FORAGING ECOLOGY OF NONNATIV

THE COLORADO RIVER, GRAND C

PREDATION ON NATIVE FISH

AND THE EFFECTS OF TURBI

Michael D. Yard 1,2, Lewis G. Coggins 1, and Col

1 Grand Canyon Monitoring and Research Center, U- Survey, Flagstaff, AZ, U.S.A.2 Stream Ecology Center, Department of Biological S- State University, Pocatello, ID, U.S.A.

Preliminary res
Adult Humpback Chub Abundance (Age 4+)

Adult Abundance (Age 4+)

Year

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

0

2000400060008000

1000012000

2002

Preliminary res
Adult Humpback Chub Abundance (Age 4+)

Adult Abundance (Age 4+)

Year

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

0

2000400060008000

1000012000

Preliminary res
OBJECTIVES

1. Determine the incidence of piscivor

trout and brown trout on native fish

2. Compare the use vs. availability of 

invertebrate and fish prey by these 

3. Evaluate how turbidity affects prey 

utilization, including the degree of p

a. Model the effects of turbidity on drift fob. Estimate the quantity of native fish con

nonnative trout under management scwithout fish suppression

Preliminary res
Preliminary res
Sampling Method 

(2003-2004)

• Fish Sampling

– Electrofishing 

• Depletion p

– 2 to 5 p– 6 trips / – 2 years 

Preliminary res
Sampling Method

• Fish Sampling• Prey Availability

– Drift Monitorin

• Sampling (

– Benthic Monit

• Sampling (

– Electrofishing 

• Sampling (

Preliminary res
Diet Analysis

• Standard Fi

– Abundan– Condition 

,

Preliminary res
Diet Analysis

• Standard Fish • Frequency of O

• Rainbow • Brown T

– Incidence o– Stomach em

Preliminary res
Diet Analysis

• Standard F• Frequency • Diet Compo

– Stratified-

• Trip• Specie• Locatio• Size (a

– Samples

• Rainbo• Brown 

– Diet propo

Preliminary res
Diet Analysis

• Standard F• Frequency • Diet Comp• Diet Indice

– Stomach – Drift Elec

,

Preliminary res
Diet Analysis

• Standard F• Frequency • Diet Comp• Diet Indice• Models

– Encounte– Piscivory 

,

Preliminary res
UPSTREAM

0

100020003000400050006000

ABUNDANCE

0

50

100150200250300

ABUNDANCE

DOW NSTREAM

0

100020003000400050006000

ABUNDANCE

0

50

100150200250300

ABUNDANCE

RAINBOW TROUT BRO

( 77% )

( 23 % )

2003 2004 2005 2006 2003 22003 2004 2005 2006 2003 2

Prelimin
UPSTREAM

0

100020003000400050006000

ABUNDANCE

0

50

100150200250300

ABUNDANCE

DOW NSTREAM

0

100020003000400050006000

ABUNDANCE

0

50

100150200250300

ABUNDANCE

RAINBOW TROUT BRO

( 98% ) 

2003 2004 2005 2006 2003 22003 2004 2005 2006 2003 2

Prelimin
RAINBOW TROUT

0.60.81.01.21.4

CONDITION FACTOR

0.60.81.01.21.4

CONDITION FACTOR

UPSTREAMDOWNSTREAM

Two-way ANOVA: F2, 21 = 13.7, p < 0.01

Season p < 0.01

Two-way 

Seaso

WINTER SUMMER WINTER SUMMER WINTER S

2003 2004 200

Preliminary results – subject to review and revision
RAINBOW TROUT

0.60.81.01.21.4

CONDITION FACTOR

0.60.81.01.21.4

CONDITION FACTOR

UPSTREAMDOWNSTREAM

Two-way ANOVA: F2, 21 = 13.7, p < 0.01

Location p < 0.02

Two-way 

Locatio

WINTER SUMMER WINTER SUMMER WINTER S

2003 2004 200

Preliminary results – subject to review and revision
Research Question

• Why were there differences amo

Preliminary res
Research Question

• Why were there differences amo• Why were fish abundance levels 

condition factors different betweand downstream sites?

