Effect of degree-days of fasting stress on rainbow trout, Oncorhynchus mykiss

• Effect of degree days of fasting on body measurements and haematological response
• The degree days of fasting affect the gut emptying and the depletion of reserves.
• The haematological response is solely affected by the water temperature.
• Fasting up to 68 degree days does not affect fish welfare prior to slaughter.

Most farm animals are fasted before slaughter to empty the digestive system but in fish, the appropriate fasting time also depends on the water temperature. To analyze how the physiological and haematological response vary with degree-days (°C days), 180 rainbow trout (Oncorhynchus mykiss) were fasted for 1, 2 or 3 days in two different trials with water at 22.7 or 11.1 °C. In general, water temperature had a significant influence on most variables measured, although the number of degree days had a less important effect on trout physiological response, being only important in the depletion of reserves rate. The condition factor decreased at 2 and 3 days of fasting. At warmer temperatures (22.7 °C), relative weight of the gut content, hepatosomatic index and plasma levels of glucose were lower while cortisol, lactate and haematocrit were higher than at colder ones (11.1 °C). Fasting up to 68 °C days did not seem to have a negative effect on stress but a high water temperature above 20 °C was stressful for trout.Statement of relevanceThis paper present data on the effect of the number of degree days of fasting on body measurements and the haematological response of rainbow trout prior to slaughter. Fasting up to 68 degree days did not have a major effect on their welfare and that their haematological response was solely affected by the water temperature and not by food deprivation