| Summary: | 1. Aqueous aluminium is described as the principal toxicant killing fish in acidified waters, along with low pH. A raising concern has been the possible effect this toxicant have on the declining Atlantic salmon population and their ability to adapt to changes in salinity. 2. River Suldalslågen in Suldal municipality has been subject to such a decline in salmon population, and numerous studies have been conducted trying to identify the role of water quality to both the smoltification process and survival. The aim of this study has been to describe the development of seawater tolerance in Atlantic salmon exposed to different water qualities in Suldalsvassdraget in spring and throughout the migration period of smolts. To achieve this, water chemistry parameters were collected, as well as physiological conditions from fish exposed to different water qualities and seawater challenge tests, during the entire period. Collected data was analysed and statistically modelled with GLMMs. An additional aim was to answer the applicability of seawater challenge testing as a tool for identifying an ongoing smoltification process and as predictor for water quality criteria in Atlantic salmon. 3. The results suggest that the Atlantic salmon in Suldalslågen never developed a satisfactory seawater tolerance, and that hatchery-produced fish from Ritland hatchery never could be characterized as smolt, prior to or during the time of migration. Thus, I have only to a limited extent achieved the aim of describing the development of seawater tolerance. The results further exposes aluminium in Suldalslågen not to be responsible for non-developing seawater tolerance in salmon, but on a contrary acting as a possible positive “stimulator” in physiological processes associated with osmoregulation in fish, where these processes, for some reason, is not developing normally. 4. Finally, this study suggest seawater challenge testing to be limited in predicting water quality criteria in Atlantic salmon, and give a cautionary notation to the ...
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