Abstract
Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017–2018 in northern Norway using stable isotopic nitrogen (δ15N: 15N/14N) and carbon (δ13C: 13C/12C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal‐eaters, herring‐eaters, and lumpfish‐eaters. Seal‐eaters showed higher δ15N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3–13.2‰, n = 10) compared to herring‐eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4–11.9‰, n = 19) and lumpfish‐eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3–11.9, n = 9). Elevated δ15N values for seal‐eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured δ15N values in the seal‐eaters, compared to that of whales that would eat solely seals (δN‐measured = 12.6 vs. δN‐expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish‐eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish‐eating individuals.
Highlights
The killer whale (Orcinus orca) is an apex predator found in all the world's oceans (Forney & Wade, 2006)
Mean contribution of harbor seal ranged from 17% (2.5%–97.5% quantiles: 2%–36%, FIGURE 5 Individual variation in the mean proportional contribution of harbor seal, herring, and lumpfish to the diet of sealeating killer whales (Cluster 1, n = 10) biopsy sampled in northern Norway, as estimated from Bayesian isotopic mixing models
Higher δ15N skin values measured throughout the year for individuals repeatedly observed feeding on seals in 1988–2016 (Jourdain et al, 2017; Vongraven & Bisther, 2014) supported persistent feeding at higher trophic level and marked preference for pinniped prey for these whales, though a wide isotopic niche indicated a diversified diet
Summary
The killer whale (Orcinus orca) is an apex predator found in all the world's oceans (Forney & Wade, 2006). A far-ranging generalist so-called Type 1 includes herring-feeding killer whales off Norway and Iceland, but with interindividual variation in the dietary proportions of contributing prey items, including high trophic level prey (Foote et al, 2009). This suggestion was supported by a variation in intrapopulation ecological niche in Iceland (Samarra et al, 2018; Samarra, Vighi, Aguilar, & Vikingsson, 2017), and field observations of a subset of individuals switching between pinniped and fish prey from both Norway (Vongraven & Bisther, 2014) and Iceland (Foote, Similä, Vikingsson, & Stevick, 2010). Results are discussed in light of predation records available for the sampled whales to assess consistency in individuals’ dietary habits and further evaluate the degree of prey specialization for these whales
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
