Prespawning migratory behaviour and spawning success of sea-ranched Atlantic salmon, Salmo salar L., in the River Gudenaa, Denmark

The migratory behaviour of sea-ranched Atlantic salmon, Salmo salar L., was analysed by radio-telemetry in the River Gudenaa, Denmark. The main object ives were to: (1) estimate mortality of returning adults through the fjord; (2) observe rate of progression and migratory pattern in the fjord and riv er; and (3) record whether spawning occurs in the river. Forty-two returnin g salmon (19 males and 23 females of total body length from 60-97 cm) reare d and released as smolts, were caught and equipped with external radio tran smitters in the outer estuary of the River Gudenaa in 1994 and 1995. Of the tagged salmon, 18 (43%) were caught in the estuary, four (10%) were not re corded after release and 20 (47%) entered the river. The mean rate of progr ession through the fjord was 7.6 km d(-1) (range 1.4-18.2) in 1994 and 5.4 km d(-1) (range 1.6-17.1) in 1995. Eleven salmon were alive at the onset of the spawning period. Eight were retrieved dead from the river during or af ter the spawning period; four with empty gonads assumed to be successful sp awners, and four with intact gonads. In 1994, unsuccessful spawners (found dead with intact gonads) entered the river earlier and had a longer total m igration distance in the river compared to successful. spawners. This sugge sts that spawning success of sea-ranched salmon is associated with time of river entry and river migration length.