Abstract Previous studies have shown that wild smolt have 2.5-4.5 times higher survival rate than smolt grown in culture and that starved reared smolt behave more like wild born smolt. The purpose of this project was to study whether there are differences in migration patterns and survival during migration to the coast between wild and reared smolt and between different types of reared smolt. In 2009, 25 smolts in three different groups were tagged with acoustic transmitters: Wild Born, conventionally reared 2-year and 1 –year old smolt. For the 2010 trial there was also added a group of reared 2 –year old smolt that were starved before release. Receivers were placed in the river and in a line outside the estuary. The receivers recorded when the tagged fish passed and which fish it was. Based on information from the receivers the survival rate and migration speed could be calculated for each group and be compared to the smolts amount of body fat. In this study the condition factor (weight/length3 *100000) was used as an indicator of the smolts body fat levels. The study was conducted in river Dalälven. The results provide some indications that the wild born smolt in accordance with previous studies had a slightly higher (not significant) survival rate than the other groups. No major signs of differences in survival were seen between the groups of the conventionally reared and the starved reared smolt. The annual smolt seemed to have a slightly higher survival rate than these two groups. Unlike the results in earlier studies the wild born smolt did not migrate significantly faster than any of the other groups. No significant difference was observed between the conventionally reared and the starved smolt. The group that migrated fastest was the 1 –year old smolt. It was the group of 1 –year old smolt that was most similar to the wild born smolt in regard to the condition factor. But the wild born smolt were still considerably leaner. There was a significant difference in the condition factor between the starved smolt and those grown conventionally. But the difference was probably too small to show any differences in migration behavior when they still were much larger than the wild born smolt. There also tended to be a negative correlation between low condition factor and higher survival. In this study the starved smolt behaved more like the conventionally reared than the wild hatched. They were not starved for a sufficient amount of time to reach as low levels of body fat as the wild hatched smolt. It would require studies in which smolt were starved for longer period to evaluate whether the condition factor is crucial for if reared smolt in river Dalälven possibly can behave as wild smolt.

Abstract Atlantic salmon is one of the most widely known migratory fish species whose populations have declined because of hydropower installations. Attempts have been made to preserve salmon stocks in regulated rivers by building fishways for adult fish migrating upstream, but downstream migration of salmon smolts has been almost totally ignored. Instead, captive breeding programmes and extensive stocking of hatchery-reared salmon smolts have been initiated to compensate for highly reduced natural production and to maintain salmon yields. In recent decades, demands to reduce the environmental effects of hydropower production, together with public awareness of decreasing recapture rate and yield of stocked salmon, have increased calls for rebuilding wild salmon stocks in rivers modified for hydropower production. As a consequence, survival of salmon smolts during downstream migration and the effects of hatchery rearing and stocking methods are now research topics of high importance. This thesis examined the need for modifications to the current standard hatchery rearing and release methods, determined the effects of commonly used tagging methods and investigated the impacts of river regulation on the survival and behaviour of downstream migrating smolts by applying telemetry techniques. Modifications made to the standard rearing processes noticeably affected the physiology, behaviour and survival of salmon smolts. In addition, the timing of release was shown to be a key factor for the survival of released smolts. Therefore, comprehensive rearing of smolts, and improving current release methods, especially to match the timing of release to the migration window of wild smolts, are high priorities. Observations in semi-natural environments indicated that commonly used tagging methods had only slight effects on the survival of smolts, so they can be freely used to examine smolt performance. However, more information on rearing and stocking processes and tagging methods is still needed to fully verify the present findings in field conditions. Finally, increasing smolt survival during their downstream migration in regulated rivers is an urgent issue, as survival of smolts was found to be six-fold lower within a river section with five hydropower dams than in a corresponding section of a free-flowing river. In future salmon stock rebuilding actions in regulated rivers, safeguarding downstream migration of smolts should be considered as an equally important issue to safeguarding upstream migration of spawners. / Tiivistelmä Lohen kannat ovat taantuneet jokien vesivoimarakentamisen seurauksena. Rakennettujen jokien katkenneita vaellusyhteyksiä on pyritty avaamaan rakentamalla kalateitä ylävirtaan vaeltavia aikuisia lohia varten, mutta alavirtaan vaeltavat kalat ovat jääneet lähes huomiotta. Samanaikaisesti kalankasvatus ja massiiviset poikasistutukset ovat olleet suuressa roolissa lohikantojen ylläpitämisessä. Viime vuosikymmeninä istutustulosten heikkeneminen ja toisaalta yleisemminkin lisääntynyt kiinnostus ympäristöasioita kohtaan ovat lisänneet halukkuutta palauttaa lohikantojen luontaista lisääntymistä rakennettuihin jokiin. Tämän seurauksena kasvatus- ja istutusmenetelmien vaikutukset lohen poikasiin sekä vaelluksen onnistuminen rakennetuilla joilla ovatkin nousseet tärkeiksi tutkimusaiheiksi. Väitöskirjani tavoitteena oli selvittää telemetriatekniikoiden avulla nykyisin käytössä olevien kasvatus- ja istutusmenetelmien, yleisesti käytössä olevien kalamerkintätapojen sekä joen patoamisen vaikutusta lohen vaelluspoikasten jokivaelluksen onnistumiseen. Poikaskasvatuksen aikaiset muutokset vaikuttivat vaelluspoikasten kokoon, fysiologiaan ja käyttäytymiseen. Lisäksi kasvatuskäytäntöjä muuttamalla saatiin tuotettua enemmän luonnonpoikasia muistuttavia istukkaita kuin perinteisellä kasvatusmenetelmällä. Istutusajankohta osoittautui myös tärkeäksi selittäjäksi vaelluspoikasten eloonjäännissä. Kasvatusmenetelmien muuttaminen paremmin luonnonolosuhteita vastaaviksi ja istutusajankohdan sovittaminen luonnonpoikasten luonnolliseen vaellusaikaan olisivat tarpeellisia muutoksia nykyisiin kasvatus- ja istutuskäytäntöihin. Yleisesti käytössä olevat kalamerkintätavat soveltuvat vaelluspoikastutkimuksiin, sillä eri merkintämenetelmien vaikutukset vaelluspoikasten kasvuun ja käyttäytymiseen olivat vähäisiä kokeellisissa olosuhteissa tehdyssä tutkimuksessa. Lisää tietoa kuitenkin tarvitaan kasvatus- ja istutusmenetelmien sekä merkintätapojen vaikutuksista luonnonolosuhteissa. Lohen vaelluspoikasten selviytymistä alasvaelluksesta rakennetuilla joilla tulee parantaa merkittävästi, sillä vapaasti virtaavassa joessa vaelluspoikasten selviytymisen havaittiin olevan kuusinkertainen rakennettuun jokeen verrattuna. Lohikantojen palauttamishankkeissa on palaavien aikuislohien kutuvaelluksen rinnalla kiinnitettävä erityistä huomiota myös vaelluspoikasten jokivaelluksen onnistumiseen.