urea-N Excretion Rate Research Articles

The physiological consequences of a very large natural meal in a voracious marine fish, the staghorn sculpin (Leptocottus armatus).

Little information exists on physiological consequences when wild fish eat natural food. Staghorn sculpins at 10-13°C voluntarily consumed 15.8% of their body mass in anchovies. Gastric clearance was slow with >60% of the meal retained in the stomach at 48 h, and was not complete until 84 h. At 14-24 h post-feeding, pH was depressed by 3 units and Cl- concentration was elevated 2-fold in gastric chyme, reflecting HCl secretion, while in all sections of the intestine, pH declined by 1 pH unit but Cl- concentration remained unchanged. PCO2and total ammonia concentration were greatly elevated throughout the tract, whereas PNH3and HCO3- concentration were depressed. Intestinal HCO3- secretion rates, measured in gut sacs in vitro, were also lower in fed fish. Whole-animal O2 consumption rate was elevated approximately 2-fold for 72 h post-feeding, reflecting 'specific dynamic action', whereas ammonia and urea-N excretion rates were elevated about 5-fold. Arterial blood exhibited a modest 'alkaline tide' for about 48 h, but there was negligible excretion of metabolic base to the external seawater. PaCO2and PaO2remained unchanged. Plasma total amino acid concentration and total lipid concentration were elevated about 1.5-fold for at least 48 h, whereas small increases in plasma total ammonia concentration, PNH3and urea-N concentration were quickly attenuated. Plasma glucose concentration remained unchanged. We conclude that despite the very large meal, slow processing with high efficiency minimizes internal physiological disturbances. This differs greatly from the picture provided by previous studies on aquacultured species using synthetic diets and/or force-feeding. Questions remain about the role of the gastro-intestinal microbiome in nitrogen and acid-base metabolism.

Gut transport characteristics in herbivorous and carnivorous serrasalmid fish from ion-poor Rio Negro water.

Three closely related characids, Tambaqui (omnivore), black Piranha (carnivore), and Pacu (herbivore), all Serrasalmidae, inhabit the ion-poor, acidic Rio Negro. We compared O2-consumption and N excretion rates in vivo, and sodium, chloride, glucose, and ammonia transport characteristics of gut sac preparations in vitro. The Pacu had a significantly higher weight-specific oxygen consumption, and a lower N/Q ratio than the omnivorous Tambaqui, and a significantly lower urea-N excretion rate than the carnivorous black Piranha, suggesting N-limitation in the herbivorous Pacu. With a value of 2.62±0.15, gut to fork length ratio in the Pacu was about 2.5 times higher than in the black Piranha, and 2.0 times higher than in the Tambaqui. Anterior intestinal activities of three enzymes involved in N-fixation for amino acid synthesis (glutamate dehydrogenase, glutamate-oxaloacetate transferase, and glutamate-pyruvate transferase) were generally greatest in the carnivore and lowest in the herbivore species. In all three species, sodium, chloride, glucose, and ammonia were taken up at high rates from the intestine, resulting in an isosmotic fluid flux. Comparing the area-specific fluid flux of the anterior, mid, and posterior gut sections, no difference was detected between the three sections of the Pacu, while in the Tambaqui, it was highest in the anterior section, and in the black Piranha highest in the middle section. Overall, the area-specific uptake rates for sodium, chloride, glucose, and ammonia of anterior, mid, and posterior sections were similar in all three species, indicating that there is no difference in the area-specific transport rates associated with trophic position. The net ammonia uptake flux from gut interior was not significantly different from the net ammonia efflux to the serosal fluid, so that the ammonia removed from the intestine by the mucosal epithelium was quantitatively transferred through the tissue to the serosal side in all three species. Thus, metabolic activity of gut tissue did not significantly influence the net ammonia transfer. Due to the much higher gut to fork length ratio, the overall transport capacity of the gut of the herbivorous Pacu by far exceeded the transport capacity of their carnivorous and omnivorous relatives, thus compensating for the lower digestibility and the low Na(+), Cl(-), and N-content of the plant diet. Accordingly, in order to cope with the more difficult digestible plant material and the very low nitrogen content of plants, herbivorous fish have not evolved more effective area-specific transport capacities, but rather have increased the length of the gut.

