Abstract
Characterization of fish diets from stomach content analysis commonly involves the calculation of multiple relative measures of prey quantity (%N,%W,%FO), and their combination in the standardized Index of Relative Importance (%IRI). Examining the underlying structure of dietary data matrices reveals interdependencies among diet measures, and obviates the advantageous use of underused prey-specific measures to diet characterization. With these interdependencies clearly realized as formal mathematical expressions, we proceed to isolate algebraically, the inherent bias in %IRI, and provide a correction for it by substituting traditional measures with prey-specific measures. The resultant new index, the Prey-Specific Index of Relative Importance (%PSIRI), is introduced and recommended to replace %IRI for its demonstrated more balanced treatment of the relative measures of prey quantity, and less erroneous behavior across taxonomic levels of identified prey. As a case study, %PSIRI was used to examine the diet of the Aleutian skate Bathyraja aleutica from specimens collected from three ecoregions of the northern Gulf of Alaska (GOA) continental shelf during June-September 2005–2007. Aleutian skate were found to primarily consume the commonly abundant benthic crustaceans, northern pink shrimp Pandalus eous and Tanner crab Chionoecetes bairdi, and secondarily consume various teleost fishes. Multivariate variance partitioning by Redundancy Analysis revealed spatially driven differences in the diet to be as influential as skate size, sex, and depth of capture. Euphausiids and other mid-water prey in the diet were strongly associated with the Shelikof Strait region during 2007 that may be explained by atypical marine climate conditions during that year.
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Acknowledgements
This manuscript benefited enormously from two anonymous reviewers and from previous comments received from Sturre Hansson and Lara Ferry. We would like to thank the many people from the National Marine Fisheries Service Alaska Fisheries Science Center (NMFSAFSC) and Alaska Department of Fish and Game (ADFG) in Homer and Kodiak who provided invaluable assistance on various portions of this study. In particular, we thank Eric Brown, Sarah Gaichas, Christopher Gburski, Dan Kimura, Ned Laman, Bob Lauth, Frank Shaw, and Jim Stark from NMFS-AFSC in Seattle for their input and cooperation in allowing us to participate on survey cruises in the Gulf of Alaska. Lynne Mattes, Kally Spalinger, and Ken Goldman coordinated our participation in ADFG surveys in the northern Gulf of Alaska. Funding for this work was provided by the North Pacific Research Board project 621, the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service through the Pacific Shark Research Center and the National Shark Research Consortium, and the David and Lucile Packard Foundation. This is North Pacific Research Board contribution 306.
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Brown, S.C., Bizzarro, J.J., Cailliet, G.M. et al. Breaking with tradition: redefining measures for diet description with a case study of the Aleutian skate Bathyraja aleutica (Gilbert 1896). Environ Biol Fish 95, 3–20 (2012). https://doi.org/10.1007/s10641-011-9959-z
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DOI: https://doi.org/10.1007/s10641-011-9959-z