| Summary: | Citation : Barnes, C. L., A. H. Beaudreau, M. W. Dorn, K. K. Holsman, and F. J. Mueter. 2020. Development of a predation index to assess trophic stability in the Gulf of Alaska. Ecological Applications. Overview This repository details the methods used to calculate indices of predation for Walleye Pollock ( Gadus chalcogrammus ) in the Gulf of Alaska (MT per year; 1990 to 2015). Pollock predators included: Arrowtooth Flounder ( Atheresthes stomias ), Pacific Cod ( Gadus macrocephalus ), Pacific Halibut ( Hippoglossus stenolepis ), Sablefish ( Anoplopoma fimbria ), and Walleye Pollock conspecifics. We used predation indices to estimate synchrony in pollock consumption and make inferences about trophic stability among demersal fishes in the Gulf of Alaska. File Structure Input data (survey and food habits) and shapefiles can be found in Folder 1 ('1_Data' folder). Folders 2 through 5 contain species-specific analyses and resulting estimates for each component of the predation index. Results from distribution/density models ( i.e. , large .rda files) were not included. Folder 6 contains predation indices (all predators combined) for each of the spatial scales of interest: basin, the area encompassed by the stock assessment for Gulf of Alaska pollock, subregion, and statistical area. Script files in Folder 6 also include variance ratio calculations, which enabled estimates of synchrony and portfolio effects. Specific analyses that resulted in publication tables and figures are noted throughout. All analyses were conducted using R v3.6 (R Core Team 2018). Data Sources Total Predator Biomass: Total biomass estimates were obtained from the most recent stock assessment for each groundfish predator (Barbeaux et al. 2017, Dorn et al. 2017, Hanselman et al. 2017, Spies et al. 2017, Stewart and Hicks 2017). Coast-wide estimates for Pacific Halibut were adjusted to reflect biomass in the Gulf of Alaska. Relative Predator Densities: Bottom trawl survey data (all groundfish predators; 1990 to 2017) were collected by the ...
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