Assessment of the effectiveness of fish passage through the vertical-slot fishway at the main dam on the longest Baltic River

Abstract

In 1969, a dam was constructed on the Vistula River, the longest in the Baltic drainage basin, 276 km from its estuary, which resulted in the collapse of migratory fish populations throughout the drainage basin. The existing, ineffective fishway was rebuilt in 2014. Monitoring at the new fishway indicated that 24 fish species used it, and over a period of five years the number of individuals fluctuated from nearly 4,000 to over 23,000. Radio frequency identification (RFID) technology was used to investigate 12 fish species, mostly vimba and bream, passing through the fishway. Fish caught in a trap in the upper part of the fishway were tagged with PITs and then released below the dam. Returning fish were recorded by four loop antennas. Of the 877 fish that were tagged, 13% returned to the fishway mostly on the first and fourth days following release. Of the fish that returned, 30% turned back while passing through the fishway: 30% vimba and 52% bream. The transit time of vimba and bream did not depend on fish size or the time after which they returned. Overall, estimated fishway efficiency based on the migration of the tagged fish was high.