Ammonia and urea excretion rates of California halibut ( Paralichthys californicus, Ayres) under farm-like conditions

In the design and optimization of aquaculture water recirculation and treatment technologies, it is important to know the nitrogen excretion rates of the fish being cultured. Rates of total ammonia nitrogen (TAN) and urea-N excretion were measured for California halibut of 4.2, 7.2, 10.4, 20.1, 112 and 199 g mean mass. The fish were fed with a feed of a size-appropriate pellet containing 43–45% protein continuously over 12 h during the light hours (photoperiod of 16 h daylight and 8 h dark). Feeding was started 2 h into the light period after the tanks had been cleaned and routine water analyses had been made. Daily average TAN excretion rates ranged from 4.3 to 8.5 mg TAN/g feed, and urea-N excretion rates ranged from 0.8 to 1.8 mg urea-N/g feed. Corresponding values relative to fish mass were between 91 and 113 mg TAN/kg fish d and between 13 and 20 mg urea-N/kg fish d. Peaks of TAN excretion occurred 4–6 h after feeding had started and 6–10 h after conclusion for the 4–20 g fish. Corresponding values for the 112 and 199 g fish were 10–12 h after feeding had started and 8–10 h after feeding ended. Some minor fluctuations in the urea excretion rates were noted, but there were no consistent peaks. TAN excretion accounted for 81.6 to 88.1% of the total daily urea-N plus TAN excretion by California halibut with the balance consisting of urea-N excretion. Although urea-N makes up a relatively small portion of the total daily nitrogen excretion by California halibut, there were times during the day during when the hourly urea-N excretion rate was similar to or larger than the corresponding TAN excretion rate. All excretion measurements were made under farm-like conditions with fish reared in a marine recirculating system.

Effect of alkalinity (pH 10) on ureogenesis in the air-breathing walking catfish, Clarias batrachus

Exposure of fish to alkaline conditions inhibits the rate of ammonia excretion, leading to ammonia accumulation and toxicity. The purpose of this study was to determine the role of ureogenesis via the urea cycle, to avoid the accumulation of ammonia to a toxic level during chronic exposure to alkaline conditions, for the air-breathing walking catfish, Clarias batrachus, where a full complement of urea cycle enzyme activity has been documented. The walking catfish can survive in water with a pH up to 10. At a pH of 10 the ammonia excretion rate by the walking catfish decreased by approximately 75% within 6 h. Although there was a gradual improvement of ammonia excretion rate by the alkaline-exposed fish, the rate remained 50% lower, even after 7 days. This decrease of ammonia excretion was accompanied by a significant accumulation of ammonia in plasma and body tissues (except in the brain). Urea-N excretion for alkaline-exposed fish increased 2.5-fold within the first day, which was maintained until day 3 and was then followed by a slight decrease to maintain a 2-fold increase in the urea-N excretion rate, even after 7 days. There was also a higher accumulation of urea in plasma and other body tissues (liver, kidney, muscle and brain). The activity of glutamine synthetase and three enzymes operating in the urea cycle (carbamyl phosphate synthetase, argininosuccinate synthetase, argininosuccinate lyase) increased significantly in hepatic and extra-hepatic tissue, such as the kidney and muscle in C. batrachus, during exposure to alkaline water. A significant increase in plasma lactate concentration noticed during alkaline exposure possibly helped in the maintenance of the acid–base balance. It is apparent that the stimulation of ureogenesis via the induced urea cycle is one of the major physiological strategies adopted by the walking catfish ( C. batrachus) during chronic exposure to alkaline water, to avoid the in vivo accumulation of ammonia to a toxic level in body tissues and for the maintenance of pH homeostasis.

