Summer and polar night diets of polar cod Boreogadus saida and Atlantic cod Gadus morhua in Kongsfjorden, Svalbard, Norway

Rapid Atlantification of marine ecosystems in the Svalbard Archipelago (Norway) is posing a threat to the local polar cod ( Boreogadus saida ) population. In Kongsfjorden, the decreasing population of polar cod is experiencing a shift in their prey community from Arctic to Atlantic zooplankton and fish. Simultaneously, the abundance of Atlantic cod Gadus morhua is increasing. In this study, we explore seasonal dietary patterns, foraging strategies, and potential competition between these 2 gadids in Kongsfjorden via stomach content analyses. Polar cod diet by proportion biomass ( B ) was dominated by pricklebacks (family Stichaeidae) and conspecifics during polar night ( B = 72.7%) and by Calanus spp. ( B = 52.4%) and Themisto spp. during summer ( B = 31.2%). Atlantic cod diet was dominated by polar cod in both seasons (polar night: B = 37.4%; summer: B = 93.6%). Despite high dietary overlap (Schoener’ s D = 0.67) between similarly sized polar cod and Atlantic cod (9-25 cm) during polar night, competition for food between these species is likely relatively low due to polar cod having high individual specialization in their foraging strategies. However, continued Atlantification, and increased abundance of the now established population of Atlantic cod, is a threat to polar cod because of direct consumption of the smaller species by the larger. The high predation rates by Atlantic cod on polar cod also make it a potential competitor with other top predators in the region, which could have negative consequences for local sea birds and marine mammals.