The effect of seasonality on polar cod (Boreogadus saida) dietary habits and temporal feeding strategies in Svalbard waters

Abstract

Climate change in polar regions will likely disrupt the fine-tuned trophic interactions among organisms in Arctic marine ecosystems. Modifications in prey phenology and composition as well as increased competition and predation from boreal species expanding their range northward are expected to affect the key Arctic fish species polar cod (Boreogadus saida) and thus have important repercussions on the Arctic marine food web. Aside from climate, the extreme seasonal light variations at high latitudes are important for phenology and foraging. Endemic Arctic organisms such as polar cod may be adapted to these drastic light variations whereas, non-endemic species may be confronted with a new set of environmental variables that could limit their northward range expansion in the context of a warming Arctic climate. In order to assess the ability of polar cod to cope with future changes in marine Arctic ecosystems, it remains important to understand their dietary plasticity. The main goal of this study was to investigate the flexibility of polar cod feeding strategies across seasons by documenting its temporal position on the generalist-specialist spectrum. Polar cod were harvested on the western and northern coast of Svalbard in September, October, January, and May in fjords influenced by Arctic water masses and fjords influenced by Atlantic water masses. The organisms’ stomach contents were extracted and analysed and prey species were identified to the lowest taxonomic level possible. Specimens were expected to experience marked seasonal variability in their feeding success and foraging strategy. Polar cod feeding success was observed to be seasonally heterogenous, with winter specimens dispalying a higher probability of having empty stomachs compared to specimens from the fall or the spring. Seasonality affected polar cod diet in terms of ingested prey composition with fall specimen from Arctic domains feeding primarily on the hyperiid amphipod Themisto libellula. This suggests that larger demersal polar cod ascended in the water column in the fall to forage on this pelagic prey. The important contribution of fish prey throughout sites in the winter highlighted a flexible size-biased diet and the potential ability to switch diet to a temporarily abundant resource. Polar cod adopted a population specialist strategy in the fall and an individual specialist strategy in the winter. Therefore, the opportunistic feeding strategy adopted by polar cod is affected by seasonality insofar as diet is limited to a few preferred prey in the fall and diversified during the polar night likely as a results of visual constraints on selectivity of preferred prey.