Pelagic production and the recruitment of juvenile polar cod Boreogadus saida in Canadian Arctic seas

Mathieu Leblanc,Maxime Geoffroy,James D Reist,Andrew Majewski,Caroline Bouchard,Stéphane Gauthier,Louis Fortier

doi:10.1007/s00300-019-02565-6

Mathieu Leblanc, Maxime Geoffroy + Show 5 more

Open Access

https://doi.org/10.1007/s00300-019-02565-6

Copy DOI

Abstract

Previous work found that an earlier ice breakup favors the recruitment of juvenile polar cod (Boreogadus saida) by enabling early hatchers to survive and reach a large size by late summer thanks to a long growth season. We tested the hypothesis that, in addition to a long growth season, an earlier ice breakup provides superior feeding conditions for young polar cod by enhancing microalgal and zooplankton production over the summer months. Ice cover and surface chlorophyll a were derived from satellite observations, and zooplankton and juvenile cod biomass were estimated by hydroacoustics in ten regions of the Canadian Arctic over a period of 11 years. Earlier breakups resulted in earlier phytoplankton blooms. Zooplankton backscatter in August increased with earlier breakup and bloom, and plateaued at chlorophyll a > 1 mg m−3. Juvenile cod biomass in August increased with an earlier breakup, and plateaued at a zooplankton backscatter > 5 m2 nmi−2, supporting the hypothesis that higher food availability promotes the growth and survival of age-0 fish in years of early ice melt. However, there was little evidence that late summer biomass of either zooplankton or age-0 polar cod benefitted from ice breakup occurring prior to June. On average, zooplankton standing stock was similar in the Southern Beaufort Sea and the North Water-Lancaster Sound polynya complex, but juvenile cod biomass was higher in the Beaufort Sea. Intense avian predation could explain the lower biomass of juvenile cod in the polynya complex, confirming its reputation as a biological hotspot for energy transfer to higher trophic levels.

Highlights

The polar cod (Boreogadus saida), a small forage fish, dominates the pelagic fish assemblage in Arctic seas (Fortier et al 2015)
Based on the linear mixed-effects model, zooplankton backscatter in the epipelagic layer (13.5–100 m) in August increased with an earlier ice breakup week and an earlier phytoplankton bloom (Fig. 3a, b; Online resource 2a)
The surface phytoplankton bloom typically starts in the weeks following ice breakup and deepens towards the nitracline to form a sub-surface chlorophyll maximum (SCM) (Martin et al 2010)

Summary

Introduction

The polar cod (Boreogadus saida), a small forage fish, dominates the pelagic fish assemblage in Arctic seas (Fortier et al 2015). In the Canadian Arctic, the biomass of juvenile polar cod in August and September increases exponentially with earlier ice breakup (< 50% ice cover) and warmer spring–summer sea surface temperatures (Bouchard et al 2017). Bouchard et al (2017) suspected that the survival of early hatchers in years of early ice breakup is enhanced by biotic factors such as an advanced bloom of ice algae and phytoplankton (e.g., Kahru et al 2011) and the resulting earlier and more intense production of copepod nauplii and copepodites (Fortier et al 1995; Ringuette et al 2002; Daase et al 2013), the preferred prey of age-0 polar cod (Michaud et al 1996; Bouchard et al 2016)

Methods

Results

Conclusion