Seasonal and diel cycles of fin whale acoustic occurrence near Elephant Island, Antarctica.

Elke Burkhardt,Boris Cisewski,Sarah Zwicker,Ilse Van Opzeeland,Olaf Boebel,Karolin Thomisch,Elena Schall,Marlene Meister,Ramona Mattmüller,Stefanie Spiesecke

doi:10.1098/rsos.201142

Abstract

This study investigates the relevance of the Elephant Island (EI) region for Southern Hemisphere fin whales (Balaenoptera physalus) in their annual life cycle. We collected 3 years of passive acoustic recordings (January 2013 to February 2016) northwest of EI to calculate time series of fin whale acoustic indices, daily acoustic occurrence, spectrograms, as well as the abundance of their 20 Hz pulses. Acoustic backscatter strength, sea ice concentration and chlorophyll-a composites provided concurrent environmental information for graphic comparisons. Acoustic interannual, seasonal and diel patterns together with visual information and literature resources were used to define the period of occupancy and to infer potential drivers for their behaviour. Spectral results suggest that these fin whales migrate annually to and from offshore central Chile. Acoustic data and visual information reveal their arrival at EI in December to feed without producing their typical 20 Hz pulse. For all 3 years, acoustic activity commences in February, peaks in May and decreases in August, in phase with the onset of their breeding season. Our results emphasize the importance of EI for fin whales throughout most of the year. Our recommendation is to consider EI for establishing a marine protected area to expedite the recovery of this vulnerable species.

Highlights

Elephant Island (EI, 61° S 55° W) is the northernmost Island of the South Shetland Island chain near the tip of the Antarctic Peninsula between Bransfield Strait and Shackleton Fracture Zone, in close vicinity to the southern boundary of the Antarctic Circumpolar Current front [1]
While passive acoustic monitoring (PAM) data had been collected a decade earlier for 3 years at a location 192 nautical miles (NM) farther west at the shelf break ([27], and figure 1, label ‘WAP’), our study presents the first on-shelf recordings of fin whales in the EI coastal zone
In a more tentative approach, we explored the potential of making meaningful comparisons of the magnitude of call-specific signal-to-noise ratios (SNRs) between different recorder deployments to, at least coarsely, capture abundance trends

Summary

Introduction

Elephant Island (EI, 61° S 55° W) is the northernmost Island of the South Shetland Island chain near the tip of the Antarctic Peninsula between Bransfield Strait and Shackleton Fracture Zone, in close vicinity to the southern boundary of the Antarctic Circumpolar Current front [1]. The area is known for high krill densities [6] with large aggregations occurring mainly north of EI due to the island’s bathymetric features [8,9,10] These krill stocks probably provide an important and reliable feeding ground for a variety of top predators [11], including baleen whales like humpback whales (Megaptera novaeangliae) and, fin whales (Balaenoptera physalus) [10,12,13]. Animals were observed feeding on krill in shallow waters (less than 300 m) and travelling in deeper waters (greater than 1000 m) This increase in recent observations of large fin whale aggregations is noteworthy. Protecting their key habitats should be a primary objective of conservation efforts

Results

Discussion

Conclusion