Estimates of the relative abundance of long-finned pilot whales (globicephala melas) in the northeast Atlantic from 1987 to 2015 indicate no long-term trends

Daniel G. Pike, Thorvaldur Gunnlaugsson, Geneviève Desportes, Bjarni Mikkelsen, Gísli A. Víkingsson, Dorete Bloch

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Abstract

North Atlantic Sightings Surveys (NASS) and associated surveys, covering a large but variable portion of the North Atlantic, were
conducted in 1987, 1989, 1995, 2001, 2007 and 2015. Previous estimates of long-finned pilot whale (Globicephala melas) abundance,
derived using conventional distance sampling (CDS), are not directly comparable to one another because of differing survey coverage,
field methods and, in the case of the 1989 NASS, different survey timing. CDS was used to develop indices of relative abundance to
determine if pilot whale abundance has changed over the 28-year period from 1987 to 2015. The varying spatial coverage of the
surveys is accommodated by delineating common regions that were covered by: i) all 6 surveys, and ii) the 3 largest surveys (1989,
1995, and 2007). These “Index Regions” were divided into East and West subregions, and post-stratification was used to obtain
abundance estimates for these index areas only. Estimates are provided using the sightings from the combined platforms for surveys
that used double platforms or the primary platform only.
Total abundance in the Index Regions, uncorrected for perception or availability biases, ranged from 54,264 (CV=0.48) in 2001 to
253,109 (CV=0.43) in 2015. There was no significant trend in the numbers of individuals or groups in either the 6 or 3 Survey Index
Regions, and no consistent trend over the period. Power analyses indicate that negative annual growth rates of -3% to -5% would
have been detectible over the entire period. The Index Regions comprise only a portion of the summer range of the species and
changes in annual distribution clearly affect the results. Operational changes to the surveys, particularly in defining pilot whale
groups, may also have introduced biases. Recommendations for future monitoring of the long-finned pilot whale population are
provided.
Original languageEnglish
Number of pages14
JournalNAMMCO Scientific Publications
Volume11
Publication statusPublished - 2021

Keywords

  • pilot whale
  • Globicephala melas
  • North Atlantic
  • surveys
  • abundance
  • trends

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