Effects of a dispersal barrier on freshwater migration of the vimba bream (Vimba vimba)

To study the effects of a dispersal barrier on migration of the semi-anadromous vimba bream in the Pärnu River, Estonia, we tagged thirty fish with acoustic transmitters and released above the barrier. Tagged fish showed variation in behaviour, and 16 different spawning movement patterns were identified. Several fish moved > 25 km upstream. Batch spawning was suggested by stops in up to four different spawning areas. The fish descended to the sea after spawning in spring; females earlier than males. After spending on average 137 days in the sea, they returned to the river during autumn and stayed in the river on average 174 days until the next spawning. The fish were most active during sunrise and sunset. In conclusion, the study shows that the dam prevents a diversification of migration behaviour and the associated expansion of spawning areas. A more efficient fishway could promote population growth and improve stock status.