title: Seals and sea lions are what they eat, plus what? Determination of trophic discrimination factors for seven pinniped species.
abstract: BACKGROUND
  Mixing models are a common method for quantifying the contribution of prey sources to the diet of an individual using stable isotope analysis; however, these models rely upon a known trophic discrimination factor (hereafter, TDF) that results from fractionation between prey and animal tissues. Quantifying TDFs in captive animals is ideal, because diet is controlled and the proportional contributions and isotopic values of all prey items are known.
  
  
  METHODS
  To calculate TDFs for the Hawaiian monk seal, northern elephant seal, bearded seal, ringed seal, spotted seal, harbor seal, and California sea lion, we obtained whiskers, serum, plasma, red blood cells, and prey items from nine captive individuals. We obtained δ(13) C and δ(15) N values using continuous-flow isotope-ratio mass spectrometry. The average δ(13) C and δ(15) N values from bulk and lipid-corrected prey from the diet were subtracted from the δ(13) C and δ(15) N values of each blood and whisker sample to calculate tissue-specific TDFs for each individual (∆(13) C or ∆(15) N).
  
  
  RESULTS
  The ∆(13) C values ranged from +1.7 to +3.2‰ (bulk prey) and from +0.8 to +1.9‰ (lipid-corrected prey) for the various blood components, and from +3.9 to +4.6‰ (bulk prey) or +2.6 to +3.9‰ (lipid-corrected prey) for whiskers. The ∆(15) N values ranged from +2.2 to +4.3‰ for blood components and from +2.6 to +4.0‰ for whiskers. The TDFs tended to group by tissue, with whiskers having greater ∆(13) C values than blood components. In contrast, the ∆(15) N values were greater in serum and plasma than in red blood cells and whiskers.
  
  
  CONCLUSIONS
  By providing the first TDF values for five seal species (family Phocidae) and one otariid species (family Otariidae), our study facilitates more accurate mixing models for these species. These values are particularly important for critically endangered Hawaiian monk seals and the three Arctic seal species (bearded, ringed, and spotted) that are faced with a rapidly changing environment.
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA, 95060, USA.
authors.forename: Roxanne S
authors.initials: RS
authors.lastname: Beltran
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA, 95060, USA.
authors.forename: Sarah H
authors.initials: SH
authors.lastname: Peterson
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA, 95060, USA.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: McHuron
authors.affiliation: Institute of Marine Sciences, Long Marine Laboratory, Santa Cruz, CA, 95060, USA.
authors.forename: Colleen
authors.initials: C
authors.lastname: Reichmuth
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA, 95060, USA.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Hückstädt
authors.affiliation: Department of Ecology & Evolutionary Biology, University of California, Santa Cruz, Santa Cruz, CA, 95060, USA.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Costa
mesh_terms: D000821:Animal Feed / Q000032:analysis; D000818:Animals; D002247:Carbon Isotopes / Q000032:analysis / Q000378:metabolism; D005247:Feeding Behavior; D020387:Food Chain; D013058:Mass Spectrometry; D009587:Nitrogen Isotopes / Q000032:analysis / Q000378:metabolism; D009752:Nutritional Status; D046070:Sea Lions / Q000097:blood / Q000502:physiology*; D046048:Seals, Earless / Q000097:blood / Q000502:physiology*
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes