Jennifer M. Marsh
M.S. Fisheries Student
School of Fisheries and Ocean Sciences
University of Alaska Fairbanks
Trophic Pyramid
 Primary producers:
5 Apex Predators
4 Tertiary
Consumers
3 Secondary
Consumers
2 Primary
Consumers
1 Primary Producers
trophic level 1
 Primary consumers:
trophic level 2
 (…)
 Food webs are more
complex
 Omnivory
 Ontogenetic diet
change
Trophic Level of Catch
 Trophic level (TL) estimates of commercial
fishery catches are used as an ecosystembased indicator for sustainability.
 Fishing down the food web (Pauly et al. 1998)
 Fishing through the food web (Essington et al. 2006)
 TL based on mass-balance models and gut
content analysis
 Single TL for species
 No seasonal feeding dynamics and average length of
catch for each species
 Stable isotope analysis to estimate TL
Overall Goal
Provide high resolution baseline information on
the trophic status of the four most abundant
groundfishes (arrowtooth flounder, pollock, cod,
and halibut) in the GOA
Objectives
Examine seasonal, annual and size-class
variation of trophic role (d13C & d15N) for each
species; and to
2. Estimate trophic level of commercial removals
and biomass for each species in the GOA.
1.
Stable isotope analysis
 Gut content analysis provides only a snapshot of
diet
 SIA integrates assimilated food over time
 Isotope values are presented in delta notation:
 RSAMPLE

dX  
 1  1000
 RSTANDARD 
where X is 15N or 13C, R is the ratio of
heavy to light isotope (15N:14N or 13C:12C)
 δ15N has a consistent enrichment of 3.4‰ from
prey to consumer
 δ15N can be used to assess trophic level
Methods: Collection
• Fish were sampled in
four seasons, 20002004, off northeast
side of Kodiak
Island.
2000
Winter
Spring
X
X
Summer
Fall
X
2001
X
X
X
2002
X
X
X
X
X
X
X
2003
2004
X
All fish from Area 630
7
Chapter 2: Analyses
 Data transformations
 Fitting ANCOVA models
 Response variable: Trophic level
 Covariate: length
 Categorical variables: years
 Evaluate ANCOVAs using AIC
 Estimate trophic level using best fit models
 Catch (Area 630)
 Biomass (Area 630)
8
ANCOVA: Full Model
Yij    Ai  i ( X ij  X i )   ij
Where:
μ is the average trophic level
Ai is the year effect (i = 1 to 5 years)
Xij is the covariate (length) measured for
observation Yij (trophic level)
Xbar is the average value of the covariate for
treatment group i
βi is the slope term for length (covariate)
εij is the error term
Preliminary Results
1.7
Walleye pollock
TL = 5 1.6
ln (Trophic level)
1.5
1.4
1.3
1.2
TL = 3 1.1
1.0
0
20
40
Total length (cm)
60
80
2000
2001
2002
2003
2004
2000
2001
2002
2003
2004
5.0
Pacific cod
4.8
4.6
Trophic Level
4.4
4.2
2000
2001
2002
2003
2004
2000
2001
2002
2003
2004
4.0
3.8
3.6
3.4
3.2
0
20
40
60
Total Length (cm)
80
100
4.6
Arrowtooth flounder
4.4
Trophic Level
4.2
4.0
2000
2001
2002
2003
2004
3.8
3.6
2000
2001
2002
2003
2004
3.4
3.2
0
20
40
60
Total Length (cm)
80
100
TL = 5
1.6
Pacific halibut
Ln(Trophic Level)
1.5
1.4
2000
2001
2002
2003
2004
2000
2001
2002
2003
2004
1.3
1.2
TL = 3
1.1
0
20
40
60
80
Total Length (cm)
100
120
140
Estimated TL of commercial catch
Trophic Level
Based on length distributions of observer catch data
and total catch numbers from area 630
4.5
4.4
4.3
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
1999
64
61
63
62
59
Pacific cod
Walleye
pollock
50
2000
48
43
2001
2002
Year
43
2003
48
2004
2005
Estimated TL of commercial catch
Based on length distributions of observer catch data
and total catch numbers from area 630
4.5
4.4
Trophic Level
4.3
4.2
4.1
4.0
72
50
65
49
64
50
3.9
67
48
47
3.8
3.7
( 68 )
3.6
3.5
1999
Arrowtooth
flounder
Pacific halibut
2000
2001
2002
Year
2003
2004
2005
Estimated TL of commercial catch
Based on length distributions of observer catch data
and total catch numbers from area 630
4.5
4.4
Trophic Level
4.3
4.2
Arrowtooth flounder
4.1
4.0
Pacific cod
3.9
Pacific halibut
3.8
Walleye pollock
3.7
Weighted Mean
3.6
3.5
1999
2000
2001
2002
Year
2003
2004
2005
Estimated TL of fish populations
Based on NMFS trawl survey estimates of size
composition of fish populations from area 630
4.4
Trophic Level
4.2
4
Arrowtooth flounder
Pacific cod
Pacific halibut
Walleye pollock
Weighted Mean
3.8
3.6
3.4
3.2
1980
1985
1990 1995 2000
Survey Year
2005
2010
Population estimates: Area 630
Based on NMFS trawl survey estimates of size
composition of population abundance from area 630
2000
Abundance (millions)
1800
1600
1400
Arrowtooth flounder
Pacific cod
Pacific halibut
Walleye pollock
1200
1000
800
600
400
200
0
1980
1985
1990 1995 2000
Survey Year
2005
2010
Summary
 Pollock lowest TL, cod highest TL :
 commercial catch
 NMFS trawl survey
 Drop in TL of commercial catch in 2003 for Pacific cod,
arrowtooth flounder and Pacific halibut
 TL of fish populations fluctuates after the mid
1990s for pollock and cod
Conclusions
 Are we fishing down the food web?
 Estimated TL of commercial catch remained steady
 Only represents a fraction of the catch (higher TL
predatory groundfish)
 Continued monitoring is suggested
 Advantages of stable isotope analysis
 Allows TL to co-vary with length
 Averages diet over a longer period of time
 Provides a TL range for each species
Acknowledgements
 Rasmuson Fisheries Research Center (RFRC)
 Gulf Apex Predator prey study (NOAA Fisheries)
 Advisors and committee: Robert J. Foy, Nicola
Hillgruber, Matthew Wooller, Gordon Kruse
 Alaska Stable Isotope Facility: Norma
Haubenstock & Tim Howe
 Alexander Andrews, Lei Guo, Franz Mueter
 School of Fisheries and Ocean Sciences
Questions?
Baseline Correction
 d N  d N  
 TL
TL  
15
i


