Fish assemblage compositions after implementation of the IndVal method on the Narew River system

Indicator species index (IndVal) was used as a new method for an already published study, and allowed of a more convincing way of fish assemblages characterization in a large river system. Three sites clusters (AB, CD, EF) were distinguish using the self-organizing map (SOM, Artificial neural network algorithm) in the lowland Narew River system, which comprised the most characteristic species of the total of 36 present. AB included Pungitius pungitius, Barbatula barbatula, Gasterosteus aculeatus and Gobio gobio (natural and slightly modified sites from small rivers), EF included Leuciscus idus, Perca fluviatilis, Rutilus rutilus, Blicca bjoerkna, Esox lucius, Lota lota and Alburnus alburnus (sites from the main channel and lower courses of biggest tributaries, and CD without characteristic species (containing sites from small and large river ditches impacted by pollution, engineering and both). The IndVal method applied here gives precise and accurate information on fish species habitat preferences.