Downstream migration routes and turbine mortality in R. Kymijoki salmon.

Acoustic telemetry was applied to study the downstream migration and turbine mortality of Atlantic salmon smolts in the River Kymijoki. Smolts fitted with acoustic transmitters were released above the three principal forks of the river, and their choice of downstream migrationroute was monitored with manual hydrophones and     fixed acoustic receivers below the forks.

Out of the 75 smolts released above the main fork of the river, 25 were later observeddownstream, the majority (15, 60%) in the western Hirvikoski main branch and the minority(10, 40%) in the eastern Pernoo main branch. Out of the 32 smolts released in the western Hirvikoski main branch, 22 were later observed downstream, all of them (100%) in the Ahvenkosk iand none in the Pyhtää branch. Out of the 50 smolts released in the eastern Pernoomain branch, 43 were observed downstream, 25 in the Koivukoski and 18 in the Korkeakoski branch. Taking into account 10 individuals entering the eastern Pernoo main branch from releases further upstream, altogether 60% (25+7 = 32) chose the Koivukoski and 40% (18+3 =21) the Korkeakoski branch.

In two out of the three forks (the main fork and Ahvenkoski/Pyhtää), the division of smolts between the alternative migration routes corresponded to the simultaneous division of water discharge, suggesting passive choice of the migration route. In the third fork, however, the majority of the fish chose the Koivukoski branch in spite of its smaller discharge comparedto the Korkeakoski branch.

Turbine mortality was studied by releasing 31 smolts above the Ahvenkoski power plant. As 2 of these remained at the stocking site till the end of the 3-week monitoring period, 29 individuals at most migrated through the turbines of the plant. Out of these, 25 were later observedbelow the dam, suggesting that 4 individuals (13%) may have died during the passage.