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.