Migration of Atlantic salmon, Salmo salar, smolt through the estuary area of River Ellidaar in Iceland Sigurdur Gudjonsson, Ingi Runar Jonsson & Thorolfur Antonsson Institute of Freshwater Fisheries, Reykjavik, Iceland (e-mail: sigurdur.gudjonsson @veidimal.is) Received 12 May 2005 Accepted 17 June 2005 Key words: smolt, sea entry, migration time, ultra sonic transmitters, tracking Synopsis We tagged both wild and hatchery Atlantic salmon, Salmo salar, smolts from River Ellidaar (64 ° 08¢ N, 21 ° 50¢ W) with ultrasonic tags. We caught the wild smolts in a smolt trap and selected the largest individuals from the run. We implanted the transmitters in the abdominal cavity of the fish and then released them in River Ellidaar close to the estuary. We used four ultrasonic receivers; one in the river’s estuary, one outside the estuary and two further away on both sides of an island in the estuary zone. The receivers recorded all transmitters within a 600 m radius. The hatchery smolts were larger than the wild smolts. Some of the smolts were lost on the way through the estuary. Only 4 out of 9 wild smolts and 14 of 17 hatchery smolts were recorded all the way through. The tags and the tagging likely affected the survival of the smolts especially the smaller fish. There were no differences in the smolt migration between the 2 years of study and no differences in the migration behavior between the wild and the hatchery smolts. After being released the smolts stayed on average for 10 h in the river then migrated into the estuary were they stayed for 54 h on average. Then they migrated straight through the area to the sea at approximately 0.2 fish lengths per second. We recorded large differences in the migration. Introduction Atlantic salmon, Salmo salar, smolts undergo large physiological changes and stress as they leave freshwater and enter the sea. In the saline envi- ronment the smolts also have to show completely different behavior, they have to avoid new preda- tors and find food. It is believed that high mor- tality of smolt occurs during this phase (Scarnecchia 1984, Hvidsten & Lund 1988, Thorpe 1988, Mills 1989). Study of the ocean phase of salmon migration has been limited partly because of lack of robust, reliable methods for tracking fish movements. In recent years the technology in tracking fish at sea has advanced, making it possible to gain more knowledge on the ocean biology of salmon (Voegeli et al.1998, Welch et al. 2003). It is of great interest and importance to know how long smolts stay in the estuary areas. This is especially relevant to evaluate the importance of the estuary areas in rivers where there are large human influ- ences such as in River Ellidaar in Iceland. River Ellidaar flows through the city of Reykjavik. Sal- mon rivers are a valuable resource in Iceland both in ecological and economical terms. River Ellidaar and its salmon population are especially impor- tant, as it is a symbol for protection and conser- vation of nature. River Ellidaar has a good run of Atlantic salmon, as some thousands of salmon ascend the river every year. The average annual catch in the river is about 1200 salmon. In recent years the number of salmon returning to the river has decreased. Both the number of smolts leaving Environmental Biology of Fishes (2005) 74:291–296 Ó Springer 2005 DOI 10.1007/s10641-005-0500-0