Ammonia and urea excretion rates of juvenile Australian short-finned eel (Anguilla australis australis) as influenced by dietary protein level

This study aimed to determine excretion rates of ammonia and urea of Austra lian short-finned elvers as influenced by varying dietary crude protein int ake. Elvers (2.3 +/- 0.02 g) were fed diets containing dietary crude protei n levels of 25% (P25), 35% (P35), 45% (P45) and 55% (P55) dry matter equiva lent to 14.17, 19.24, 20.57 and 26.39 g CP/MJ, respectively (pairs of diets P25, P35 and P45, P55 were isoenergetic). Elvers were fed twice a day to a total of 6% BW/day and nitrogenous excretory products (ammonia- and urea-n itrogen) measured during the following 24 h and peak excretion rates occurr ed 4-8 h following both the morning and afternoon feed. Daily ammonia-nitro gen excretion was significantly (P < 0.05) higher on the P55 diet compared to the P35 and P45 diets. Increasing dietary protein intake resulted in inc reasing ammonia- (y = 0.022 x + 0.058; n = 12; r(2) = 0.88; P < 0.001) and urea-nitrogen (y = 0.0044x + 0.426; n = 12; r(2) = 0.55; P < 0.01) excretio n. The highest urea-nitrogen excretion as a percentage of consumed nitrogen was measured for fish fed the P25 diet (41.99 +/- 2.62%) and compared with 30.29 (+/- 3.58%), 25.76 (+/- 1.41%) and 23.57 (+/- 1.54%) for diets P35, P45 and P55, respectively. The Australian short-finned eel appeared to be s imilar to other teleost and eel species in terms of the magnitude of ammoni a-nitrogen excretion following feeding. However, higher rates of urea-nitro gen excretion indicates that urea is an important excretory end-product in this species. (C) 2001 Elsevier Science B.V. All rights reserved.