Growth, osmoregulatory response, adenine nucleotide contents, and liver transcriptome analysis of steelhead trout (Oncorhynchus mykiss) under different salinity acclimation methods

Chinese researchers have been working salmon mariculture far offshore in the Yellow sea, and steelhead trout is one of the popular Salmonidae. This study was conducted to investigate the effect of different salinity acclimation methods on the growth, osmoregulation, energy balance, and liver transcriptome of steelhead trout (Oncorhynchus mykiss). Fish (initial weight, 394.50 g) were reared for 40 days after undergoing salinity acclimation at three rates: an abrupt increase in salinity from zero to 30 ppt (T30); an abrupt increase in salinity to 14 ppt, followed by a daily increase of six ppt (T6) or two ppt (T2), until reaching 30 ppt and no salinity exposure (control group, TO). Subsequently, serum was collected to determine osmolality, iron concentration, Na+-K+-ATP (NKA) and Ca2+-Mg2+-ATP (CMA) activity; muscle and gill samples were collected for the determination of adenosine 5'-triphosphate (ATP); liver transcriptome was compared between the TO and T2 groups. The final weight, specific growth rate, food conversion efficiency, and the ATP content of fish in the T2 group were significantly higher than those in the other groups. However, the serum osmolality, serum ion concentration, and gill NKA and CMA activity in the T2 group was significantly lower than that in the T6 and T30 groups. A total of 53,679 unigenes were produced, and 1438 differently expressed genes were selected by DESeq. Among them, 587 genes were up-regulated, while 581 were down-regulated. These differential genes were mainly related to the function of metabolites and ions transportation, energy metabolism, signal transduction, immune responses, and structural assembly. The results indicate that steelhead trout with a weight of approximately 400 g is suitable for mariculture, and T2 was the best transition method for salinity acclimation.