Monthly distribution maps of cetacean occurrence in north-west European waters

The Atlas of Cetacean distribution in north-west European waters (Reid, J.B., Evans, P.G.H., & Northridge, S.P., (2003), Joint Nature Conservation Committee, Peterborough) depicts snapshots of the distribution of the cetaceans that have been recorded in these waters from the latter half of the 20th century. The data sources used to compile the annual distribution maps were the European Seabirds at Sea database, the Sea Watch database, and the SCANS(I) database (see Data Sources section in the Methods chapter in the Atlas). Data from the latter survey pertain mainly to one month – July (1994).  Data in the other two databases were collected throughout the year, albeit over a long time period.  Annual distribution maps in the Atlas were compiled from data from all months in many different years.
Monthly species distribution maps were not depicted in the Atlas for two reasons:
  1. space was limited
  2. there was concern over the possibility that the maps would be misinterpreted. 
This latter concern remains and it is with an entreaty to the reader to be cautious in his/her reading of the monthly distribution maps that are now presented here.  The caution that applies to the interpretation of the annual maps should be exercised to a much greater degree in reading the monthly maps.  Specifically, the reader should bear in mind:
  • the data used to map species distributions were collected over a period of two decades (and possibly therefore over different climatic regimes), so any inter-annual variation within this period will be hidden;
  • sightings rates are species-specific; the maps may not be used to compare inter-specific differences in relative abundance;
  • there may well be systematic bias associated with some of the data depicted; this may be more pronounced in monthly data than in the annual data;
  • monthly coverage is patchy and the consequences of wide variation in search effort are less likely to be masked at such a relatively fine temporal scale.
Monthly maps are presented here only for those species for which sufficient data exist.  Not surprisingly, these species are mostly those for which sufficient data also existed to allow estimation of correction factors, and so depict relative abundance as number of individuals per standardised hour; the only exception is killer whale.
Notwithstanding the caution with which these maps should be considered, they have many features of interest.  At the very least they show, at a coarse scale, presence in various waters off north-west Europe on a monthly, and in some cases, seasonal basis.  Although they cannot indicate with certainty the absence of species from some parts of the area covered, they may allow one to hypothesise on possible gaps in distribution.  Year-round residency of some species in some areas may also be inferred, albeit with the comfort of corroborative evidence from bespoke research and survey.
Reference must be made to the interpretation section for the annual maps and the Methods chapter of the Atlas, in order to interpret these maps appropriately.
  • Minke whale                                     Balaenoptera acutorostrata   (PDF,  1.8 mb)
  • Common bottlenose dolphin        Tursiops truncates                  (PDF,  3.2 mb)
  • Short-beaked common dolphin   Delphinus delphis                   (PDF,  2.3 mb)
  • White-beaked dolphin                    Lagenorhynchus albirostris  (PDF,  3.4 mb)
  • Atlantic white-sided dolphin          Lagenorhynchus acutus        (PDF,  3.3 mb)
  • Risso's dolphin                               Grampus griseus                     (PDF,  3.0 mb)
  • Killer whale                                      Orcinus orca                              (PDF,  2.1 mb)
  • Long-finned pilot whale                 Globicephala melas               (PDF,  1.8 mb)
  • Harbour porpoise                           Phocoena phocoena               (PDF,  3.3mb)
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