Native-nonnative Interactions; 

Factors Influencing Predation and 

Competition

David Ward, Rylan Morton-Starner, Ben Vaage, 

Grand Canyon Monitoring and Research Center
Project G: Trout/chub interactions

• Effects of competition between rainbow and 

brown trout and humpback chub

• Relative predation vulnerability of juvenile chub 

to adult conspecifics and to rainbow and brown 

trout

• Effects of water temperature, fish size and 

turbidity on predation vulnerability to rainbow 

and brown trout
Confluence of the Colorado River and Little Colorado Rivers in Grand Canyon
Yard et al. 2011. Transactions of the American Fisheries Society 140: 471-486

Rainbow and brown trout do consume native 

fish in Grand Canyon 

but population level impacts are difficult to assess because 

predation vulnerability is highly variable depending on the 

sizes of the prey and predators and physical conditions under 

which the predation interactions take place
Rocky 

Mountain 

Research 

Station
How can lab data tell us anything useful?

Lab data does not tell us what humpback chub predation 

vulnerability is in the mainstem Colorado River

- but it can tell us what environmental factors are most 

important and how relationships are likely to change
Number and sizes of fish used to evaluate the relationship between water 

temperature, fish size, turbidity and predation vulnerability

Number of Number of Trout Total Length 

(mm) Prey Species Number of

Chub Total Length (mm)

Replicates Trout Mean (Range) *(All captive reared) Chub Mean (Range)

Rainbow 12 48 299 (283 - 319) Bonytail 144 77 (70 - 85)

Trout 24 96 275 (261 - 291) Humpback chub 288 65 (45 - 87)

12 48 305 (281 - 324) Roundtail chub 144 63 (47 - 71)

24 96 288 (260 - 330) Bonytail 288 66 (60 - 70)

80 320 289 (261 - 321) Bonytail 960 66 (60 - 70)

Brown 12 48 246 (230 - 273) Bonytail chub 144 77 (67 - 90)

Trout 36 144 263 (218 - 341) Humpback chub 432 70 (45 - 91)

12 48 231 (214 - 245) Roundtail chub 144 58 (44 - 67)

24 96 288 (260 - 330) Bonytail 288 66 (60 - 70)

64 256 260 (210 - 399) Bonytail 786 67 (60 - 70)

Totals 300 1152 2658

Preliminary data, do not cite
 

 

Chub Total Length (mm)

45 50 55 60 65 70 75 80 85

Probability of Survival (%)

0

20

40

60

80

100

10 °C

15 °C

20 °C

Rainbow Trout

Percent (%) probability that a juvenile chub will survive predation by a 285 

mm rainbow trout as chub size increases from 45-85 mm TL at 10, 15 and 

20 °C, with trout size held constant at 285 mm TL. 

Preliminary data, do not cite
 

 

Chub Total Length (mm)

45 50 55 60 65 70 75 80 85

Probability of Survival (%)

0

10

20

30

40

50

10 °C

15 °C

20 °C

Brown Trout

Percent (%) probability that a juvenile chub will survive predation by a 285 mm 

brown trout as chub size increases from 45-85 mm TL at 10, 15 and 20 °C, with 

trout size held constant at 285 mm TL. Note that the y-axis on the brown trout is 

reduced by half.

Preliminary data, do not cite
Roundtail chub grown at three water temperatures in the laboratory for 9 months

C. Moran – PhD Student at NAU – 2013 unpublished data

Its not just about instantaneous predation vulnerability 

its also about growth!
 

 

Trout Total Length (mm)

220 230 240 250 260 270 280 290 300

Probability of Survival (%)

0

20

40

60

80

100

10 °C

15 °C

20 °C

Rainbow Trout

Percent (%) probability that a juvenile chub at 55 mm TL will survive predation 

by rainbow trout as trout size increases from 220 – 300 mm TL at 10, 15 and 20 

°C, with chub size held constant at 55 mm TL.

Preliminary data, do not cite
 

 

Trout Total Length (mm)

220 230 240 250 260 270 280 290 300

Probability of Survival (%)

0

10

20

30

40

50

10 °C

15 °C

20 °C

Brown Trout

Percent (%) probability that a juvenile chub at 55 mm TL will survive 

predation by brown trout as trout size increases from 220 – 300 mm TL at 

10, 15 and 20 °C, with chub size held constant at 55 mm TL. Note that the yaxis on the brown trout is reduced by half.

