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Production of Atlantic salmon smolts in recirculation aquaculture systems (RAS) is growing, and novel production protocols using continuous light in RAS are being implemented in the industry. In the present study, Atlantic Salmon parr were exposed to either a traditional protocol (short‐day winter signal [12:12 L:D] for 6 weeks) or to continuous light. Both photoperiods were applied in freshwater (FW) and brackish water RAS. Salmon from all treatments were transferred to seawater pens at 200 and 600 g and grown until slaughter size. A control group was smoltified with a 6‐week short‐day winter signal and kept in FW until sea transfer at 100 g. Continuous light gave a higher growth rate in RAS but reduced feed intake and growth and increased feed conversion ratio during the first 8 weeks in seawater. However, at slaughter, fish exposed to continuous light was bigger than fish given a winter signal because of the higher growth rate in RAS. Slaughter weight was lowest in fish transferred to sea at 600 g, despite having the highest day‐degree sum during their life span. The best performing group was the control group transferred at 100 g. All treatments handled transfer to seawater and survival and maturation were not affected by the treatments in RAS. The immune status was examined with a multigene expression assay on BioMark HD platform from parr stage to 5–7 months after seawater transfer. Overall, there was no significant effect of photoperiod or salinity on the expression of the selected immune genes. In sum, the results from this study indicate that using continuous light in RAS may have negative effects on performance shortly after transfer in fish transferred to sea at 200 g, whereas at 600 g, all treatments had reduced growth after transfer irrespective of treatment in RAS. [ABSTRACT FROM AUTHOR]

The objective of this study was to determine the long-term effects of ambient unionized ammonia nitrogen ( NH3-N) combined with different feeding regimes on Atlantic salmon Salmo salar L parr growth, welfare and smoltification. Previous studies on the parr stage of Atlantic salmon have mostly focused on acute exposure, or at low temperatures. Atlantic salmon parr were exposed for 105 days (at 12°C, pH 6.8) to four sublethal ammonia concentrations ranging from 0.1 to 35 μg L−1 NH3-N (0.1-25 mg L−1 TAN) at two feeding levels: full feed strength (+20% overfeeding) and 1/3 of full feed strength. After 21 days, it was observed that 32 μg L−1 NH3-N reduced growth rate of parr fed full ration, but this effect was not evident at the end of the exposure. Feed utilization was not affected by ammonia exposure at any sampling point. Increasing ammonia levels were associated with a higher prevalence and severity of gill damage at 22 days but not at the end of the exposure. The examination of welfare indicators revealed only a few pathologies, not related to ammonia exposure. In addition, higher ammonia concentrations did not appear to influence the development of hypo-osmoregulatory ability during parr-smolt transformation. [ABSTRACT FROM AUTHOR]

Abstract: During the sea water phase, Atlantic salmon is exposed to shifting environmental conditions affecting the physiology and metabolism of the fish. Strategic dietary supplements may enable the fish to sustain a higher growth rate during particular periods. In this experiment, a combination of two amino acids (l-arginine and l-glutamate, with supplementation levels 1.1% and 0.75%, respectively) was used as dietary supplements since earlier observations have indicated that these amino acids have versatile functions, influencing growth, reproduction and the immune response in animals. In the case of arginine, the control diet was set at assumed requirement level, while the test diet was supplemented further. The effects of this supplementation on feed intake and growth parameters were studied, as well as selected organ weight, biochemical, molecular and hormonal indicators. During the first experimental period (May–July), few significant effects were found. However, in the second period of the experiment (July–September), a higher specific feeding rate (SFR), thermal growth coefficient (TGC) and specific growth rate (SGR) (p <0.05) occurred in fish fed the supplemented diet, together with a trend (p <0.1) for increased final body weight. The measured gene expression levels of Insulin-like growth factor (IGF) and growth hormone receptor gene did not correlate with the increased growth observed during the second period in fish fed the supplemented diet. Furthermore, no difference in total plasma IGF-I concentrations was found after the first period. After the second period, fish fed the supplemented diet had in fact a significantly lower plasma IGF-I level. Additionally, we found a higher relative weight of gastrointestinal sections and elevated levels of plasma arginine, urea and ornithine after the second period in fish fed the supplemented diet. We conclude that dietary supplementation of l-arginine and l-glutamate significantly increased feeding rate and growth during the second feeding period in autumn. This response appeared to occur when the two conditions of rapidly decreasing day lengths and high feed intake, were met. [Copyright &y& Elsevier]