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

Abstract

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.