Preliminary res
Research Question

• Why were there differences amo• Why were fish abundance levels 

condition factors different betweand downstream sites?

• Were these spatial differences r

food availability?

Preliminary res
Research Question

• Why were there differences amo• Why were fish abundance levels 

condition factors different betweand downstream sites?

• Were these spatial differences r

food availability?

• And how did prey availability an

contribute to the incidence of pis

Preliminary res
INCIDENCE OF PREDATIO

(Rainbow and Brown Trout)

P

ORIGIN TYPE PROP

AQUATIC FISH 9

OTHER LIZARDS BIRDS BATS 

UNKNOWN VERTEBRATE 

TERRESTRIAL

VERTEBRATE PREY

Preliminary res
INCIDENCE OF PISCIVOR

OBSERVATIONS

Season Location xP N MIP 

Upstream 46 5,347 0.9% Downstream 25 1,260 2.0% Upstream 22 2,742 0.8% Downstream 39 1,528 2.6% Upstream 13 2,382 0.5% Downstream 11 1,030 1.1% Upstream 5 924 0.5% Downstream 23 772 3.0% Upstream 4 84 8.3% Downstream 16 48 33.3Upstream 4 42 9.5% Downstream 29 63 46.0Upstream 6 59 10.2Downstream 50 109 45.9Upstream 4 25 16.0Downstream 11 40 27.5

WINTER 2004SUMMER 2004

 BROWN TROUT RAINBOW TROUT SUMMER 2004 WINTER 2004 SUMMER 2003 WINTER 2003 WINTER 2003 SUMMER 2003

Prelimin
INCIDENCE OF PISCIVOR

OBSERVATIONS

Season Location xP N MIP 

Upstream 46 5,347 0.9% Downstream 25 1,260 2.0% Upstream 22 2,742 0.8% Downstream 39 1,528 2.6% Upstream 13 2,382 0.5% Downstream 11 1,030 1.1% Upstream 5 924 0.5% Downstream 23 772 3.0% Upstream 4 84 8.3

Downstream 16 48 33.3Upstream 4 42 9.5Downstream 29 63 46.0Upstream 6 59 10.2Downstream 50 109 45.9Upstream 4 25 16.0Downstream 11 40 27.5

WINTER 2004SUMMER 2004

 BROWN TROUT RAINBOW TROUT SUMMER 2004 WINTER 2004 SUMMER 2003 WINTER 2003 WINTER 2003 SUMMER 2003

Prelimin
INCIDENCE OF PISCIVOR

OBSERVATIONS

Season Location xP N MIP 

Upstream 46 5,347 0.9% 

Downstream 25 1,260 2.0

Upstream 22 2,742 0.8% 

Downstream 39 1,528 2.6

Upstream 13 2,382 0.5% 

Downstream 11 1,030 1.1

Upstream 5 924 0.5% 

Downstream 23 772 3.0

Upstream 4 84 8.3% 

Downstream 16 48 33.3

Upstream 4 42 9.5% 

Downstream 29 63 46.0

Upstream 6 59 10.2

Downstream 50 109 45.9

Upstream 4 25 16.0

Downstream 11 40 27.5

WINTER 2004SUMMER 2004

 BROWN TROUT RAINBOW TROUT SUMMER 2004 WINTER 2004 SUMMER 2003 WINTER 2003 WINTER 2003 SUMMER 2003

Prelimin
INCIDENCE OF PISCIVOR

(Rainbow and Brown Trout)

TYPE COMMON NAME

SPECIE

NATIVE FISH FLANNELMOUTH SUCKER (Catostomus lati

BLUEHEAD SUCKER (Catostomus dis

UNIDENTIFIABLE SUCKER (Catostomus sp

HUMPBACK CHUB (Gila cypha) 

