Year-Round Dive Characteristics of Male Beluga Whales From the Eastern Beaufort Sea Population Indicate Seasonal Shifts in Foraging Strategies

Luke Storrie,Alex Nunes,David G Barber,Nigel E Hussey,Greg O’Corry-Crowe,Shannon A Macphee,Lisa L Loseto,John Iacozza

doi:10.3389/fmars.2021.715412

Abstract

Dive behavior represents multiple ecological functions for marine mammals, but our understanding of dive characteristics is typically limited by the resolution or longevity of tagging studies. Knowledge on the time-depth structures of dives can provide insight into the behaviors represented by vertical movements; furthering our understanding of the ecological importance of habitats occupied, seasonal shifts in activity, and the energetic consequences of targeting prey at a given depth. Given our incomplete understanding of Eastern Beaufort Sea (EBS) beluga whale behavior over an annual cycle, we aimed to characterize dives made by belugas, with a focus on analyzing shifts in foraging strategies. Objectives were to (i) characterize and classify the range of beluga-specific dive types over an annual cycle, (ii) propose dive functions based on optimal foraging theory, physiology, and association with environmental variables, and (iii) identify whether belugas undergo seasonal shifts in the frequency of dives associated with variable foraging strategies. Satellite-linked time-depth-recorders (TDRs) were attached to 13 male belugas from the EBS population in 2018 and 2019, and depth data were collected in time series at a 75 s sampling interval. Tags collected data for between 13 and 357 days, including three tags which collected data across all months. A total of 90,211 dives were identified and characterized by twelve time and depth metrics and classified into eight dive types using a Gaussian mixed modeling and hierarchical clustering analysis approach. Dive structures identify various seasonal behaviors and indicate year-round foraging. Shallower and more frequent diving during winter in the Bering Sea indicate foraging may be energetically cheaper, but less rewarding than deeper diving during summer in the Beaufort Sea and Arctic Archipelago, which frequently exceeded the aerobic dive limit previously calculated for this population. Structure, frequency and association with environmental variables supports the use of other dives in recovery, transiting, and navigating through sea ice. The current study provides the first comprehensive description of the year-round dive structures of any beluga population, providing baseline information to allow improved characterization and to monitor how this population may respond to environmental change and increasing anthropogenic stressors.

Highlights

Cetaceans spend the vast majority of their lives at various depths below the ocean surface, vertical movement must be considered if not more so than horizontal movement to fully understand the ecological significance of animal behavior
Optimal foraging theory predicts that central place foragers, such as cetaceans which must return to the surface to breathe, should maximize the time spent at depth as the foraging depth increases to offset the energetic costs associated with greater travel distance (Mori, 1998)
Time series depth data from satellite-linked and pop-up archival tags revealed that Eastern Beaufort Sea (EBS) male belugas employ diverse dive profiles; with differing time spent at various depths, descent and ascent rates, vertical movements during the bottom phases of dives, and post-dive behavior to exploit the three-dimensional habitat they occupy over an annual cycle

Summary

Introduction

Cetaceans spend the vast majority of their lives at various depths below the ocean surface, vertical movement must be considered if not more so than horizontal movement to fully understand the ecological significance of animal behavior. Seasonal foraging intensity varies among species and populations depending on their geographic distribution and degree of interconnectedness with sea ice (Laidre and HeideJørgensen, 2005; Breton-Honeyman et al, 2016; Watt et al, 2016; Castellote et al, 2020). Considering the time and depth structures and seasonal frequency of dives in the context of optimal foraging theory and ADL can provide insight on dive function (Heerah et al, 2019), seasonality of behaviors (Blanchet et al, 2015), energetics (Arce et al, 2019; Friedlaender et al, 2020), and vertical prey distribution (Tennessen et al, 2019)

Objectives

Methods

Results

Conclusion