Nitrogen utilisation and ureogenesis as affected by dietary nucleic acid in rainbow trout ( Oncorhynchus mykiss ) and turbot ( Psetta maxima )

We evaluated the influence of dietary nucleic acid (NA) supplied either as an RNA extract (RNA) or as brewer's yeast (BY) on nitrogen (N) utilisation, N excretion and ureogenesis in a freshwater teleost, the rainbow trout (Oncorhynchus mykiss) and a marine teleost, the turbot (Psetta maxima). Five diets containing two levels of NA or BY were formulated for each species, and fed to juvenile rainbow trout (initial body weight (IBW): 21.7 g) and turbot (IBW: 11.8 g) over 8–10 weeks. Besides growth and N utilisation, we monitored total ammonia-N and urea-N excretion rates and measured the activities of selected enzymes (glutamate dehydrogenase, ornithine carbamoyltransferase, arginase and uricase). There was no clear N sparing effect of dietary NA or BY supplementations in either species. Feeding diets containing the RNA extract led to an almost two-fold increase in postprandial plasma urea-N and uric acid concentrations and in urea-N excretion rates. Glutamate dehydrogenase and arginase activities were decreased in fish fed NA supplemented diets. Uricase activities were inversely related to dietary NA levels. Data on N excretion and enzyme activities clearly suggest that ureogenesis is influenced by dietary NA in both species.

Role of ureogenesis in the mud-dwelled Singhi catfish ( Heteropneustes fossilis) under condition of water shortage

The air-breathing Singhi catfish Heteropneustes fossilis was kept inside moist peat for 1 month mimicking their normal habitat in summer and the role of ureogenesis for their survival in a water-restricted condition was studied. The ammonia excretion rate by the mud-dwelled fish increased transiently between 6 and 12 h of re-immersion in water to approximately between eight and 10-fold, followed by a sharp decrease almost to the normal level at the later part of re-immersion. The urea-N excretion by the mud-dwelled fish increased to approximately 11-fold within 0–3 h of re-immersion, followed by a gradual decrease from 9 h onwards. The rate of urea-N excretion by the mud-dwelled fish, however, remained significantly higher (approx. threefold more) than the control fish even after 36–48 h of re-immersion. Although there was a significant increase of both ammonia and urea levels in the plasma and other tissues (except ammonia in the brain), the level of accumulation of urea was higher than ammonia in the mud-dwelled fish as indicated by the decrease in the ratio of ammonia: urea level in different tissues including the plasma. The activities (units/g tissue and /mg protein) of glutamine synthetase and three enzymes of the urea cycle, carbamyl phosphate synthetase, argininosuccinate synthetase and argininosuccinate lyase increased significantly in most of the tissues (except the brain) of the mud-dwelled fish as compared to the control fish. Higher accumulation of ammonia in vivo in the mud-dwelled Singhi catfish is suggested to be one of the major factors contributing to stimulation of ureogenesis. Due to this physiological adaptive strategy of ureogenesis, possibly along with other physiological adaptation(s), this air-breathing amphibious Singhi catfish is able to survive inside the moist peat for months in a water-restricted condition.

Stimulation of ureogenesis in the perfused liver of an Indian air-breathing catfish, Clarias batrachus, infused with different concentrations of ammonium chloride

