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Migration performance of Atlantic salmon post-smolts in a Norwegian fjord system Plantalech Manel-la, Núria


The migratory behaviour of hatchery-reared Atlantic salmon (Salmo salar) postsmolts during their seaward migration was examined using acoustic telemetry. The first study focused on the swimming depths of eight post-smolts relative to the water-column salinity and temperature using depth sensitive acoustic transmitters. Individuals were released at the mouth of the Eio River and manually tracked during the first hours of their sea migration in a Norwegian fjord system. Vertical salinity and temperature distributions were recorded along the tracked post-smolts' trajectory. Post-smolts were tracked for a mean of 11.4 ± 1.9 hours after release over a mean distance of 8.7 ±3.4 km from release into the fjord. Mean swimming depth was 1.6 m with a range of 0.5 - 2.1 m. The results of this study suggest no overall preference for salinity concentration with post-smolts spending on average 68% of the time in salinity concentrations lower than 20 practical salinity units (psu) during the sea migration. The second study compared the behaviour of two distinct populations with different inherent migratory distances to the ocean in a common-garden experiment. Atlantic salmon post-smolts of Rivers Flekke (20 km from the open ocean, n = 80) and Lærdal (144 km from the open ocean, n = 79) origin were tagged with acoustic transmitters and released in spring of 2005 and 2006 at the mouth of the river Opo, in Sørfjorden, Western Norway. The post-smolts' migratory behaviour was continuously monitored using acoustic listening stations at three areas in Sørfjorden (2.5 - 35 km from the release point). The high percentage of post-smolts detected 35 km from release site (84% in 2005 and 81% in 2006) suggested a high survival and was independent of the fish origin. The post-smolts' progression rate from release to the end of area 3 (means, 2005: 0.6 bl s ̄¹ and 2006: 0.7 bl s ̄¹) was not dependent on fish origin in either of the two years. This study is the first to describe the swimming behaviour, including swimming depth, of fjord-migrating post-smolts

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