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 20
th 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:
- space was limited
- 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.