SPECKLED DACE (Rhinichthys osc

NON-NATIVE FISH FATHEAD MINNOW (Pimephales pro

RAINBOW TROUT (Oncorhynchus 

IDENTIFIABLE FISH PREY

PREY C

TYPE PROPORTIONS CO

NATIVE FISH 85.0% 

NON-NATIVE FISH 15.0% 

IDENTIFIABLE FISH PREY

Preliminary res
Empty Stomachs

• Rainbow trout stomach

– Upstream 

• Summer 15.9% • Winter 3% 

– Downstream 

• Summer 66.7% • Winter - 10.2%

• Brown trout stomachs 

– Upstream 

• Summer 59.1% • Winter 58.7% 

– Downstream 

• Summer 81.2% • Winter 74.6%

Preliminary res
Empty Stomachs

• Rainbow trout stomach

– Upstream 

• Summer 15.9% • Winter 3% 

– Downstream 

• Summer 66.7% • Winter - 10.2% 

• Brown trout stomachs 

– Upstream 

• Summer 59.1% • Winter 58.7% 

– Downstream 

• Summer 81.2% • Winter 74.6%

Preliminary res
Empty Stomachs

• Rainbow trout stomach

– Upstream 

• Summer 15.9% • Winter 3% 

– Downstream

• Summer 66.7% • Winter - 10.2% 

• Brown trout stomachs 

– Upstream 

• Summer 59.1% • Winter 58.7% 

– Downstream

• Summer 81.2% • Winter 74.6%

Preliminary res
Primary 

Aquatic Invertebrate 

Prey Available

Preliminary res
RAINBOW TROUT

0%

20%40%60%80%

100%

January

February

March

July

August

September

January

February

March

July

August

Upstream Dow nstream

DIET PROPORTION -

2003-2004

(% Weight)

Preliminary res
BROWN TROUT

0%

20%40%60%80%

100%

January

February

March

July

August

September

January

February

March

July

August

Upstream Dow nstream

DIET PROPORTION -

2003-2004 

(% Weight)

Preliminary res
RAINBOW TROUT

0123456789

10

JAN FEB MAR JUL AUG SEP JAN FEB 

2003 

STOMACH FULLNESS 

(mg dry mass / g fish wet mass)

Two-way ANOVA: F3 , 841 = 50, p < 0.01 Post-hoc tests Season (p < 0.01, higher in winte

Location (p < 0.01, higher upstreaYear (p = 0.75) 

Preliminary res
BROWN TROUT

0123456789

10

JAN FEB MAR JUL AUG SEP JAN FEB 

2003 

STOMACH FULLNESS 

(mg dry mass / g fish wet mass)

Two-way ANOVA: F3 , 369 = 7.2, 

Post-hoc tests Season (p < 0.04, higher 

Location (p = 0.06) Year (p = 0.20) 

Preliminary res
• Why are there spatial differences for rainbow trout? • Is food availability limited downstream?

Preliminary res
INVERTEBRATE DRIFT

0

0.5

1

1.5

2

2.5

3

3.5

4

January

February

March

April

May

June

CONCENTRATION (mg/L.

UpstreamDow nstream

Two-way ANOVA: F2, 119 = 16, p < 0.0

Post-hoc tests 

Seasons (p < 0.01, higher in sLocations (p = 0.68) 

2003

ZooAquTerr

10%

51%

39%

Preliminary res
Gammarus

ChironomidaeSimulidae

UPSTREAM

-0.261 

0.290 

RAIN

0.042 

ELEC

BAS

Preliminary res
Simulidae -0.158 

Gammarus 0.400 Chironomidae -0.283 

BRO

ELEC

UPSTREA

BAS

Preliminary res
Simulidae -0.158 

Gammarus 0.400 Chironomidae -0.283 

BRO

ELEC

UPSTREA

Preliminary res
JAN FEB MAR APR MAY JUN J

0

100200300400500600700800900

1000

SIMULID DENSITY / MULTI-PLATE SAMPLER

 UPST DOW

Two-way ANOVA: F3, 315 = 

Post-hoc tests 

Seasons (p < 0.01Locations (p < 0.01

Benthic Invertebrate Samples 2004

Preliminary results – subject to review and revision
Rainbow Trout

DIET PATTERNS• Abundance is less downstream (23%)• Condition factor is less downstream • Stomachs are frequently empty downstream • Stomachs are frequently empty in summer• Stomach fullness is less downstream• Stomach fullness is less in summer • Diet composition remains the same upstream aPREY AVAILABILITY• Drift prey availability is higher in summer• Drift prey availability remains the same upstrea• Benthic prey availability is higher in winter • Benthic prey availability is higher downstream

