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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...