Freshwater fishes represent about one quarter of the worlds vertebrates. They are also the most species-rich group amongst European vertebrates.
Biogeographers usually classify freshwater fishes according to their tolerance to salt water. Primary division families are those whose members are strictly intolerant of salt water, at present and historically. Secondary division families are those whose members now live in freshwater but are able, or supposedly once were able, to tolerate seawater for a short period. In addition to these categories that apply to entire families, many species that occur in freshwaters do actually belong to primarily marine families:
The majority of Europe's freshwater fish species belong to one of the following two taxonomic groups;
Both groups are well known for including many local endemics at species and subspecies levels. A third major group are the Percimorphs. Even though rather species-poor, they often occupy a key ecological role in freshwater ecosystems.
All species of freshwater fishes recorded from European freshwater waters were included in the assessment. Marine migrants and species of brackish waters were also included if they regularly enter freshwater habitats. In total 531 species were assessed, 382 of which are present in the 27 member states of the European Union.