Diet and isotopic niche overlap elucidate competition potential between seasonally sympatric phocids in the Canadian Arctic
Interspecific interactions may be altered as a result of poleward species range shifts caused by climate change. In recent decades, Cumberland Sound, Nunavut, in the Canadian Arctic, has undergone concurrent increases in the availability of the forage fish capelin (Mallotus villosus) and the number of migratory harp seals (Pagophilus groenlandicus) during the open-water period; however, the impacts of these changes on endemic Arctic species, such as ringed seals (Pusa hispida), have received little attention. We coupled stomach contents with stable isotope analysis (δ13C and δ15N) of muscle and liver to determine the extent of potential competition between ringed seals (n = 91) and harp seals (n = 39) in Cumberland Sound. Isotopic niche breadth was greater for ringed seals (95% ellipse: 10.08‰2) than for harp seals (95% ellipse: 8.00‰2), and harp seal isotopic niche overlapped more with ringed seal isotopic niche than vice versa, suggesting asymmetrical competition potential. Although there was high overlap in isotopic niche breadth (range 50.3–91.0%) and prey species consumed (Schoener’s Index 0.60), stomach content analysis revealed differences in prey species proportions and size composition, thereby reducing the degree of realized niche overlap. Harp seals consumed a higher biomass of fish (66.7%) than did ringed seals (31.9%), and harp seals also consumed larger capelin (64–200 mm), polar cod (Boreogadus saida; 28–194 mm), and Liparidae (55–115 mm) than ringed seals (63–154 mm, 20–189 mm, and 16–128 mm, respectively). With climate change and range shifts predicted to continue into the future, our results provide an important baseline for future studies examining interspecific interactions.
The authors gratefully acknowledge the Hunters and Trappers Organization in Pangnirtung and its hunters for the collection of seal samples. We also thank Blair Dunn and Brent Young for sample collection and preparation, and Wojciech Walkusz and Rick Wastle for their assistance with seal prey identification. Thanks also to Justine Hudson, Tera Edkins, Maha Ghazal, Kevin Crook, Mikala Epp, and Alison Loeppky for help in the lab. We thank the Chemical Tracers Lab at the Great Lakes Institute for Environmental Research (University of Windsor) for stable isotope analysis. This study was supported by funding from ArcticNet, Fisheries and Oceans Canada, the Natural Sciences and Engineering Research Council Discovery Grants (GKD and SHF), the University of Manitoba Graduate Enhancement of Tri-Council Funds (WRO), Nunavut Wildlife Management Board, Ocean Tracking Network, and the W. Garfield Weston Foundation (DJY). We thank two anonymous reviewers for their feedback which improved the quality of this manuscript.
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Conflict of interest
Funding for this research was provided by ArcticNet, Fisheries and Oceans Canada, the Natural Sciences and Engineering Research Council Discovery Grants (GKD and SHF), the University of Manitoba Graduate Enhancement of Tri-Council Funds (WRO), Nunavut Wildlife Management Board, Ocean Tracking Network, and the W. Garfield Weston Foundation (DJY). The authors declare that they have no conflicts of interest and that all research was carried out under compliance with the current laws in Canada.
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