Nitrogen excretion and determination of nitrogen and energy budgets in rainbow trout (Oncorhynchus mykiss R.) under different feeding regimes

The postprandial excretion pattern of ammonia in dependence of feeding regime (fasting, 1 ×/day, 2 ×/day, 4 ×/day and continuous feeding), was determined in rainbow trout ( Oncorhynchus mykiss ), using continuous flow analysis. In fed fish ammonia peaked c. 7 h after the first meal with no differences in pattern between treatments. Fasting fish did not show a pattern. Overall production rates (NH₄ and NO₂+ NO₃) ranged from 0.29–0.31 g kg⁻¹ BW/d in fed fish and were around 0.07 g kg⁻¹ BW/d in fasting fish. Additionally determined total N (Kjeldahl) showed much higher values in fed fish (0.78–1.05 g kg⁻¹ BW/d) but only slightly higher values in fasting fish (0.11 g kg⁻¹ BW/d). Budgets of nitrogen (N) and energy (E) showed low recoveries ( c. 50% and between 50% and 70%, respectively). When correcting ammonia excretion (NH₄ and NO₂+ NO₃) using literature data on urea excretion of O. mykiss and assuming that total N partly stemmed from uneaten but undetected feed, both N and E budgets reached a recovery of around 100% in all four fed groups. Implications of this approach are discussed in the light of incomplete budgets as determined in earlier studies.