Objective criteria are needed for ranking marine sites when examining candidate areas for protection measures. We suggest a Marine Classification Criterion (MCC) which allows the application of the widely used Ramsar 1% criterion for wetlands for seabirds with clustered distribution in offshore habitats. The maximum size of an area considered to be internationally important has not been defined by the Ramsar Convention. Terrestrial and coastal sites generally have obvious hydrological or physical boundaries, whereas such boundaries are less obvious at sea. The smallest unit which would pass the demands set by the MCC is 1% of the bio-geographic population of a particular species concentrated in an area (site) supporting a density exceeding a value equivalent of four times the average density of the species in the investigated regional sea. The effect of choosing smaller or larger reference densities is tested. The results indicate that the chosen threshold density is a suitable requirement for the inclusion of the most important areas for seabird species with at least 25% of their bio-geographic population occurring in the studied regions of the North Sea and the Baltic Sea. The test cases indicate that provided the MCC is based on geo-statistical analyses of un-biased survey data the boundaries of areas holding large concentrations of seabirds can be estimated with confidence. The MCC could be used to identify concentrations of seabirds and other marine animals of conservation priority and to rank marine areas by their cumulative importance to different species.