Metabolic rate measurements
Mass-specific metabolic rates (mg O2 per g fish WM per h) of adult zebrafish (age 217 dpf) were estimated from the oxygen consumption of the fish measured in an intermittent-flow respirometer connected to a fibre-optic oxygen sensor (Loligo Systems 3-channel OXY-4 sensors). Fish were fasting 24 hours before the measurement as feeding increases oxygen consumption (Ferreira et al., 2019). Oxygen consumption was measured at 25.3 °C. We placed one rearing group (N=5) at a time in the respirometer chamber (mean volume 313.5 ml). Oxygen consumption was measured for three minutes with a three-minute flush and a two-minute wait period over a 20–21 h period resulting in altogether a total of 120–130 observations per group per measurement period. The chambers were kept for the first two hours in light and the following 18–19 hours in the dark. There are different ways to define standard metabolic rate (SMR) and maximum metabolic rate (MMR; e.g., Hvas & Oppedal 2019; Lucas et al., 2014; Svendsen et al., 2017). Here, SMR was based on the mean of the three lowest oxygen consumption values during the measurement period in the dark while MMR was assumed as the mean of the three highest oxygen consumption values in light. To exclude potential bacterial oxygen consumption, the oxygen consumption of empty chambers was measured before and after each measurement with fish in the chambers. If the difference between these two measurements of bacterial oxygen consumption was more than 1 mg O2 per min, a linear interpolation was applied over the data. After the respirometer measurements, we measured the SL and WM of each fish.