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

AUTHOR/S: G. L. MIHALCA, O. TITA, M. TITA, A. MIHALCA
Sunday 1 August 2010 by Libadmin2010

10th International Multidisciplinary Scientific GeoConference - SGEM2010, www.sgem.org, SGEM2010 Conference Proceedings/ ISBN 10: 954-91818-1-2, June 20-26, 2010, Vol. 2, 693-700 pp

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.

Keywords: Phosphorus; Salmonid; Diet; Urine; Plasma; Kidney