i
15
3.4
ref


ref
Where:
TLi is the trophic level of organism i,
d15Ni is the measured d15N value for organism i,
d15Nref is the measured d15N value for the baseline
organism and
TLref is the TL of the baseline organism
Baseline organism = EULACHON
• assigned a trophic 3.52 based on GOA Ecopath
models (Aydin et al. 2007)
Trophic level of catch in the GOA
Walleye pollock
Osmeridae
5%
Misc.
fish
7%
Misc.
prey
8%
Walleye
pollock
3%
 Percent weight of

Calanoid
18%
Shrimp
13%
Amphipod
2%
Euphausiid
44%



prey items
Weighted average
of 1999 & 2001
Stomachs
analyzed:
1263
Length range:
7 – 75 cm
Average length:
≈ 38 ± 5.5 cm
Data from: Food Habits of Groundfishes in the GOA in 1999 and 2000. Yang et al.
(2006)
Pacific cod
Flatfish
3%
Walleye
pollock
7%
Ammodytidae
12%
Misc. fish
10%
Other crab
16%
Misc. prey
11%
 Percent weight
of prey items
 Stomachs
Polychaete
8%
analyzed:
Euphausiid
6%
1256
 Length range:
9 – 104 cm
Shrimp
17%
 Average length:
≈ 52 ± 5 cm
Tanner crab
10%
Arrowtooth flounder
Walleye
pollock
7%
Flatfish
9%
 Percent weight
Misc. prey
10%
Euphausiid
14%
Osmeridae
22%
Shrimp
14%
Ammodytidae
11%
Misc. fish
13%
of prey items
 Stomachs
analyzed:
1858
 Length range:
9 – 81 cm
 Average
length:
≈ 39 ± 5 cm
Pacific halibut
Flatfish Misc. prey
7%
8%
Walleye
pollock
21%
Shrimp
2%
Hermit crab
11%
Tanner crab
4%
Ammodytida
e
11%
Other crab
20%
Misc. fish
16%
 Percent weight
of prey items
 Stomachs
analyzed:
942
 Length range:
13 – 126 cm
 Average
length:
≈ 59 ± 7 cm