Towards an understanding of Bonneville cutthroat trout responses to riparian grazing

exclosures

Scott Miller1,2 and Phaedra Budy1,3

1Intermountain Center for River Rehabilitation and Restoration, Department of Watershed Sciences, Utah State University

2Bureau of Land Management3USGS Utah Cooperative Fish and Wildlife Research Unit

Introduction

Habitat Degradation

Invasive SpeciesDisease

Bonneville cutthroat trout (BCT)

Introduction

• Livestock grazing is leading cause of riparian and instream habitat degradation in western U.S.

• > 80% of western riparian areas affected

Introduction

Habitat Degradation

Invasive SpeciesDisease

Grazing

Bonneville cutthroat trout (BCT)

Introduction

• Riparian grazing exclosures widely implemented on public lands

• UT public lands: > 150 riparian exclosures

• Goals: Restore degraded habitat and facilitate the coexistence of grazing and native fish populations Source: ICRRR Restoration Database

Introduction

• Despite widespread implementation, few studies assess restoration efficacy

• Cutthroat: Equivocal or conflicting results

• What factors contribute to differential responses among system?

Introduction

Saunders and Fausch 2007

Grazing

Riparian vegetative cover/biomass

Terrestrial arthropod inputs

Salmonid biomass

(-)

(-)

(-)

IntroductionQuestions:1. Do BCT populations differ between grazed and ungrazed

reaches?

2. What reach-scale factors are related to differences in BCT populations between grazed and ungrazed reaches?

3. Do recovery patterns vary as a function of grazing regime, exclosure age, or exclosure size?

Site selection• 10 paired grazed and ungrazed

reaches- BCT- 2nd & 3rd order reaches- Maintained exclosure- Active grazing- Minimize geomorphic differences

between pairs

• System characteristics- Grazing regime: season long vs.

rotational- Age: 4 – 39 years- Size: 0.06 – 96%

Sampling

Upstream: Grazed Downstream: Ungrazed

Min. 1 km buffer

• Reach lengths: 20x bankfull• Sampled once: summer 2008 or 2009

Paired study design

Response variables

• Fish assemblages- 3-pass depletion- Reaches: 20x bankfull- Response variables: Density,

biomass, condition, composition

• Diet- Stomach contents - gastric lavage- Stable isotopes (δ13C and δ15N)

Response variables

• Physical habitat measurements:

ContinuousUndercut banks

Overhanging vegetationHabitat units (e.g., riffle, run, pool)

Response variables

• Physical habitat measurements:

ContinuousUndercut banks

Overhanging veg.Habitat units (e.g., riffle, run, pool)

PointDepth

VelocitySubstrate

Temperature*Width

Response variables

• Prey availability- Terrestrial arthropod prey

- 5 pan traps/reach- 48 hours

- Aerial aquatic insects- 5 pan traps/reach- 48 hours

- Benthic aquatic arthropod prey- 8 composite Surbers/reach

Results

Do BCT populations differ between grazed and ungrazed reaches?

Response Variable% change =

100* ((Ungrazed-Grazed)/Grazed)

Statistical TestWilcoxon signed rank test

-100

0

100

200

300

400

500

600

700

% C

hang

e: B

CT D

ensi

ty

-100

0

100

200

300

400

500

600

700

% C

hang

e: B

CT B

iom

ass

NA NA

NA NA

Results

• On average, density and biomass significantly greater within exclosures

• Consistent directional changes, while magnitude of change highly variable

• No differences in condition, age structure, or assemblage composition

-100

0

100

200

300

400

500

600

700

% C

hang

e: B

CT D

ensi

ty

-100

0

100

200

300

400

500

600

700

% C

hang

e: B

CT B

iom

ass

NA NA

NA NA

W+ = 34.0, P = 0.030, df =8

W+ = 33.0, P = 0.042, df =8

ResultsWhat reach-scale factors are related to differences in BCT

populations between grazed and ungrazed reaches?

