Dietary characteristics of co-occurring polar cod (Boreogadus saida) and capelin (Mallotus villosus) in the Canadian Arctic, Darnley Bay

Reduction in sea ice due to climate change is expected to have a negative impact on habitat availability for Arctic marine fishes and induce range expansion of species from southern environments. Such an effect will likely be observed in the abundance of polar cod, Boreogadus saida (Lepechin, 1774), as well as interspecific interactions of this intermediate-level trophic taxon, particularly in more southerly fringing seas in the Arctic. Polar cod and capelin, Mallotus villosus (Muller, 1776), are pelagic, planktivorous forage fishes, which occupy similar dietary niches and are the primary prey of marine predators. Co-occurring polar cod and capelin were collected at seven stations in Darnley Bay, NT, during August 2013. Standard length (SL), used as a proxy for age, suggested that polar cod (mean ± 1 SD: 71.1 ± 10.3 mm) were predominantly age 1+ and capelin (96.2 ± 13.4 mm) were mostly age 2+. Stomach content analyses indicated that both species feed extensively on calanoid copepods (Calanus hyperboreus, C. glacialis, Metridia longa) and amphipods (Themisto libellula). There was high dietary overlap between capelin and polar cod, evidenced by Schoener’s index (0.80). Additionally the quantity of dietary items, biomass and energetic content consumed differed among size classes in both capelin (SL, 70.5–132.0 mm) and polar cod (SL, 42.1–114.4 mm). This study illustrates that the diets of these sympatric forage fishes in an Arctic ecosystem are very similar, indicating a high potential for interspecific competition as the sub-Arctic capelin expands its range into Arctic regions with climate change.