Fish distribution and ecological state of the Siesartis River, Lithuania

The article presents investigation data on the ecological state of the Siesartis River and the diversity, distribution, community structure of fish therein. During the study period, 23 fish species belonging to nine fish families were caught. Nine of these species are protected. River lamprey Lampetra fluviatilis (L.), brook lamprey Lampetra planeri (Bloch.), salmon Salmo salar (L.) are protected under the EU Habitats Directive and the Bern Convention, grayling Thymallus thymallus (L.), vimba Vimba vimba (L.) and riffle minnow Alburnoides bipunctatus (Bloch.) are under the protection of the Bern Convention; bitterling Rhodeus sericeus amarus (Bloch.) and spined loach Cobitis taenia (L.) are protected under the Bern Convention; the bullhead Cottus gobio (L.) is protected under the EU Habitats Directive. Salmon (20.3%), riffle minnow (20.1%), minnow (17.2%) and brown trout (12.2%) dominated the Siesartis River fish community in abundance. Protected species accounted for 48.5% of the fish community abundance. In biomass, the fish community of the Siesartis River was dominated by brown trout (20.8%), sea trout (18.8%) and salmon (15.3%). The mean fish density was 34.3 ind/100 m² and the mean biomass 731.2 g/100 m². Correlations between some environmental factors and indices of fish communities were established. Fish communities were mainly impacted by the river depth, flow velocity, bottom substrate structure and the vegetation coverage rate. Judging by species diversity indices, fish abundance, density, biomass and the Lithuanian Fish Index, the ecological state of the community is very good.