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.