TRGD: Trout Recruitment, Growth 

and Population Dynamics

Project Element: H.1

Glen Canyon Dam Adaptive Management Program

Annual Reporting Meeting 2020

Tuesday, January 14, 2020

Michael Yard, USGS GCMRC

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) rainbowand 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. 
GLEN CANYON STUDY AREA, TRGD SAMPLING DESIGN

TRGD SAMPLING DESIGN

• Years 2017 – P

• 4 Full trips per year

• Trips: Jan, Apr, Sep, & Oct

• Sample 3 Subreaches

• 6 Nights / trip of sampling

• Subreach - 3 km

• 1 Single mid-summer trip 

• 1C Subreach

• 2 passes (2 nights)

• 3 km

1C

Lowest subreach

1B

Middle subreach

1A

Upper subreach
RAINBOW TROUT ABUNDANCE AND CONDITION

1C - Subreach

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

Preliminary data, do not cite

(2017)
INVERTEBRATE PRODUCTION

(Prey: chironomids, gammarus, and simuliids)

0

5

10

15

20

25

30

35

0

0.05

0.1

0.15

0.2

0.25

12 13 14 15 16 17

Annual Invertebrate Production - (Prey Items)

(g afdm m-2 yr-1 )

Daily Invertebrate Production - (Prey Items)

(g afdm m-2 da-1 )

YEAR

Daily Prey Production (Feeding

Efficiency Between 50% to 90%)

Annual Prey Production

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 (1A-1B)

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

downstream (1C)

• Condition Factor higher upstream

1A

(Upstream)

1B

(Middle)

1C

(Lower)

Preliminary data, do not cite

LARGE RAINBOW TROUT

Crash Post-Crash

Preliminary 

data, do 

not cite
COMPARISON BETWEEN RAINBOW TROUT AND BROWN TROUT

RAINBOW TROUT

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 - MONTHLY GROWTH RATES, slide 1

Mean Monthly Growth Rate 

(g/month)

RAINBOW TROUT

• RBT growth higher in lateWinter/Spring and Summer

• RBT growth is more 

reduced in lateSummer/Fall and earlyWinter

Preliminary data, 

do not cite
2019 - MONTHLY GROWTH RATES, slide 2

Mean Monthly Growth Rate 

(g/month)

BROWN TROUT

• 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 November-January,

whereas, Rainbow Trout (RBT) around mid-March. 

• 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 effected 

by TMFs in late Spring and Summer

(2017)

2015

2016

2017

2018

2019

Preliminary data, do not cite
Fall-High Flow Experiment -- Rainbow Trout

Mean growth for 266 g 

RBT (typical weight for 

a 300 mm fish)

2015

NO-HFE

2016

HFE

2017

NO-HFE

2018

HFE

Population Crash

2019

NO-HFE

?

Preliminary data, 

do not cite
Stable Flows – “Bug Flows”

Rainbow Trout

Mean growth for 266 g 

RBT (typical weight for a 

300 mm fish)

No Bug Flows

2015 2016 2017

Bug Flows

2018 2019

Preliminary data, 

do not cite
Stable Bug-Flows

Rainbow Trout

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

No Bug Flows

2015 2016 2017

Bug Flows

2018 2019

Preliminary data, 

do not cite
Trout Management Flows, slide 1

Purpose of TMFs is to regulate large 

recruitment events (excess age-0 trout) to 

avoid downstream dispersal

Problems are:

• Uncertain 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, slide 2 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 SeptemberOctober. 

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 et al. 2011 Preliminary data, do not cite
Trout Management Flows, slide 3

0

200

400

600

800

1000

Age-0 abundance (x 1000)

2003

2004

2006

2007

2008

2009

2010

Apr May Jun Jul Aug Sep Oct Nov Dec

• Purpose of TMFs is to regulate 

large recruitment events 

(excess age-0 trout) to avoid 

downstream dispersal

• Density dependent 

compensation

Preliminary data, do not cite
RAINBOW TROUT – ANNUAL RECRUITMENT

1C 

- Subreach

2012 2015 2016 2017 2018 2019

Preliminary 

data, do not 

cite
Egg Production – Weight Condition factor - Maturity

Brian Healey, NPS

unpublished data

Both Sexes

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 reservoir dynamicslikely 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 is 

limited by researchers inability to forecast 

recruitment prior to, or during the ideal time 

period (specified by LTEMP design)

Preliminary data, do not cite
GLEN CANYON STUDY AREA, NATAL ORIGIN SAMPLING DESIGN 

NATAL ORIGIN 

GLEN CANYON SAMPLING DESIGN 

• Years 2012 – 2016

• 4 seasonal trips per year

• Trips: Jan, Apr, Jun/Jul, & Sep

• Sampled 1 Subreach (1C)

• 2 nights per trip

• 2 Single passes (4 km 

lengths)

• 2 km overlap (Robust 

Design)

• Sampled all of Glen Canyon

• 1 to 2 trips per year

• 7 to 10 days per trip

• Late-Oct/Nov & Dec/Jan

• 25 km length