranding in 

low angle-habitat.

• Based on RBT distribution RBT recruits 

would be most vulnerable to stranding in 

May-June.

Problems are:

1) Unable to forecast RBT recruitment until 

July.

2) By July, only 50% of RBT recruits remain 

in low angle-habitat.

3) BNT recruits don’t appear to occupy 

wetted shoreline (low-angle or highangle habitat) until September-October. 

4) By September-October, BNT recruits are 

large in size, and not likely vulnerable to 

stranding.

0

20

40

60

80

100

Month

% of population in low-angle habitat

May June July Aug. Sept. Oct. Nov. De

2004

2006

2007

2008

2009

2010

Korman and others, 2011 Preliminary data, do not cite
RELATIONSHIP BETWEEN CONDITION FACTOR AND MATURITY RATE

Egg Production - Weight Condition factor - Maturity

Brian Healey, NPS

unpublished data

Both Sexes

Preliminary data, do not cite

Condition factor - Maturity

Preliminary data, do not cite.
CONCLUSIONS

Population Status and Trends

• Rainbow trout abundance of catchable sized fish has 

increased and remained stable for the last 3-years.

• Since the RBT population crash, RBT relative 

condition is good, above 1.0

• 2017 was the last moderately large RBT recruitment 

(age-0) event.

• Since 2015, brown trout relative abundance (CPUE) 

has progressively increased.

• BNT relative condition and growth remains good and 

appears higher than RBT.

• Since 2015, annual recruitment events for BNT have 

been relatively large with the exception of 2017 (i.e., 

the only year that RBT had a moderately large 

recruitment year since 2011).

• Difference in electrofishing catch between the two 

trout species suggest that BNT are not occupying the 

near shoreline (wetted edge) when smaller in size 

(i.e., vulnerable only during fall and winter.

Experimental Flow Status and Trends

• Reduction in invertebrate prey 

production/growth rates due to decreased SRP 

likely led to the RBT population crash (2014- 2015).

• Fall High-Flow Experiments may have an effect on 

trout growth and condition, but at this time the 

effect appears to be weak and inconclusive.

• Stable Summer Flows (“Bug Flows”) appears not 

to have had a positive effect on RBT growth and 

overall condition (i.e, other extrinsic factors).

• The implementation of Trout Management Flows 

- Researchers are currently limited in being able 

to forecast recruitment prior to, or during the 

ideal time period (specified by LTEMP design).

Preliminary data, do not cite