Preliminary res
Reactive Distance

SwekBarreGerrit

DT i EncounterR = ⋅ pSVate ⋅

Preliminary res
0 50 100 150 200 250 300 

ENCOUNTER RATE (g A FDM / Day)

0.00.10.20.30.40.50.60.70.80.91.0

REACTIVE DISTANCE 

(M)

 LEES FER (LITTLE C

EncounterR DTpSVate i = ⋅ ⋅

Lees F

< 1 mg/L

Prey 

Preliminary results – subject to review and revision
0 50 100 150 200 250 300 

ENCOUNTER RATE (g A FDM / Day)

0.00.10.20.30.40.50.60.70.80.91.0

REACTIVE DISTANCE 

(M)

EncounterR DTpSVate i = ⋅ ⋅

Reactiv

LCR (

Preliminary results – subject to review and revision
Jan-2003

Feb-2003

Mar-2003

Apr-2003

May-2003

Jun-2003

Jul-2003

Aug-2003

Sep-2003

Oct-2003

Nov-2003

Dec-2003

Jan-2004

Feb-2004

Mar-2004

02468

1012141618202224262830

UPSTREAM ENCOUNTER 

RATES (g/da)

Jan-2003

Feb-2003

Mar-2003

Apr-2003

May-2003

Jun-2003

Jul-2003

Aug-2003

Sep-2003

Oct-2003

Dec-2003

Jan-2004

Feb-2004

Mar-2004

02468

1012141618202224262830

DOWNSTREAM ENCOUNTER

RATES (g/da)

Preliminary res
Preliminary res
WIN SUM WIN SUM

2003 2004

WIN 

20

UPSTREAM OF THE LCR

0.0000.0020.0040.0060.0080.0100.0120.0140.0160.0180.0200.0220.024

CPUE (FISH <150 mm / 1 min)

DOW

0.0000.0020.0040.0060.0080.0100.0120.0140.0160.0180.0200.0220.024

CPUE (FISH <150 mm / 1 min)

NATIVE SMALL FISH 

WIN SUM WIN SUM

2003 2004 

WIN 

2

Preliminary res
Piscivory Estimat

∧

⋅

∧

=

∧

pNC

N Is the product of the estimated numbe

ˆ

pˆ Is the probability that a predator had c

Cˆ Is the estimated number of fish consum

Preliminary res
a Rainbow trout picivory rates expanded by largest abundance estimate (upstream = 

 Brown trout picivory rates expanded by largest abundance estimate (upstream = 10

FISH CONSUMPTION ESTIMA

Year Species Upstream Downstream Total Upstream 

2003 4,334 5,751 10,086 9,701 2004 1,389 4,682 6,071 6,830 Total 5,724 10,433 16,157 16,530 2003 626 7,088 7,713 1,948 2004 311 5,181 5,491 2,017 Total 936 12,269 13,205 3,965 2003 4,960 12,839 17,799 11,649 2004 1,700 9,863 11,563 8,847 Total 6,660 22,702 29,362 20,496 

RAINBOW BROWN COMBINED

FISH SUPPRESSION WITHO

Preliminary res
a Rainbow trout picivory rates expanded by largest abundance estimate (upstream = 

 Brown trout picivory rates expanded by largest abundance estimate (upstream = 10

FISH CONSUMPTION ESTIMA

Year Species Upstream Downstream Total Upstream 

2003 4,334 5,751 10,086 9,701 

2004 1,389 4,682 6,071 6,830 

Total 5,724 10,433 16,157 16,530 

2003 626 7,088 7,713 1,948 

2004 311 5,181 5,491 2,017 Total 936 12,269 13,205 3,965 

2003 4,960 12,839 17,799 11,649 2004 1,700 9,863 11,563 8,847 Total 6,660 22,702 29,362 20,496 

