During the compilation of the red list on marine fishes and lampreys of the Wadden Sea area, several problems in the definition of criteria have to be faced which are important for the classification of the taxa. One major problem was the concept of 'taxa regularly reproducing within the area' on which the categories were based. For most of the North Sea species, populations in the sense of reproductive units comprise large circulation systems (like the complete North Sea or, for the eel Anguilla anguilla, even the whole North Atlantic), as the species disperse over long distances by large-scale migration or planktonic eggs and larvae. The only taxon living exclusively in the Wadden Sea area is Coregonus lavaretus oxyrhinchus. Other fish and lamprey species living in the Wadden Sea area are usually not isolated reproductive units, but part of larger populations. Therefore, for Wadden Sea species, the term ,,population" is not used in the sense of isolated reproductive unit. The presence of a species in the Wadden Sea area is treated as a population if the Wadden Sea is an important part of the reproductive unit in which a decline of the population size may be a threat for the whole reproductive unit. Therefore, anadromous, catadromous, and other migrants are considered, even if they mainly reproduce outside the Wadden Sea area.