Growth experiment
To explore the differences in growth, which we used as a proxy for
fitness (e.g., Perez & Munch, 2010), between the selection lines, we
measured the average growth rate of fish reared in different
MnSO4 concentrations. The standard length (SL) of each
fish was measured once a week to the nearest 0.1 mm. For that, fish were
anesthetized and photographed, and SL measured from the photos with
ImageJ image processing software
(www.imagej.nih.gov). The measurements
started at the age of 70 dpf, when fish were robust enough for
anesthesia, and continued weekly until the age of 161 dpf. Because fish
at age 70 dpf were assumed to be too fragile to survive individual
marking, we did not have individual-level body size data but used
rearing group averages for SL. A final body size (SL) together with wet
mass (WM) was measured at age of 218 dpf. Then, we calculated relative
condition factor (K ) for each fish.