Two trials with rainbow trout have been performed to study the effect of dietary protein and phosphorus levels on growth rate and N and P balance. In the first trial, 702 rainbow trout (RT) (106.4 ± 1.26 g initial live weight) were fed for 86 days with three diets with the same ether extract (EE) content [20.45 % dry matter (DM)] but differing in protein and P content: [(36.7, 0.86 (A); 39.0, 0.97 (B) and 43.0%, 1.04% DM (C)]. Specific Growth Rate (SGR) and Feed Conversion Efficiency (FCE) were 0.94 and 1.31; 0.98 and 1.27; 1.05 and 1.17, respectively for treatments A, B and C. Feeding level significantly influenced SGR but not FCE. Nitrogen released into the environment was 46.0, 47.6 and 49.7 kg/t of fish produced for diets A, B and C respectively with no significant differences between treatments. P released into the environment was significantly lower with diet A (6.5 kg/t fish produced vs 7.5 and 7.6 with diets B and C). In the second trial, 360 RT (175 ± 2.5 g initial live weight) were fed 3 extruded diets at 0.94% live weight/d for 56 days. Each diet contained 28% EE and 39.4 (D), 42.0 (E) or 45.0 (F) % crude protein (CP). SGR and FCE increased significantly as dietary protein increased (1.03 and 0.94; 1.07 and 0.90; 1.15 and 0.84, respectively for treatments D, E and F, P < 0.05). N load in the effluents was not affected by dietary treatment (D: 29.9, E: 29.8 and F: 29.1 kg/t) whereas P load per t produced fell from diet E to F (D: 7.3, E: 6.7 and F: 5.9 kg, P < 0.05). The results of these experiments where food intake was restricted showed that the dietary level of N and P play an important role in determining the effluent load of these nutrients. At the same time, extrusion is a valid means of controlling N and P discharge, favourably improving growth rate, feed utilisation and gross protein retention.

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