Phosphorus utilization by rainbow trout ( Oncorhynchus mykiss): estimation of dissolved phosphorus waste output

Abstract

Phosphorus (P) waste output, notably in the dissolved form (DWP), is a major concern for many fish culture operations. Fish are believed to excrete DWP via the urine but this aspect has never been examined in detail. A better understanding of P utilization and renal P handling of fish could aid development of nutritional strategies for the management and reduction P waste. Rainbow trout were fed high corn gluten meal diets, supplemented with dibasic calcium phosphate, containing increasing P levels (0.75, 1.15, 1.66 and 2.19%). P utilization was examined in a 16-week growth trial. A second trial was conducted to determine urinary inorganic P (Pi) excretion using a non-invasive technique involving the use of a glomerular filtration marker and spot-sampling of urine. A third trial was conducted to measure DWP output through P accumulation in water. Increasing dietary P intake had no significant effect on growth and feed efficiency but significantly increased whole carcass and vertebrae P content. Efficiency of P retention decreased with increasing P intake. DWP represented 25, 47, 63 and 71% of digestible P intake as digestible P increased from 0.29, 0.62, 0.94 to 1.27%. Above a “threshold” plasma inorganic P (Pi) concentration (86 mg Pi l −1), urinary Pi excretion was related to plasma Pi in a linear fashion and could be estimated as follows: urinary Pi output [mg kg −1 body weight (BW) day −1]=−360+4.2 plasma Pi (mg l −1); ( R 2=0.82, P<0.001). DWP output estimates, based on P accumulation, were 10, 63, 75 and 112 mg kg −1 BW day −1 for fish fed the diets with 0.29, 0.62, 0.94 and 1.27% digestible P, respectively. The DWP output of these fish, estimated from the difference between digestible P intake and expected P retention, were 7, 29, 58 and 92 mg kg −1 BW day −1. Urinary Pi excretion rates, estimated based on plasma Pi of the fish, were 0, 21, 94, 101 mg kg −1 BW day −1. This study suggests that plasma Pi is the main factor determining DWP output of fish and that plasma Pi measurements could help in the estimation of P adequacy of the diet or DWP output.