The emergence of efficient ecosystem modelling is severely hampered by the need for an ecological classification of fishes. An earlier concept of ecological groups is here expanded to cover all living fishes. This ecological classification is based mainly on form and function in early developmental intervals, on preferred spawning grounds, and on features of reproductive behavior. Within the adaptations for reproduction and embryonic development two factors prevail — predators and the oxygen regime; consequently, spawning behavior and spawning grounds determine ensuing respiratory conditions and available protection from predators. The remaining factors are more or less associated with those two. From the point of view of resource exploitation, factors other than feeding habits governing density, biomass, and production are more important. In this sense reproductive guilds are more meaningful than feeding strategies, to which one usage of "guild" was limited.Thirty-two guilds putatively encompass all the 30,000 living fish taxa (or about 20,000 species) and form an ecological classification unrelated to the Linnean classification. The guilds retain phyletic significance in respect to reproductive systems and form a basis for the construction of models on natural and cultural succession. Number of guilds and frequency of taxa within guilds in a given area reflect geochronological sequences and invasion abilities of members of different guilds.
