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In the early 1990s, the high bycatch of harbour porpoise in gillnet fisheries in the North and Celtic Seas prompted researchers from Range States to develop a project to survey these waters with the aim of obtaining the first comprehensive estimates of abundance of harbour porpoise. As a result, the first large-scale line transect (distance) sampling survey for cetaceans (Small Cetaceans in European Atlantic Waters and the North Sea, known as SCANS) was conducted in summer 1994 (Hammond et al. 2002). SCANS generated abundance estimates for harbour porpoise that allowed bycatch (and other anthropogenic pressures) to be assessed in a population context. Abundance was also estimated for white-beaked dolphin and minke whale in the North Sea.

SCANS 1994 was envisaged to be the first in a series of large-scale, long-term surveys with an approximately decadal frequency. Accordingly, a second survey covering all European Atlantic shelf waters was conducted in 2005 (SCANSII 2008; Hammond et al. 2013), supplemented by a survey in offshore waters in 2007 (CODA 2009). A third survey, SCANS-III, followed in 2016 (Hammond et al. 2021) covering the same area as SCANS-II and CODA combined but excluding waters to the south and west of Ireland, which were surveyed by the ObSERVE project in 2015 and 2016 (Rogan et al. 2018).