Short-term fasting and welfare prior to slaughter in rainbow trout, Oncorhynchus mykiss

•We evaluated the effect of fasting and slaughter time on stress in rainbow trout.•Cortisol, glucose, lactate, hematocrit and leucocytes were measured.•Rainbow trout can cope well with fasting up to 58.0 °C days (three days).•We did not find clear differences in stress levels throughout the day.

Fasting fish before slaughter is a common practice in aquaculture but it is not clear how long rainbow trout can be starved before suffering unnecessary stress, nor at what moment of the day slaughter is least stressful. We fasted 90 rainbow trout (Oncorhynchus mykiss; initial average weight 215.0 ± 22.6 g) for 24, 48 and 72 h (19.5, 38.8 and 58.0 °C days) and slaughtered them in the morning (08 h00), afternoon (14 h00) and night (20 h00) to observe the effect of fasting duration and slaughter time on welfare indicators, including plasma cortisol, glucose and lactate concentrations as well as hematocrit and leucocyte count. The values of the fasted fish were compared with 90 control fish kept under similar conditions but not fasted. Body weight was not significantly different between fasted fish and controls during the trial but the relative weight of the gut was higher in control trout. Cortisol levels were similar between fasted and control fish and among the treatment groups. Similar results were found for glucose and lactate concentrations in plasma. Hematocrit values were also normal and similar between fasted fish and controls throughout the experiment, but leucocyte count was slightly lower in fasted fish by day three. There were no clear differences in any of the stress parameters in the morning, afternoon and night in either treatment. These results suggest that rainbow trout can cope with fasting up to three days (58.0 °C days) prior to slaughter and that their welfare is therefore not seriously compromised.