Influence of environmental temperature on flow rate of foodstuffs through the gastrointestinal tract of rainbow trout

Experiments were carried out with rainbow trout ( Salmo gairdneri) to determine the rate of passage of foodstuffs through the digestive tract, before and after a rise in environmental temperature from 9–10°C to 18°C in 1 day. Transit rate was analyzed from the rate of fecal excretion of a marker (Cr 2O 3) incorporated in the diet. In a first experiment on trout weighing about 80 g, mean retention time (MRT) (Castle, 1956) and mean transit time (MTT) (Zierlier, 1958) of the marker, starting from the 9°C acclimated values (34 h for MRT and MTT), decreased as early as the first day after the temperature change (24 and 26 h respectively) and reached the 18°C acclimated values (19 and 21 h respectively) in 4–7 days. In a second experiment on trout weighing about 140 g the pattern of response of MRT and MTT to the temperature increase was similar to that observed in the first experiment: they were 27 and 29 h at 10°C, 16 and 19 h on the second day after the temperature change, and 13 h and 15 h after acclimation at 18°C, respectively. These changes related to environmental temperature were independent of any increase in food consumption. The differences in transit rate of marker between the two experiments could be attributed to the higher body weight and food consumption in the second experiment. Excretion of Cr 2O 3 in the feces was analyzed in the second experiment through a four-compartment model involving the stomach, anterior and posterior intestine, and feces. Fractional evacuation rate of marker in each digestive tract compartment was calculated according to the model. All compartments were affected by the temperature increase. There was a transitory acceleration of transit rate, observed principally in the stomach. These alterations were achieved by the eighth day after the temperature change and it was concluded that acclimation to the new temperature was accomplished in 1 week.