Fueling phocids: Divergent exploitation of primary energy sources and parallel ontogenetic diet switches among three species of subarctic seals

Authors

  • Strahan Tucker,

    Corresponding authorCurrent affiliation:
    1. Pacific Biological Station, Department of Fisheries and Oceans, Nanaimo, British Columbia, Canada
    • Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada
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  • Garry B. Stenson,

    1. Northwest Atlantic Fisheries Centre, Department of Fisheries and Oceans, St John's, Newfoundland, Canada
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  • W. Don Bowen,

    1. Population Ecology Division, Bedford Institute of Oceanography, Department of Fisheries and Oceans, Dartmouth, Nova Scotia, Canada
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  • Sara J. Iverson

    1. Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada
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    • Author contributions: GBS designed field collections for harp and hooded seals; ST, WDB and SJI collected gray seal samples; ST analyzed the data and wrote the manuscript with the assistance of the other authors.

Corresponding author (e-mail: strahan.tucker@dfo-mpo.gc.ca).

Abstract

Determining how marine predators partition resources is hindered by the difficulty in obtaining information on diet and distribution. Stable isotopes (SI) of carbon (13C/12C, δ13C) and nitrogen (15N/14N, δ15N) provide a two-dimensional estimate of the dietary space of consumers; an animal's isotopic composition is directly influenced by what they consume and where they feed. Harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals are abundant phocid species found in the North Atlantic. We measured and contrasted SI values between seals sampled at nearshore and offshore sites to test for effects of sampling location, sex, age-class, and body size to gain insight into how these species partition space and prey resources. In addition we contrasted previously published results for gray seals (Halichoerus grypus). Isotope values differed significantly by age class and location in harp and hooded seals. We found significant differences in SI values (mean δ13C and δ15N ± SE) between all species. Hooded seals, a continental shelf-edge, deep-diving species, exhibited low SI values (juveniles: −20.9‰ ± 0.03‰, 13.36‰ ± 0.05‰; adults: −20.41‰ ± 0.03‰, 14.81‰ ± 0.04‰) characteristic of feeding on meso- to bathypelagic prey. Harp seals, which dive to moderate depths primarily on the shelf had intermediate SI values (juveniles: −20.53‰ ± 0.01‰, 13.91‰ ± 0.01‰; adults: −20.13‰ ± 0.01‰, 14.96‰ ± 0.01‰) characteristic of feeding on epipelagic prey, whereas gray seals, which feed on or near the sea floor in shallow shelf waters, had high SI values (juveniles: −19.74‰ ± 0.04‰, 17.51‰ ± 0.05‰; adults: −18.86‰ ± 0.01‰, 17.23‰ ± 0.02‰) characteristic of feeding on demersal prey. In all species, δ13C values increased with body size and age in the same manner, indicating that seals exploit or forage in deeper habitats as they get larger and older. We hypothesize that the consistent ontogenetic shift in foraging niche, despite large differences between species in their diving behavior, geographic range and habitat use, not only reflects increased access to different prey due to increased diving capacity, but a progressive adjustment to balance energy budgets by reducing foraging costs.

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