Scholarly record

PHOSPHORUS UTILIZATION BY RAINBOW TROUT (ONCORHYNCHUS MYKISS): ESTIMATION OF DISSOLVED PHOSPHORUS WASTE OUTPUT

G. L. Mihalca, O. Tita, M. Tita, A. Mihalca

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); (R2 = 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 those plasma Pi measurements could help in the estimation of P adequacy of the diet or DWP output.

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