Experimental design
The experimental setup consisted of two 80 L. tanks (30 or 0 ‰), equipped with a grey plastic wall with punctures, dividing each experimental tank into two 40 L compartments. At the start of the experiment, eight randomly selected fish that appeared healthy were collected from each acclimation tank and placed directly in separate, freshly prepared, experimental compartments. Each salinity/ecotype was tested in their native salinity; saltwater control (SwC) and freshwater control (FwC), and in their non-native salinity; salt water ecotype exposed to freshwater (SwFw) and freshwater ecotype exposed to salt water (FwSw) (Figure 2 ). The fish were kept in the experimental tanks for 6 hours before they quickly were netted out, immediately killed by a swift blow to the head and directly processed for gill tissue sampling. Gills were used as they play an important role in the maintenance of blood ion and acid–base balance (Evans et al. 2005; Hoar and Randall 1984). The experiment was approved by the Norwegian Animal experimentation and care committee (permit no ID 2705) and all efforts were made to minimize suffering. Large and sudden changes in salinity can influence survival and growth (Bachman and Rand 2008); however, no fish died during the six-hour exposure, and results from the same experimental setup demonstrate very low mortality rates also when exposing fish for up to three weeks (Taugbøl et al. 2014a). Also, the same stickleback populations did not express differences in oxygen consumption rate after 14 days of exposure (Grøtan et al. 2012), indicating that long-term salinity change has reverted the organisms back to steady states through adaptive physiological responses.