Jennifer M. Marsh M.S. Fisheries Student School of Fisheries and Ocean Sciences University of Alaska Fairbanks

Trophic Pyramid Primary producers: trophic level 1 Primary consumers: trophic level 2 (…) Food webs are more complex Omnivory Ontogenetic diet change 5 Apex Predators 1 Primary Producers 2 Primary Consumers 3 Secondary Consumers 4 Tertiary Consumers

Trophic Level of Catch Trophic 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 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 Objectives 1. Examine seasonal, annual and size-class variation of trophic role (  13 C &  15 N) 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 analysis Gut content analysis provides only a snapshot of diet SIA integrates assimilated food over time Isotope values are presented in delta notation: δ 15 N has a consistent enrichment of 3.4‰ from prey to consumer δ 15 N can be used to assess trophic level where X is 15 N or 13 C, R is the ratio of heavy to light isotope ( 15 N: 14 N or 13 C: 12 C)

Fish were sampled in four seasons, , off northeast side of Kodiak Island. Methods: Collection WinterSpringSummerFall 2000XXX 2001XXX 2002XXX 2003XXX 2004XX

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 Where: μ is the average trophic level A i is the year effect (i = 1 to 5 years) X ij is the covariate (length) measured for observation Y ij (trophic level) Xbar is the average value of the covariate for treatment group i β i is the slope term for length (covariate) ε ij is the error term

Preliminary Results

TL = 5 TL = 3

TL = 5

Estimated TL of commercial catch Based on length distributions of observer catch data and total catch numbers from area

Estimated TL of commercial catch Based on length distributions of observer catch data and total catch numbers from area 630 ( )

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

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

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 Where: TL i is the trophic level of organism i,  15 N i is the measured  15 N value for organism i,  15 N ref is the measured  15 N value for the baseline organism and TL ref 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 Data from: Food Habits of Groundfishes in the GOA in 1999 and Yang et al. (2006)

Pacific cod Percent weight of prey items Stomachs analyzed: 1256 Length range: 9 – 104 cm Average length: ≈ 52 ± 5 cm

Arrowtooth flounder Percent weight of prey items Stomachs analyzed: 1858 Length range: 9 – 81 cm Average length: ≈ 39 ± 5 cm

Pacific halibut Percent weight of prey items Stomachs analyzed: 942 Length range: 13 – 126 cm Average length: ≈ 59 ± 7 cm