Kendall’s tau: % Change BCT Density BCT BiomassProportion overhanging vegetation 0.57 0.46Aquatic benthic prey biomass 0.21 0.43Terrestrial prey biomass 0.50 0.57Aerial aquatic prey biomass 0.28 0.36Width-to-Depth ratio 0.21 0.14Proportion undercut banks 0.71 0.5Residual Pool Depth -0.07 0.14Habitat Diversity 0.21 -0.14Temperatue -0.21 0.29Substrate (D16) -0.43 -0.36

Results

BCT biomass responses proportional to increases in terrestrial prey availability

BCT density responses scaled with increases in

cover availability

-200

-100

0

100

200

300

400

500

600

-100 -50 0 50 100 150

Per

cen

t ch

ange

: BC

T b

iom

ass

Percent change: Terrestrial prey inputs

-100

0

100

200

300

400

500

600

700

-10.00 0.00 10.00 20.00 30.00 40.00

Per

cen

t ch

ange

: BC

T d

ensi

ty

Percent change: Undercut banks

R2 = 0.53

R2 = 0.41

Results

• Terrestrial prey comprised only 11% of prey availability by biomass

• BCT foraging behavior: strong preference of terrestrial prey

• 50% of ingested preyHuff SpawnTemple Dry Big Big2 Rock Rock2

Che

sson

's A

lpha

(T

erre

stri

al)

0.0

0.2

0.4

0.6

0.8

1.0

1.2UngrazedGrazed

All Sites

Ave

rage

Per

cent

cha

nge

-100

-80

-60

-40

-20

0

C.

Huff Spawn Temple Dry Big Big2 Rock Rock2 Randolph LMC

Pro

port

ion

terr

estr

ial b

iom

ass

0

5

10

15

20

25UngrazedGrazed

All Sites

Per

cent

cha

nge

10

20

30

40

50

*

ResultsDo recovery patterns vary as a function of grazing regime,

exclosure age, or exclosure size? Kendall’s tau: % Change

Grazing regime Exclosure Age Exclosure SizeAquatic benthic prey biomass 0.06 0.05 0.28Terrestrial prey biomass 0.46 0.18 0.11Aerial aquatic prey biomass 0.09 -0.3 -0.03Width-to-Depth ratio -0.43 -0.1 0.63Proportion undercut banks 0.12 -0.32 -0.14Proportion overhead vegetation 0.43 0 0.13Residual Pool Depth 0.06 0.14 -0.04Habitat Diversity 0.43 0.41 0.17Substrate 0.37 0.18 -0.2Temperature -0.3 -0.1 -0.21BCT Density -0.47 -0.07 -0.14BCT Biomass -0.47 0 -0.07Averages 0.33 0.17 0.19

Results

Weak relationships with age…

Grazed

Ungrazed

1978 (SL) 1978 (SDR)

Overriding influence of grazing practices

Results

Differential responses between grazing regimes and among abiotic and biotic variables

Terrestria

l Prey

BCT Biomass

Overhead Vegetation

Habitat Diversity

Width:Depth

Ave

rage

% C

hang

e

0

100200

300

400RotationalSeason-long

Grazed: SL Ungrazed: SL

Results

Differential responses between grazing regimes and among abiotic and biotic variables

Terrestria

l Prey

BCT Biomass

Overhead Vegetation

Habitat Diversity

Width:Depth

Ave

rage

% C

hang

e

0

100200

300

400RotationalSeason-long

Grazed: SDR Ungrazed: SDR

Results

Passive restoration bang for your buck: BCT responses

0

100

200

300

400

500

Grazed -SL

Ungrazed -SL

Grazed -SDR

Ungrazed -SDR

BCT

Den

sity

(fish

/km

)

95%

312%

560%*% Change relative to Grazed - SL

Results

Putting responses in a regional context

0

200

400

600

800BC

T D

ensi

ty (

fish/

km)

Median population level

Take home points

• Variable fish responses to grazing management are likely predictable

• Both habitat and prey resource availability (terrestrials) likely facilitate BCT recovery

• Grazing management at larger spatial scales will greatly increase efficacy of passive restoration efforts

• Landscape versus local-scale processes in facilitating abiotic and biotic recovery trajectories

Future questions

• What factors control the recovery of terrestrial arthropod assemblages

• Do exclosures facilitate increased growth and survival = source populations

• Identify interactions among grazing regime, exclosure age, and size

• How robust are these patterns

Acknowledgements

Funding• UDWR Endangered Species Mitigation Fund (# 0809)• Intermountain Center for River Rehabilitation and

Restoration• Bureau of Land Management

Field and lab assistance• Paul Mason, Gary Thiede, Reed Chaston, Dave Fowler,

Katrina Langenderfer, Ben Marett, Ellen Wakeley, and Hilary Whitcomb

Logistics and permitting• Wyoming Game and Fish Department, Idaho Fish and Game,

Utah Department of Wildlife Resources