Non-fecal phosphorus (P) excretion in freshwater rainbow trout measured in a recirculating tank system (Fig. 3A; Sugiura, 1998). Non-fecal P excretion, primarily urinary when properly sampled, provides a rapid and clear response (Fig. 3B), making it useful for estimating dietary P requirements of large or adult fish. In this experiment, 13 fish (average body weight 203 g / fish) were placed in a 150-liter tank for 24 hours without an external water supply. The recirculating tank system continuously collected feces into a long, unstirred water column within minutes of defecation. Although some soluble components in the feces may have leached in the unstirred water column, they remained separated from the tank water. The collected feces, along with water from the unstirred column, were considered fecal excretion, while any changes in the tank water were classified as non-fecal excretion.