Recent changes in distribution and relative abundance of cetaceans in the Norwegian Sea and their relationship with potential prey

Leif Nã¸Ttestad,Bjã¸Rn A Krafft,Matteo Bernasconi,Herdis L Mã¸Rk,Valantine Anthonypillai,Lise Langã¥Rd,Anders Fernã¶

doi:10.3389/fevo.2014.00083

Abstract

This study aimed to assess possible shifts in distributional patterns of cetaceans residing in the Norwegian Sea, and if possible relate the distribution to their feeding ecology during the summer seasons of 2009, 2010 and 2012. During this same period, historically large abundances in the order of 15 million tonnes pelagic planktivorous fish such as Norwegian spring-spawning herring (Clupea harengus), northeast Atlantic mackerel (Scomber scombrus) and blue whiting (Micromesistius poutassou), have been reported feeding in the Norwegian Sea during the summer. There is also observed elevated average surface temperatures and a reduction in zooplankton biomasses. Such changes might influence species composition, distribution patterns and feeding preferences of cetaceans residing the region. Our results show higher densities of toothed whales, killer whales (Orcinus orca) and pilot whales (Globicephala melas), than the previous norm for these waters. Baleen whales, such as minke whales (Balaenoptera acutorostrata) and fin whales (Balaenoptera physalus), which is often associated with zooplankton, displayed a distribution overlap with pelagic fish abundances. Humpback whales (Megaptera novaeangliae) were observed in low numbers, indicating shift in habitat preference, compared to sighting data collected only few years earlier. Our study illustrate that both small and large cetaceans that reside in the Norwegian Sea have the capability to rapidly perform shifts in distribution and abundance patterns dependent of the access to different types and behaviour of prey species.

Highlights

Marine mammal distributional shifts as a consequence of a warming climate are reported worldwide (Würsig et al, 2002; Moore and Huntington, 2008; Ferguson, 2009; Simmonds and Elliot, 2009; Smith and Reeves, 2010; Kovacs et al, 2011)
A compelling result from this study is the more frequent toothed whale sightings, in particular of killer- and pilot whales, in the Norwegian Sea compared to previous historic sighting surveys (Hammond and Lockyer, 1988; Buckland et al, 1993; Abend and Smith, 1999; Nøttestad and Olsen, 2004; Øien, 2013)
Fin whales seemed to have switched toward a fish prey vs. a diet consisting of krill and amphipods only few years earlier (Nøttestad et al, 2014a)

Summary

Introduction

Marine mammal distributional shifts as a consequence of a warming climate are reported worldwide (Würsig et al, 2002; Moore and Huntington, 2008; Ferguson, 2009; Simmonds and Elliot, 2009; Smith and Reeves, 2010; Kovacs et al, 2011). A number of recent reviews concerning the possible impact of climate change upon marine mammals predict that their distribution, prey preference and long term recruitment will be affected (IWC, 1997, 2009; Learmonth et al, 2006; Burek et al, 2008; Laidre et al, 2008; Moore and Huntington, 2008; MacLeod, 2009; Evans et al, 2010). Despite ascertainable changes in the whale prey communities in the Norwegian Sea, no clear changes in fin- (Balaenoptera physalus) or humpback whale (Megaptera novaengliae) distribution patterns were detected in a study investigating sighting data collected during 2006 and 2007 (Nøttestad et al, 2014a), compared to distributional trends reported 10–15 years earlier

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