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Trophic level of commercial groundfish catch: are we fishing down the food web in the Gulf of Alaska?. Jennifer M. Marsh M.S. Fisheries Student School of Fisheries and Ocean Sciences University of Alaska Fairbanks. Trophic Pyramid. 5 Apex Predators .

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trophic level of commercial groundfish catch are we fishing down the food web in the gulf of alaska

Trophiclevel of commercial groundfish catch: are we fishing down the food web in the Gulf of Alaska?

Jennifer M. Marsh

M.S. Fisheries Student

School of Fisheries and Ocean Sciences

University of Alaska Fairbanks

trophic pyramid
Trophic Pyramid

5Apex Predators

  • Primary producers: trophic level 1
  • Primary consumers:

trophic level 2

  • (…)
  • Food webs are more complex
    • Omnivory
    • Ontogenetic diet change

4 Tertiary Consumers

3Secondary Consumers

2 Primary Consumers

1Primary Producers

trophic level of catch
TrophicLevel of Catch
  • Trophiclevel (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 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
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
  • Estimate trophic level of commercial removals and biomass for each species in the GOA.
stable isotope analysis
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:
  • δ15N has a consistent enrichment of 3.4‰ from prey to consumer
    • δ15N can be used to assess trophic level

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

methods collection
Methods: Collection
  • Fish were sampled in four seasons, 2000-2004, off northeast side of Kodiak Island.
chapter 2 analyses
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)
ancova full model
ANCOVA: Full Model

Where:

μ is the average trophic level

Ai is the year effect (i = 1 to 5 years)

Xijis the covariate (length) measured for observation Yij(trophic level)

Xbar is the average value of the covariate for treatment group i

βiis the slope term for length (covariate)

εijis the error term

slide11

TL = 5

TL = 3

slide14

TL = 5

TL = 3

estimated tl of commercial catch
Estimated TL of commercial catch

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

63

64

61

62

59

48

48

50

43

43

estimated tl of commercial catch16
Estimated TL of commercial catch

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

72

65

50

64

48

49

50

67

47

( )

68

slide17

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
Estimated TL of fish populations

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

population estimates area 630
Population estimates: Area 630

Based on NMFS trawl survey estimates of size composition of population abundance from area 630

summary
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
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
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
baseline correction
Baseline Correction
  • 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)
walleye pollock
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

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

pacific cod
Pacific cod
  • Percent weight of prey items
  • Stomachs analyzed:

1256

  • Length range:

9 – 104 cm

  • Average length:

≈ 52 ± 5 cm

arrowtooth flounder
Arrowtooth flounder
  • Percent weight of prey items
  • Stomachs analyzed:

1858

  • Length range:

9 – 81 cm

  • Average length:

≈ 39 ± 5 cm

pacific halibut
Pacific halibut
  • Percent weight of prey items
  • Stomachs analyzed:

942

  • Length range:

13 – 126 cm

  • Average length:

≈ 59 ± 7 cm