Exotic brown trout impacts: the case of a “novel” predator

Gary P. Thiede, Utah State University

Phaedra Budy, US Geological Survey, Utah Cooperative Fish and Wildlife Research Unit

Christy Meredith, Utah State University, now with US Forest Service

W. Carl Saunders, Utah State University

Identifying extent and degree of effects of invasive species is critical

Understanding factors that determine success

Ability to effectively prioritize control efforts

Often, effects remain poorly understood and overlooked when invader has “value”

What makes a successful invasive species ?

Depends on invader traits

Depends on characteristics of environment

Broad feeding niche

+ Choose high-energy prey

+ Establish high densities

____________________

“ SUCCESS ”

Brown trout

Wide array of life-history expressions and plasticity

Occupy wide niche for a trout

Optimal temps 12 – 18 oC; however, max thermal tolerance near 30 oC

Perform greatest in prey-rich streams, but perform well in low productivity streams

And... extremely high propagule pressure (introduction effort)

Objective

Determine the predatory impact of this exotic predator on the native fish community in the Logan River, Utah

Study area Stronghold for Bonneville cutthroat trout Cold, snowy winters Hot, dry summers Spring-snowmelt floods (15.7 cms) Base flow @ approx 2.8 cms Average summer stream temperatures

range from 9.2 - 12.1 oC Summer diel fluctuations are as high as

8.8 oC Long-term monitoring since 2001

%U

%U

%U

%U

%U

%U

%U

%U

City of Logan

ThirdDam

LowerLogan

RightHandFork

TwinBridges Temple

Fork

ForestryCamp

RedBanks

FranklinBasin

CacheNationalForest

0 2 4 6 8 10 Km

N

UTAH

Salt LakeCity

Area ofDetail

1350 m

2000 m

Methods

Field experiments: McHugh, Saunders, et al.

Laboratory experiments: Saunders, Burbank

Field evaluations: Wood, Meredith, Saunders, Gordon, Hafen, et al.

Long-term monitoring: Budy, et al.

Diet analyses: de la Hoz Franco, McHugh, VanDyke, Wood, Meredith, Gordon, et al.

Methods

Field experiments: McHugh, Saunders, et al.

Laboratory experiments: Saunders, Burbank

Field evaluations: Wood, Meredith, Saunders, Gordon, Hafen, et al.

Long-term monitoring

Diet analyses: compilation of findings from 2001 - present

Methods

Long-term monitoring

Collect fish during annual e-fishing surveys

Diet analyses

Sample fish at all elevations: take guts, gastric lavage

In laboratory: identify, weigh, and measure prey

Using bioenergetics model, estimate predatory impact (how many prey fish can brown trout consume)

Results

Very high densities (some of the highest in the universe)

High growth rates

High condition

High survivalX Data

Red Bank

Forest Camp

Temple Fk

Twin Bridge

Right H F3 Dam

Lower Log

Bro

wn

trou

t per

km

0

500

1000

1500

2000

2500

3000

3500

4000

Results mice snakes hot dogs

invertebrates80%

sculpin16%

organics 4%brown trout

0.8%

cutthroat trout0.2%

Brown trout length (mm)200 250 300 350 400 450

Leng

th (

mm

) of

scu

lpin

eat

en

0

50

100

150

200

Results

Capable of eating large prey fish

Eat prey fish smaller than gape limit

Gape limit

X Data

Red Bank

Forest Camp

Temple Fk

Twin Bridge

Right H F3 Dam

Lower Log

Bro

wn

trou

t per

km

0

500

1000

1500

2000

2500

3000

3500

4000

Consume cutthroat Cannibalism

No sculpinpresent

Consume sculpin where present

Bioenergetics modeling

Temperature Growth

Diet

Output = biomass of prey consumed

Results

Bioenergetics modeling of consumption: An average age-2 brown trout (18 – 26 cm;

7 – 10 inches) consumes 13 sculpin a year Expanded by abundance, that is up to 2500

sculpin in a single stream reach

Results

Distinct negative relationship between cutthroat and brown density

Not so clear with sculpin and whitefish density (between dams)

Cutthroattrout

Native fish density (fish/m2)

0.000.05

0.100.15

0.20

Bro

wn

trou

t den

sity

(fis

h/m

2 )

0.00

0.05

0.10

0.150.30

0.35

0.40

0.45

Sculpin

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Whitefish

0.000.01

0.020.03

Considerations

Sculpin eat small brown trout and likely eat brown trout eggs

Are these naturalized, non-native brown trout fulfilling the niche of extirpated fluvial cutthroat trout ?

Comparison: diets of stream-dwelling brown trout in native vs exotic locations

USA west

Aquatic invertebratesTerrestrial invertebratesFish

USA south

Norway Denmark Spain

USA midwestNew Zealand

Comparison: maximum size of brown trout in native vs exotic locations

Major geographical area

DenmarkNorway

Spain - AstSpain C-L

USA midwestUSA west

New ZealandUSA south

Max

imum

siz

e (m

m)

100

200

300

400

500

600

700

One of the world’s most successful and “deleterious” invaders, brown trout are superior competitors are highly aggressive are cannibalistic prey on native fish consume thousands of sculpin are restricted to lower elevations ... but for how long ?

are not as benign as previously thought

Summary

Very preliminary findings

After three years of removal work “between the dams”, where all species co-occur Sculpin abundance continues to fluctuate widely

(likely due to sculpin behavior, removal techniques) Stocked rainbow trout numbers have increased Whitefish abundance has increased

( first sign of whitefish recruitment in over 10 years ? ) Cutthroat trout abundance has increased

Management

Anglers love them

Science shows they impact native fishes

Managers must balance the trade off

Acknowledgements• Utah DWR• Sport Fish Restoration• USGS Utah Coop Unit• S.J. Quinney Foundation• NSF Advance Program• Ecology Center @ USU• ICRRR @ USU• Fish Ecology Lab @ USU

• Pete McHugh• Jeremiah Wood• E. de la Hoz Franco• Erin Van Dyke• Wes Gordon• Konrad Hafen• Nora Burbank• Dan Weber

AFS Policy Statement –#15 Introduction of Aquatic Species:

...Documentation of predation by introduced species on native species serves as the most definitive example of impacts on communities...