Spatial segregation and similar trophic-level diet among eastern Canadian Arctic/north-west Atlantic killer whales inferred from bulk and compound specific isotopic analysis

Abstract

Killer whales in the Eastern Canadian Arctic (ECA) prey on narwhal, beluga, bowhead whales and seals, while further south in the north-west Atlantic (NWA), killer whales off the coast of Newfoundland and Labrador prey on both marine mammals and fish. Bulk and amino acid (AA) specific isotopic composition of dentinal collagen in teeth of 13 ECA/NWA killer whales were analysed to assess the degree, if any, of dietary specialization of killer whales across the region. Dentine was sampled from within annual growth layer groups (GLGs) to construct chronological profiles of stable nitrogen (d 15 N) and carbon (d 13 C) isotopic compositions for individual whales spanning 3-25 years. Interannual isotopic variation across GLGs was less than that among individuals, and median bulk d 15 N values differed by up to 5‰ among individuals. Significant correlation between bulk d 15 N values and baseline (source AA) d 15 N values indicates much of the observed isotopic variation among individuals reflects foraging within isotopically distinct food webs, rather than diet differences. This interpretation is supported by consistent differences in bulk d 13 C values between the two individuals with lowest source AA d 15 N values and the remaining whales. After accounting for baseline isotopic variation, comparable d 15 N values among individuals indicates similar trophic-level diet, although uncertainties in relative trophic 15 N enrichment of individual AAs currently limits trophic position estimates for top consumers. Further research is required to clarify seasonal movement patterns and possible diet shifts of ECA/NWA killer whales to better define their role in marine ecosystems across the region.

Embracing variability in amino acid δ15N fractionation: mechanisms, implications, and applications for trophic ecology

“…However, even the multi‐TDF Glu‐ Phe approach still appeared to underestimate wild penguin TP CSIA values. Several recent studies on marine mammals similarly found that while a multi‐TDF equation improved estimates of TP CSIA , the calculated TP CSIA values were still ecologically unrealistic (Matthews and Ferguson , Ruiz‐Cooley et al. ).…”

“…In recent years, the basic assumption of a constant TDF Glu-Phe value has come under increasing scrutiny. A number of field studies calculating TP CSIA in upper trophic-level consumers (including cephalopods, teleost fishes, elasmobranchs, marine mammals, and penguins) have noted that assuming a constant TDF Glu-Phe value of 7.6‰ often led to substantially underestimated TP CSIA relative to expected values from a whole host of other metrics, including gut content analysis and feeding observations (Lorrain et al 2009, Dale et al 2011, Choy et al 2012, ruiz-Cooley et al 2013, Matthews and Ferguson 2014. Bradley et al (2015) used an elegant linear regression approach to back-calculate a TDF Glu-Phe value of 5.7‰ ± 0.3‰ from 224 wild-caught teleost fishes (47 species) based on the difference in consumer δ 15 N Glu and δ 15 N Phe values and predicted TP from published stomach content analyses.…”

Dentine oxygen isotopes (δ18O) as a proxy for odontocete distributions and movements

“…Ecological interpretations of tissue δ 13 C and δ 15 N values can be complicated by the confounding influences of trophic and physiological factors, as well as baseline variation on multiple spatial and temporal scales (see Matthews and Ferguson ). In contrast, the oxygen isotope composition of freshwater and marine vertebrate tissues primarily reflects that of ambient water (Yoshida and Miyazaki ; Coulson et al.…”

“…; Riofrío‐Lazo et al. ; Matthews and Ferguson ). To assess the applicability of δ 18 O analysis of dentine hydroxyapatite (Ca 10 [PO 4 ,CO 3 ] 6 [OH,CO 3 ] 2 ) in this context, we measured δ 18 O values of structural carbonate and phosphate ( δ 18 O SC and δ 18 O P ) in dentine of odontocete species from marine water bodies ranging in surface water δ 18 O values (LeGrande and Schmidt ).…”

“…Isotopic analysis of dentine, which grows continuously and retains its isotopic composition indefinitely (Bloom and Fawcett 1975), has recently proven useful for reconstructing individual marine mammal distributions and movements from d 13 C and d 15 N values of isolated collagen (Mendes et al 2007a,b;Martin et al 2011;Riofr ıo-Lazo et al 2012;Matthews and Ferguson 2014). To assess the applicability of d 18 O analysis of dentine hydroxyapatite (Ca 10 [PO 4 ,CO 3 ] 6 [OH,CO 3 ] 2 ) in this context, we measured d 18 O values of structural carbonate and phosphate (d 18 O SC and d 18 O P ) in dentine of odontocete species from marine water bodies ranging in surface water d 18 O values (LeGrande and Schmidt 2006).…”