Abstract
Resume Brown trout and rainbow trout (average weight 100 g) were reared in fresh water at 12OC under the same conditions beforc transfer of brown trout to sea water, in order to compare nitrogen utilisation in the two species. Apparent protein digestibility (ADC), nitrogen (ammonia and urea) excretion, protein productive value (PPV) and actual observed nitrogen mass balance were determined. Rainbow trout raised in fresh water had a higher growth rate (1.1 vs 0.8Yb.d-'), better food conversion ratio (0.7 vs 1.0), better ADC (91 vs 85%) and PPV (45 vs 35%) and lower ammonia excretion rates than brown trout reared in fresh water. Transfemng brown trout to sea water induced lower PPV (30%) and ammonia and urea excretion. Salinity did not rnodify metabolic efficiency in brown trout. Fat content was higher in brown trout (7.7-8.9% ww) than in rainbow trout (5.7-7.6% ww). Nitrogen mass balance indicated that compounds other than arnmonia and urea were produced in higher quantities by seawater brown trout. Behaviour, less domestication and specific ability to utilise protein could explain the differences between the two species.
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