Abstract
Understanding the feeding ecology of polar cod (Boreogadus saida) during its first year of life is crucial to forecasting its response to the ongoing borealization of Arctic seas. We investigated the relationships between diet composition and feeding success in 1797 polar cod larvae and juveniles 4.5–55.6 mm standard length (SL) collected in five Arctic seas from 1993 to 2009. Prey were identified to species and developmental stages when possible, measured, and their carbon content was estimated using taxon-specific allometric equations. Feeding success was defined as the ratio of ingested carbon to fish weight. Carbon uptake in polar cod larvae < 15 mm was sourced primarily from calanoid copepods eggs and nauplii which were positively selected from the plankton. With increasing length, carbon sources shifted from eggs and nauplii to the copepodites of Calanus glacialis, Calanus hyperboreus and Pseudocalanus spp. Calanus glacialis copepodites were the main carbon source in polar cod > 25 mm and the only copepodite positively selected for. Pseudocalanus spp. copepodites became important replacement prey when C. glacialis left the epipelagic layer at the end of summer. Calanus glacialis was the preferred prey of polar cod, contributing from 23 to 84% of carbon uptake at any stage in the early development. Feeding success was determined by the number of prey captured in larvae < 15 mm and by the size of prey in juveniles > 30 mm. As Arctic seas warm, the progressive displacement of C. glacialis by the smaller Calanus finmarchicus could accelerate the replacement of polar cod, the dominant Arctic forage fish, by boreal species.
Highlights
Thanks to large biomasses and a high degree of trophic connectivity, polar cod (Boreogadus saida) plays a pivotal role in Arctic marine ecosystems
Assuming that Calanus spp. prey were C. glacialis, larvae < 15 mm obtained most of their carbon from copepod eggs and the nauplii of C. glacialis and Pseudocalanus spp
Polar cod 15–25 mm preyed on eggs and calanoid nauplii, and added the copepodites of C. glacialis, Pseudocalanus spp., Oithona similis and the infrequent C. hyperboreus as carbon sources
Summary
Thanks to large biomasses and a high degree of trophic connectivity, polar cod (Boreogadus saida) plays a pivotal role in Arctic marine ecosystems. Alterations of its ecology and abundance by climate change may cascade in the pelagic food web and directly affect the many services provided to local communities by the predatory fish, marine mammals and seabirds feeding on polar cod Climate warming is altering the distribution of Arctic zooplankton in ways that could negatively affect the feeding success of young polar cod. A general decrease in the proportion of large zooplankton in Arctic seas (e.g., Balazy et al 2018; Møller and Nielsen 2019) and the northward penetration of the smaller C. finmarchicus (e.g., Beaugrand et al 2013) could reduce the availability of high-energy prey to polar cod larvae and juveniles
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