Reconciling Behavioural, Bioenergetic, and Oceanographic Views of Bowhead Whale Predation on Overwintering Copepods at an Arctic Hotspot (Disko Bay, Greenland)

Neil S Banas,Torkel Gissel Nielsen,Ingrid H Ellingsen,Kristin L Laidre,Eva Friis Møller,Malene Simon

doi:10.3389/fmars.2021.614582

Abstract

Bowhead whales (Balaena mysticetus) visit Disko Bay, West Greenland in winter and early spring to feed onCalanusspp., at a time of year when the copepods are still mostly in diapause and concentrated in near-bottom patches. Combining past observations of copepod abundance and distribution with detailed observations of bowhead whale foraging behaviour from telemetry suggests that if the whales target the highest-density patches, they likely consume 26–75% of theCalanusstanding stock annually. A parallel bioenergetic calculation further suggests that the whales' patch selection must be close to optimally efficient at finding hotspots of high density copepods near the sea floor in order for foraging in Disko Bay to be a net energetic gain. AnnualCalanusconsumption by bowhead whales is similar to median estimates of consumption by each of three zooplankton taxa (jellies, chaetognaths, and predatory copepods), and much greater than the median estimate of consumption by fish larvae, as derived from seasonal abundance and specific ingestion rates from the literature. The copepods' self-concentration during diapause, far from providing a refuge from predation, is the behaviour that makes this strong trophic link possible. Because the grazing impact of the whales comes 6–10 months later than the annual peak in primary production, and because Disko Bay sits at the end of rapid advective pathways (here delineated by a simple numerical particle-tracking experiment), it is likely that theseCalanuspopulations act in part as a long-distance energetic bridge between the whales and primary production hundreds or thousands of km away.

Highlights

Calanus spp. and other calanoid copepods are a crucial link in mid- and high-latitude marine food webs, often dominating the biomass of primary consumers that transfer primary production to predators like fish, seabirds, and marine mammals (FalkPetersen et al, 2007; Aarflot et al, 2017; Kimmel et al, 2017; Møller and Nielsen, 2020)
To make room in these approaches for the biology of particular predator species, or for that matter the biology of the prey. In this Brief Report we describe an alternate approach, intended to clarify the questions raised by past observations of a distinctive, short Arctic food chain, from phytoplankton to copepods to bowhead whales, the largest Arctic marine mammal (Laidre et al, 2007; Simon et al, 2009; Moore, 2016)
The magnitude of annual bowhead whale predation on copepods in Disko Bay was estimated by two semi-independent calculations, the first based on information in the literature about whale diving and foraging behaviour from telemetry, the second based on bioenergetics, following Laidre et al (2007)

Summary

Introduction

Calanus spp. and other calanoid copepods are a crucial link in mid- and high-latitude marine food webs, often dominating the biomass of primary consumers that transfer primary production to predators like fish, seabirds, and marine mammals (FalkPetersen et al, 2007; Aarflot et al, 2017; Kimmel et al, 2017; Møller and Nielsen, 2020). Calanus spp. have many predators, both they and their predators have patchy distributions and complex patterns of behaviour and selectivity, and it is possible that there is no system where the data exist to properly, empirically close the Calanus carbon budget Often this problem is dealt with using inverse food web models or other mass-balance approaches (Vézina and Platt, 1988; Aydin et al, 2002; Heath, 2012), which by definition harmonise incomplete and mismatched information into a smooth, best-guess picture. In this Brief Report we describe an alternate approach, intended to clarify the questions raised by past observations of a distinctive, short Arctic food chain, from phytoplankton to copepods to bowhead whales, the largest Arctic marine mammal (Laidre et al, 2007; Simon et al, 2009; Moore, 2016)

Methods

Results

Discussion

Conclusion