Trophic level of commercial groundfish catch: are we

fishing down the food web in the Gulf of Alaska?

Jennifer M. MarshM.S. Fisheries Student

School of Fisheries and Ocean SciencesUniversity of Alaska Fairbanks

Trophic PyramidPrimary producers:

trophic level 1Primary consumers:

trophic level 2(…)Food webs are more

complexOmnivoryOntogenetic diet

change

5 Apex Predators

1 Primary Producers

2 Primary Consumers

3 Secondary Consumers

4 Tertiary Consumers

Trophic Level of CatchTrophic level (TL) estimates of commercial

fishery catches are used as an ecosystem-based 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 analysisSingle TL for speciesNo seasonal feeding dynamics and average length

of catch for each speciesStable isotope analysis to estimate TL

Overall Goal

Objectives1. 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.

Provide high resolution baseline information on the trophic status of the four most abundant groundfishes (arrowtooth flounder, pollock, cod, and halibut) in the GOA

Stable isotope analysisGut content analysis provides only a snapshot

of dietSIA integrates assimilated food over time

Isotope values are presented in delta notation:

δ15N has a consistent enrichment of 3.4‰ from prey to consumerδ15N can be used to assess trophic level

10001RR

XSTANDARD

SAMPLE

where X is 15N or 13C, R is the ratio of heavy to light isotope (15N:14N or 13C:12C)

• Fish were sampled in four seasons, 2000-2004, off northeast side of Kodiak Island.

Methods: Collection

Winter Spring Summer

Fall

2000 X X X

2001 X X X

2002 X X X

2003 X X X

2004 X X

All fish from Area 630

7

8

Chapter 2: AnalysesData transformations

Fitting ANCOVA modelsResponse variable: Trophic levelCovariate: lengthCategorical variables: years

Evaluate ANCOVAs using AIC

Estimate trophic level using best fit modelsCatch (Area 630)Biomass (Area 630)

ANCOVA: Full Model

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

ijiijiiij XXAY )(

Preliminary Results

Walleye pollock

Total length (cm)

0 20 40 60 80

ln (

Tro

phic

leve

l)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

2000200120022003200420002001200220032004

TL = 5

TL = 3

Pacific cod

Total Length (cm)

0 20 40 60 80 100

Tro

phic

Lev

el

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

20002001 2002 20032004 20002001200220032004

Arrowtooth flounder

Total Length (cm)

0 20 40 60 80 100

Tro

ph

ic L

eve

l

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

20002001 200220032004 20002001200220032004

Pacific halibut

Total Length (cm)

0 20 40 60 80 100 120 140

Ln(T

roph

ic L

evel

)

1.1

1.2

1.3

1.4

1.5

1.6

2000 2001 2002 2003 2004 20002001200220032004

TL = 3

TL = 5

Estimated TL of commercial catch

1999 2000 2001 2002 2003 2004 20053.53.63.73.83.94.04.14.24.34.44.5

Pacific codWalleye pol-lock

Year

Tro

ph

ic L

evel

Based on length distributions of observer catch data and total catch numbers from area 630

61

64

636

2 59

484

343

48

50

Estimated TL of commercial catch

1999 2000 2001 2002 2003 2004 20053.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

Arrowtooth flounderPacific halibut

Year

Tro

ph

ic L

evel

50

72

6549

50

64

47

486

7

68

Based on length distributions of observer catch data and total catch numbers from area 630

( )

1999 2000 2001 2002 2003 2004 20053.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

Arrowtooth flounderPacific codPacific halibutWalleye pollockWeighted Mean

Year

Tro

ph

ic L

evel

Estimated TL of commercial catch Based on length distributions of observer catch data

and total catch numbers from area 630

Estimated TL of fish populations

1980 1985 1990 1995 2000 2005 20103.2

3.4

3.6

3.8

4

4.2

4.4

Arrowtooth flounderPacific codPacific halibutWalleye pollockWeighted Mean

Survey Year

Tro

ph

ic L

evel

Based on NMFS trawl survey estimates of size composition of fish populations from area 630

Population estimates: Area 630

1980 1985 1990 1995 2000 2005 20100

200000000

400000000

600000000

800000000

1000000000

1200000000

1400000000

1600000000

1800000000

2000000000

Arrowtooth flounderPacific codPacific halibutWalleye pollock

Survey Year

Ab

un

dan

ce (

million

s)

Based on NMFS trawl survey estimates of size composition of population abundance from area

630

SummaryPollock 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

ConclusionsAre 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 analysisAllows TL to co-vary with lengthAverages diet over a longer period of timeProvides a TL range for each species

AcknowledgementsRasmuson 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

refrefi

i TLNN

TL

4.3

1515

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

Percent weight of prey items

Weighted average of 1999 & 2001

Stomachs analyzed: 1263

Length range: 7 – 75 cm

Average length:≈ 38 ± 5.5 cm

Misc. prey8%

Calanoid18%

Am-phipod

2%

Euphausiid44%

Shrimp13%

Misc. fish7%

Osmeridae5%

Walleye pollock3%

Data from: Food Habits of Groundfishes in the GOA in 1999 and 2000. Yang et al. (2006)

Pacific codMisc. prey

11%

Poly-chaete

8%

Euphausiid6%

Shrimp17%

Tanner crab10%

Other crab17%

Misc. fish11%

Am-modytidae

12%

Flatfish3%

Walleye pollock7%

Percent weight of prey items

Stomachs analyzed: 1256

Length range: 9 – 104 cm

Average length:≈ 52 ± 5 cm

Arrowtooth flounderPercent weight

of prey itemsStomachs

analyzed: 1858

Length range: 9 – 81 cm

Average length:≈ 39 ± 5 cm

Misc. prey10%

Euphausiid14%

Shrimp14%

Misc. fish13%

Am-modyti

dae11%

Osmeridae22%

Walleye pollock

7% Flatfish9%

Pacific halibutPercent

weight of prey items

Stomachs analyzed: 942

Length range: 13 – 126 cm

Average length:≈ 59 ± 7 cm

Misc. prey8% Shrimp

2%

Hermit crab11%

Tanner crab4%

Other crab20%

Misc. fish16%

Am-modytidae

11%

Walleye pol-lock20%

Flatfish7%