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.