RAINBOW BROWN COMBINED

FISH SUPPRESSION WITHO

Preliminary res
a Rainbow trout picivory rates expanded by largest abundance estimate (upstream = 

 Brown trout picivory rates expanded by largest abundance estimate (upstream = 10

FISH CONSUMPTION ESTIMA

Year Species Upstream Downstream Total Upstream 

2003 4,334 5,751 10,086 9,701 2004 1,389 4,682 6,071 6,830 Total 5,724 10,433 16,157 16,530 

2003 626 7,088 7,713 1,948 2004 311 5,181 5,491 2,017 Total 936 12,269 13,205 3,965 2003 4,960 12,839 17,799 11,649 2004 1,700 9,863 11,563 8,847 Total 6,660 22,702 29,362 20,496 

RAINBOW BROWN COMBINED

FISH SUPPRESSION WITHO

Preliminary res
a Rainbow trout picivory rates expanded by largest abundance estimate (upstream = 

 Brown trout picivory rates expanded by largest abundance estimate (upstream = 10

FISH CONSUMPTION ESTIMA

Year Species Upstream Downstream Total Upstream 

2003 4,334 5,751 10,086 9,701 2004 1,389 4,682 6,071 6,830 Total 5,724 10,433 16,157 16,530 2003 626 7,088 7,713 1,948 2004 311 5,181 5,491 2,017 Total 936 12,269 13,205 3,965 

2003 4,960 12,839 17,799 11,649 2004 1,700 9,863 11,563 8,847 Total 6,660 22,702 29,362 20,496 

RAINBOW BROWN COMBINED

FISH SUPPRESSION WITHO

Preliminary res
a Rainbow trout picivory rates expanded by largest abundance estimate (upstream = 

 Brown trout picivory rates expanded by largest abundance estimate (upstream = 10

FISH CONSUMPTION ESTIMA

Year Species Upstream Downstream Total Upstream 

2003 4,334 5,751 10,086 9,701 2004 1,389 4,682 6,071 6,830 Total 5,724 10,433 16,157 16,530 

2003 626 7,088 7,713 1,948 2004 311 5,181 5,491 2,017 Total 936 12,269 13,205 3,965 2003 4,960 12,839 17,799 11,649 2004 1,700 9,863 11,563 8,847 Total 6,660 22,702 29,362 20,496 

RAINBOW BROWN COMBINED

FISH SUPPRESSION WITHOU

Preliminary res
FISH CONSUMPTION ESTI

• Piscivory appears to be a large source of mortality • Consumption estimates represent a single predat

conservative (based on 1 fish prey/24 h.)

– 14% of Rainbow trout consumed more than on– 32% of Brown trout consumed more than one 

• 77% of all fish were consumed downstream• 85% of all fish consumed were native fishes• We estimate that 20,000 humpback chub would h

2003-2004 had trout removal not occurred 

Preliminary res
Conclusion

• Detect ability rather than food availability appear to exp

rainbow spatial distribution and condition factors.

• Drift feeding appears to be an inadequate strategy for p• Higher electivity for larger prey items• Foraging strategy may shift from visual sight feeding to 

searching strategy under increased turbidity

• At high densities cumulative effects from piscivory may 

RAINBOW TROUT

Preliminary res
• Highly piscivorous, but the least abundant trout

• Brown trout distribution and condition are not correlated • Diet is not correlated with invertebrate drift availability• Incidence of piscivory is correlated with prey availability • Incidence of piscivory is not influenced by turbidity• Brown trout use a mobile foraging strategy that includes 

and piscivory

Conclusion

BROWN TROUT

Preliminary res
THE END
• Rainbow Trout

– MIP was low and varied with location and sea

• Seasons (p <0.01, summer 1.7%, and winter 1• Locations (p < 0.01, upstream 0.61%, downstre• Years (p = 0.59)

• Brown Trout

– MIP was high and varied with location

• Seasons (p =0.09) • Locations (p < 0.01, upstream 11.6%, downstre• Years (p = 0.6) 

INCIDENCE OF PISCIVOR

Preliminary res