The migration of four salmonid species through fish bypass channels depending on environmental factors

The upstream migration of four potamodromous salmonid species (brown trout Salmo trutta fario , grayling Thymallus thymallus , Danube salmon Hucho hucho , rainbow trout Oncorhynchus mykiss ) through five fish bypass channels in southern Germany was investigated almost daily with fish counting pools from April 2017 to December 2019. In addition, 14 abiotic environmental factors (water temperature, change of the water temperature, discharge, change of the discharge, suspended matter content, change of the suspended matter content, day length, day of the lunar cycle, wind speed, global radiation, air pressure, change of air pressure, fluctuations of the earth’s magnetic field, earth tremors) of the project area were recorded. Using generalized linear models, the environmental factors that significantly affect the ascension rates of the investigated species were determined. In addition, juvenile and adult individuals were considered separately. All significant environmental factors were then evaluated in detail. The individual species react clearly differently to the investigated environmental factors. Especially the ascent rates of grayling and Danube salmon showed strong correlations with water temperature and day length. While high ascent rates of adult individuals were observed mainly during the typical water temperature and season for their spawning season, juvenile individuals were found most frequently in midsummer at high water temperatures. The ascent numbers of rainbow trout showed a much less clear correlation with the environmental factors than the other species considered.