Expression level of the rnf19b gene in gill tissue normalized by the most stable reference genes (rpl8 and hprt) in comparison to normalization with the least stable reference genes (actb and tuba). (A) Relative expression of rnf19b in individuals from site IM, (B) individuals from sites MEL, (C) PEL and (D) SFM. Control bar represents the relative expression of the calibrator (site SFM). Whisker caps represent the minimum and maximum values of the 2−ΔΔCq. *P < 0.05.