As part of a scheme to monitor the distribution of otters throughout the UK, four England otter survey have been carried out covering the years 1977-79, 1984-85 and 1991-94, and 2000-02. In the first national survey, alternate 50-km squares - the north-west and the south-east quarters of each 100-km square - of the National Grid were surveyed to geographically represent cover of all regions (giving a total of 2940 in the first survey). The second survey (funded by The Vincent Wildlife Trust) in 1984-86 added four more 50-km squares (that were deemed unlikely to have otters in the first survey) giving a total of 3189 sites, all of which were re-surveyed in 1991-94 (The Vincent Wildlife Trust). In the 2000-02 (The Environment Agency) surveyed all alternate 50-km squares in England were surveyed, making the total of 38 50-km (and partial) squares and 3327 sites. Sites were selected by accessibility and likelihood of finding signs of otter activity - typically road bridges and the confluences of watercourses. The number of sites visited per square varied. The number of spraints and presence of mink was also recorded.
There are also Otter Survey databases for Scotland, Wales and Northern Ireland following the same protocol at differing resolutions.