Map Interpretation
The maps of cetacean occurrence in the study area presented
here are of three types:
- those that depict the distribution, relative abundance and
associated effort for those species for which sufficient data
allowed estimation of correction factors (individuals/standardised
hour). Maps of this type are presented for the following species:
humpback whale, minke whale, sei whale, fin whale, short-beaked
common dolphin, white-beaked dolphin, Atlantic white-sided dolphin,
Risso's dolphin, long-finned pilot whale and harbour porpoise;
- those that depict similar information for those species for
which sufficient data did not exist to allow estimation of
correction factors (individuals/hour). Maps of this type are
depicted for sperm whale, northern bottlenose whale, selected
beaked whales Mesoplodon spp., common bottlenose dolphin,
striped dolphin and killer whale; and
- sightings locations of rarely recorded species; two maps of
this type are presented showing locations of northern right whale,
blue whale and pygmy sperm whale, and of Cuvier's beaked whale,
Sowerby's beaked whale and false killer whale.
No maps are presented for species very rarely recorded or
identified in north-west European waters, namely beluga, narwhal,
Fraser's dolphin and melon-headed whale. The data used for each map
span all years of data collection, from 1979 to 1997, so possible
inter-annual shifts or differences in distribution are not
reflected in the maps; rather, the maps represent an integrated
picture or 'snapshot' over this 20-year period. Neither can these
annual maps indicate any seasonal differences in distribution; maps
depicting monthly distributions will be presented elsewhere.
On all maps except those for rare species, each individual
grid cell, i.e. ¼ ICES rectangle, is shaded to indicate the level
of search effort achieved in that cell. The darker the shading, the
more survey coverage achieved, while empty cells or blocks of cells
indicate no survey coverage. The red dots that overlie this grid of
rectangular cells indicate by their size the relative sightings
rates for each species. Note that the minimum and maximum sightings
rates on each map vary, as there are order of magnitude differences
in the sightings rates between species. The maps cannot therefore
be used to compare inter-specific differences in relative density.
The red dots are intended solely to indicate relative density for a
particular species, thereby enabling comparisons to be made only
between areas for individual species.
The fact that sightings rates for some of the species
presented here have been derived from search effort data that have
been standardised for sea state differences also means that the
grey effort shading differs among the species maps. Thus, for each
species where sea state corrections have been applied to actual
search effort, the corrected search effort is species-specific.
Hence, search effort for the same cell in different species maps
may differ (and frequently does).
It is worth noting that there are many potential biases
associated with these data as presented. We have assumed that none
of these biases is systematic, but in fact this is unlikely to be
the case and only a detailed statistical analysis of the data will
establish the nature and extent of any biases. The patchy nature of
the observational coverage, however, should always be borne in mind
when interpreting these maps. A chance sighting of a large school
of animals during a relatively short observation period may lead to
an apparently high relative density in a local area. However, the
grey background shading indicating effort levels should enable the
reader to filter the more extreme examples of such cases.
Nevertheless, it would be inappropriate to infer too much about
local densities of animals at the individual cell level. In our
interpretation of these maps we have tried to avoid any such
over-interpretation, and rely instead on general statements about
relative animal densities at a regional level.