Encouraging encounters: unusual aggregations of bowhead whales Balaena mysticetus in the western Fram Strait

Mn De Boer,G Nijs,N Janinhoff,H Verdaat

doi:10.3354/esr00948

Abstract

The subpopulation of the bowhead whale Balaena mysticetus in the East Greenland-Svalbard-Barents Sea is endangered and until recently was believed to number in the tens. Recent studies have suggested that this subpopulation appears to be increasing. Here, we report on unusual aggregations of bowhead whales within the Fram Strait. We present opportunistic and effort-corrected observations of bowhead whales made from a small expedition vessel during cruises in June (2015−2018). Bowhead whales were sighted on 85 occasions (220−227 whales). An aggregation in 2015 (n = 84 whales) and high numbers in 2018 (n = 104−110) exceeded all previous records. The index of whale abundance was significantly higher in open water-leads (1.08−1.14 whales km−1 of survey effort) compared to areas with drift-ice (0.51−0.53 whales km−1). The highest abundance index was measured in deep waters where the bottom slope was relatively steep. Our findings highlight the temporal and spatial consistency of this species in areas with relatively loose ice cover (open water-leads) and steep slopes. It is unknown how global warming and resultant changes in ice-extent are going to affect bowhead whales within the Strait and whether they will find new feeding grounds due to an expanding open-ocean habitat. These slopes may become increasingly important to bowhead whales and Arctic top predators as a spring/early summer feeding ground. These relatively large numbers of bowhead whales are encouraging and can help direct future research monitoring programs to study the population ecology of these endangered whales.

Highlights

Bowhead whales Balaena mysticetus were once common in the Arctic and Subarctic Seas until they were severely depleted by commercial whaling
During the 2015−2018 cruises, 85 sightings of bowhead whales Balaena mysticetus were made in total, involving 220−227 individuals
The number of bowhead whales Balaena mysticetus observed during the cruises (85 sightings involving 220−227 bowhead whales), the unusual aggregation observed in 2015 (n = 84 whales) and the high numbers in 2018 (n = 104−110), exceed all previous published observations

Summary

Introduction

Bowhead whales Balaena mysticetus were once common in the Arctic and Subarctic Seas until they were severely depleted by commercial whaling. 4 different bowhead whale subpopulations are recognized based on their geographic distribution: (1) Bering-Chukchi-Beaufort Seas; (2) East Canada-West Greenland (3) East Greenland-SvalbardBarents Sea; and (4) Okhotsk Sea The preexploitation abundance based on catch data has been estimated between 24 000 and 100 000 whales (Allen & Keay 2006) They were the first Arctic whale species to be exploited by whalers around Spitsbergen (Svalbard) in 1611 (Ross 1993, Allen & Keay 2006). Within the Northeast Atlantic alone, an estimated 91 000 whales were harvested (Ross 1993), which depleted the Svalbard subpopulation to near extinction, and its current status is regarded as Endangered according to the IUCN (Cooke & Reeves 2018)

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