A tracer (51Cr-EDTA) study was undertaken with juvenile (20 g) freshwater rainbow trout, Oncorhynchus mykiss, using an experimental design that minimized stress effects during feeding and drinking trials. A calculation procedure was developed, where feed intake (pellet number), tracer intake (mL), water in stomach contents (g) and drinking rate (mL·kg-1·h-1) are essential to discriminate between the major stomach water sources prandially and postprandially: water bound in food; initial water absorption of pellets; prandial water intake; postprandial water intake and endogenous stomach secretion. We put forward the hypothesis that intake of dry food with a minor water content (10%) may impose a demand for water to moisture the feed up to the level in natural feed (75%) as preparation for gastric emptying, whereafter food is ready to pass from the stomach through the pyloric sphincter. Moisture content of pellets increased from 9.4 to 24.9% in the pre-meal period. The pellets were ingested with 4.0 to 19.3 μL water per pellet, reflecting high inter-individual variation. Prandially moisture content rose to 52% and further increased to 56% in the delay period. Moisture content was ca. 65%, when pellets began to disintegrate and move through the sphincter in accordance with the hypothesis. Stomach secretion contributed 34-44% of the stomach water and ingested water 25-35%. The sampling and calculation procedure gave convincing evidence for the detailed stomach water budget and this individual approach can be very useful in comparisons of artificial and natural diets. © 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Éditions scientifiques et médicales Elsevier SAS.
Kristiansen, H. R., & Cliff Rankin, J. (2001). Discrimination between endogenous and exogenous water sources in juvenile rainbow trout fed extruded dry feed. Aquatic Living Resources, 14(6), 359–366. https://doi.org/10.1016/S0990-7440(01)01131-7