Preliminary data, do not cite

Turbidity

Sifted mud down to <63 

microns and blended before 

mixing in trial tanks

Paria
50 FNU 200 FNU 500 FNU 1000 FNU

500 FNU

50 FNU

Preliminary trails 
Turbidity (FNU)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Probability of Survival (%)

0

20

40

60

80

100

Mean

95 % C.I.

Percent probability of survival for juvenile humpback chub (60 mm TL) 

exposed to predation by rainbow trout (285 mm TL) as turbidity 

increases from 0 – 150 FNU at 15° C. 

Rainbow trout

Preliminary data, do not cite
Rainbow Trout Summary

25 NTU 50 NTU 75 NTU 100 

NTU

150 

NTU

46 % 

survival

62 %

survival

60 %

survival

75 %

survival

80 %

survival

Clear trials: 0 % survival Preliminary data, do not cite
Brown trout

Turbidity (FNU)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Probability of Survival (%)

0

10

20

30

40

50

Mean

95 % C.I.

Percent probability of survival for juvenile humpback chub (60 mm TL) 

exposed to predation by brown trout (285 mm TL) as turbidity 

increases from 0 – 150 FNU at 15° C. 

Brown trout

Preliminary data, do not cite
Total length of chub as prey (mm)

40 50 60 70 80 90

% Probability of survival

0

20

40

60

80

100

Roundtail chub

Rainbow trout

95 % Confidence Intervals

Percent probability of survival for juvenile chub exposed to predation by adult 

roundtail chub (260 mm TL) or rainbow trout (260 mm TL) at 20° C in overnight 

laboratory trials.

20 °C

Preliminary data, do not cite
Percent probability of survival for juvenile chub exposed to predation by 

adult roundtail chub (260 mm TL) at 15° C or rainbow trout (260 mm TL) at 

10° C in overnight laboratory trials.

Chub mm (TL)

40 50 60 70 80 90

% Probability of survival

0

20

40

60

80

100

15 °C

20 °C

Preliminary data, do not cite
Chub size (TL mm)

40 50 60 70 80 90

% Probability of survival

0

20

40

60

80

100

Percent probability of survival for juvenile chub exposed to predation by adult 

roundtail chub (260 mm TL) at 10° C or rainbow trout (260 mm TL) at 10° C in 

overnight laboratory trials.

10 °C

20 °C

Preliminary data, do not cite
Preliminary data, do not cite
Lets evaluate the lab data in terms of 

what we see in the field

1. Differences in water temperature

2. Changes in Prey Size

3. Changes in average trout size

4. Annual variability in turbidity
Average = 11.1 °C

2005

Average = 16 °C

5 °C increase

1998

(UGSG gage 09383100: http://www.gcmrc.gov/discharge_qw_sediment/station/GCDAMP/09383100)

Water Temperatures: Colorado River near Little Colorado River Confluence
Humpback chub TL (mm)

20-2930-3940-4950-5960-6970-7980-8990-99

Frequency

0

200

400

600

800

1000

1200

1400

Humpback chub TL (mm)

20-2930-3940-4950-5960-6970-7980-8990-99

Frequency

0

200

400

600

800

1000

1200

1400

2013

2014

10 mm increase

In YOY HBC this year

Dzul and Yakulic, GCMRC, 2014 unpublished Data, do not cite

Length Frequency histograms for Age-0 humpback chub in July in the Little Colorado River
RBT above and below the LCR 2014

Total Length (mm)

40-49

50-59

60-69

70-79

80-89

90-99

100-109

110-119

120-129

130-139

140-149

150-159

160-169

170-179

180-189

190-199

200-209

210-219

220-229

230-239

240-249

250-259

260-269

270-279

280-289

290-299

300-309

310-319

320-329

330-339

340-349

350-359

360-369

370-379

380-389

390-399

400-409

Frequency

0

20

40

60

80

Preliminary data, do not cite
Annual variability in turbidity

Turbidity at Grand Canyon gauge July 1 to Nov 1 

Year

2008 2009 2010 2011 2012 2013 2014

# of Days with Turbidity above 40 FNU

20

40

60

80

100

120

Preliminary data, do not cite
Evaluate Scenarios in JMP
Conclusions

• Brown trout are bad news for native fish!

• A small chub is a vulnerable chub!

• Increases in chub size greatly reduce 

predation vulnerability!
Conclusions

• Relatively small changes in turbidity may be 

sufficient to alter predation dynamics of 

rainbow trout on humpback chub!