Historical records indicate that Nucella lapillus was once present, on suitable substrates, around the whole of the North Sea coastline. Wild dogwhelks could be collected in all countries except Belgium and Germany, where the species is present only on Helgoländ (in numbers too small to be sampled).

All wild populations of dogwhelks showed imposex. Near sites of intense boating activity, all populations examined contained some sterile females. At all sites where transplants were recovered, the dogwhelks were affected by TBT. Prolonged survival of dogwhelks at any of these sites would not be expected, except perhaps at Tørsminde in Denmark.

All sites studied were categorized with reference to their potential for the maintenance of self-sustaining Nucella populations. All sites in the southern part of the North Sea (i.e. the coastlines of Belgium, The Netherlands, Germany and southern Denmark) were placed in Category C (reduced egg capsule production) or D (adult females expected to be sterile).

In Norway, sites near large harbours had only sterile females in the population (Category D), whilst those further from harbours probably had reduced egg capsule production (Category C). In Sweden, all wild sites were affected but probably had no reduction in egg capsule production (Category B).