Influences of hydrogeomorphology and chemical water quality on fish assemblages in the Nevėžis River, Lithuania: implications for river basin management plans in the Baltics

Further resolving physicochemical-fish associations would be of considerable benefit to advancing both scientific research and monitoring programs in the Baltic states. We collected 3 years of coordinated hydrogeomorphic, water-chemistry, and fish assemblage data at 11 study reaches along the Nevėžis River of central Lithuania and assessed their relative influence on fish assemblages. Of the 23 fish species surveyed in the Nevėžis River, omnivorous and tolerant species were most common. Both water chemistry and physical, hydrogeomorphic characteristics emerged as predictors of fish assemblage descriptors. The strength of evidence for biological oyxgen demand as a strong environmental driver was compelling for both the Lithuanian Fish Index (LFI) and percentage of simple lithophils. Channel substrate emerged in multiple models as a strong predictor variable (LFI, % intolerant species, % simple lithophils, % omnivores). Measures of channel size (drainage area, mean depth) contributed to models for multiple fish metrics including percentage of lithophils, percentage of omnivores, and percentage of intolerant species. This research represents novel work in the region, and our results are an important step in supporting the development of a comprehensive physicochemical research and monitoring program in Lithuania.