The present analysis is based upon effort-based systematic survey observations (including SCANS and SCANS II, European Seabirds at Sea, and Sea Watch Foundation surveys, as well as several survey datasets from other research groups). These span a period of 30 years (1980-2009), with effort well distributed across all 5-year time periods with greatest consistency in the North Sea, West of Scotland, and Irish Sea. Cetacean survey effort was calculated on the basis of observation hours rather than survey distance in order to integrate data from both static and mobile platforms. Corrections are made for the effect of sea state, and the resulting relative abundance indices assigned to grid cells at a resolution of 15 seconds latitude by 30 seconds longitude before plotting using ArcGIS.
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