COMPARISON OF BROWN TROUT (SALMO-TRUTTA) REARED IN FRESH-WATER AND SEA-WATER TO FRESH-WATER RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) .1. GROWTH AND NITROGEN-BALANCE

Brown trout and rainbow trout (average weight 100 g) were reared in fr esh water at 12 degrees C under the same conditions before transfer of brown trout to sea water, in order to compare nitrogen utilisation in the two species. Apparent protein digestibility (ADC), nitrogen (ammo nia and urea) excretion, protein productive value (PPV) and actual obs erved nitro en mass balance were determined. Rainbow trout raised in f resh water had a higher growth rate (1.1 vs 0.8%. d(-1)), better food conversion ratio (0.7 vs 1.0), better ADC (91 vs 85%) and PPV (45 vs 3 5%) and lower ammonia excretion rates than brown trout reared in fresh water. Transferring brown trout to sea water induced lower PPV (30%) and ammonia and urea excretion. Salinity did not modify metabolic effi ciency 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 indica ted that compounds other than ammonia and urea were produced in higher quantities by seawater brown trout. Behaviour, less domestication and specific ability to utilise protein could explain the differences bet ween the two species.