Influence of salinity on rainbow trout (Oncorhynchus mykiss) smolt development and postsmolt performance

Rainbow trout aquaculture has large potential for growth, but requires better understanding of aspects like smoltification and its effect on trout performance after seawater transfer. The present study examined the effect of three different salinities (fresh water, 3–5 ‰ and 15–16 ‰) on smoltification and growth of rainbow trout (70 ± 10 g) reared under constant light between March–June 2018; it also documented hypoosmoregulatory performance and growth of trout from all salinity treatments transferred to full seawater at three different times (Transfer 1: 26 April; Transfer 2: 24 May; Transfer 3: 21 June). Smolt development was assessed with markers commonly used for Atlantic salmon like gill Na+, K+-ATPase activity, gill mRNA transcription of key genes (nkaα1a, nkaα1b and nkcc1a) and plasma concentration of Cl− and Na+. Results evidenced that trout in brackish water experienced an earlier onset of smoltification, displayed by rapid changes in mRNA transcription of the ATPase α-isoforms nkaα1a (freshwater isoform) and nkaα1b (seawater isoform), which, in turn, induced an earlier peak of the Na+, K+-ATPase (NKA) activity. However, smoltification also commenced and progressed in fresh water most likely as a result of reaching a certain size. The salinity treatment did not influence growth during the smoltification period. After seawater transfer, all treatments in Transfer 1 (26 April) displayed the best capacity to acclimatize to full seawater (with clear upregulations of nkaα1b and nkcc1a after seawater entry) and the highest growth, compared to all treatments in Transfers 2 and 3. In contrast, few differences in growth and smoltification markers occurred between salinity treatments within each of the seawater transfers. This suggests that trout in the first transfer had the best smolt condition (were at the peak of smolt window), which allowed them to adapt and grow better in seawater. Overall, such results indicate that timing of seawater transfer can be more important for trout performance in the grow-out phase in the sea than the water salinity during the early rearing period. As such, although brackish water can be used to induce rainbow trout smoltification, transferring the fish at the peak of the smolt window can increase chances for best seawater performance.