More than just a pinch of salt? The impact of increasing salinity on maraena whitefish in different husbandry systems

The present study investigated the influence of different salinity concentrations on gene expression patterns in maraena whitefish ( Coregonus maraena ) from three aquaculture facilities. Over a period of 10 weeks, we compared maraena whitefish from the same batch of fertilised eggs that were kept in a freshwater-pond system and a net-cage facility in the southwestern Baltic Sea, where periodic saline intrusions from the North Sea occur. Having demonstrated differential expression of the two variants of the well-established atp1a1 biomarker gene in gills of maraena whitefish from both habitats, we profiled the expression levels of a further 22 seawater-tolerance biomarkers in the gills, skin, head kidney and liver. The expression profiles were largely consistent across whitefish from both habitats, with only sporadic modulations. A principal component analysis revealed that these differential expression patterns were dominated by eef2 , ghr and wasf1. A subsequent salinity experiment, carried out in a recirculating aquaculture system under constant environmental conditions, revealed that increases in salinity from 0 to 8 Practical Salinity Units (PSU) resulted in significantly elevated transcript levels of fkbp5 and atp1a1a in the gills. The reduced levels of eef2 , ghr, wasf1, nampt and uba1 in the head kidney, skin or both tissues indicated a response mechanism to higher salinity (20 PSU). Altogether, only the quantification of the atp1a1a gene in the gill tissue provided reliable evidence of successful acclimation from low- to high-salinity water in all three facilities investigated. Based on the overall transcriptional data and the generally good fish physical constitution, we assume that maraena whitefish were not negatively affected by salinities up to 20 PSU suggesting their ability to tolerate dynamic brackish-water environments.