The pattern of changes of activity of the urea cycle enzymes and the rate of urea-N excretion were studied in the perfused liver of an Indian air-breathing ureogenic walking catfish, Clarias batrachus. The liver was perfused with different concentrations of NH4Cl for a period of 60 min to determine the role of ammonia for stimulation of hepatic ureogenesis and the threshold level of ammonia loading needed to cause such stimulation. Both the urea-N excretion and the ammonia uptake by the perfused liver were found to be a saturable process. Ammonia accumulated significantly in the liver infused with 1.25 μmoles g liver −1 min−1 of NH4Cl, followed by a maximum accumulation of about 28.5 μmoles g wet wt−1 with the infusion of 5.08 μmoles g liver−1 min−1. The Vmax of the urea-N excretion (0.47 μmol g liver−1 min−1) was obtained with the addition of 5.08 μmoles g liver−1 min−1 of NH4Cl. Both the tissue and the specific activity of the urea cycle enzymes, except ornithine transcarbamylase and arginase, were stimulated significantly with the infusion of either 1.25 or 5.08 μmoles g liver−1 min−1 of NH4Cl. Maximum stimulation of tissue activity of carbamoyl phosphate synthetase (about 120%) was seen with the infusion of 5.08 μmol g liver−1 min−1, and for argininosuccinate synthetase (about 135%), and argininosuccinate lyase (about 50%) with the infusion of 10.81 μmol g liver−1 min−1 of NH4Cl. Higher accumulation of ammonia of about 10–15 μmol g wet wt−1 from the physiological level in the perfused liver while infusing with NH4Cl was suggested to be one of the major causes of stimulation of ureogenesis. The presence of such physiological adaptive strategy is probably necessary in this unique group of air-breathing walking catfish to survive under hyper-ammonia stress in their normal habitat or while living outside water or while burrowing inside mud.

Toxicity of ammonia to turbot juveniles: 1. effects on survival, growth and food utilisation

Long-term effects of constant exogenous ammonia concentrations were investigated in two different batches of turbot juveniles (53 and 73 g) under controlled environmental and feeding conditions. Over the 84-day experiments, ambient un-ionized ammonia (UIA-N, NH3) concentrations were steady (coefficient of variation, 12-16 %) and water pH range was 7.88-7.99. Survival was maximum up to 0.33 mg.l(-1) UIA-N and at 0.73, 50 % mortalities were observed on day 52 (73 g) or day 77 (53 g). No-growth concentration was 0.73 mg.l(-1) UIA-N in the two groups and the 84-day lowest-observable-effect concentration range 0.14-0.33 mg.l(-1). In the most tolerant group (53 g), the 84-day no-observable-effect concentration was 0.14 mg.l(-1) UIA-N. The estimated 28-day efficient concentration that gave 50 % of the specific growth rate of controls ranged from 0.60 to 0.75 mg.l(-1) UIA-N. Specific growth rate to ambient ammonia concentration patterns were different in the two groups for the lowest concentrations (0.14-0.34) and similar to food intake to ammonia pattern. Food efficiency was negatively correlated to ambient ammonia concentration, with major changes in food conversion ratio, and protein utilisation observed above 0.33-0.70 mg.l(-1) UIA-N. The main change in body composition was an increase in water content in all ammonia-exposed fish. A significant increase in urea-N excretion rate was also observed over 0.33 mg.l(-1) UIA-N. Thresholds for growth were affected by exposure duration. Turbot juveniles growth capacity may be depressed by usual ambient ammonia concentrations under intensive farming conditions.

Induction of ureogenesis in perfused liver of a freshwater teleost, Heteropneustes fossilis, infused with different concentrations of ammonium chloride

The induction pattern of urea cycle enzymes and the rate of urea-N excretion were studied with relation to ammonia load in the perfused liver of a freshwater ammoniotelic teleost, Heteropneustes fossilis, when infused with different concentrations of ammonium chloride for 60 min. Both urea-N excretion and uptake of ammonia by the perfused liver were found to be a saturable process. The V max of urea-N excretion (0.45 μmol/g liver/min) was obtained at ammonium chloride addition of 1.18 μmol/g liver/min. The maximum induction of carbamyl phosphate synthetase (ammonia dependent), ∼200%, and of ornithine transcarbamylase, ∼120%, was seen by the addition of 0.58 μmol/g liver/min, and for argininosuccinate synthetase and argininosuccinate lyase of ∼150% and 115%, respectively, by the addition of 2.8 μmol/g liver/min of ammonium chloride. However, arginase activity did not alter in any of the concentrations of ammonium chloride added. An increase of ammonia load of 3–5 μmol/g wet wt from the physiological level in the perfused liver was sufficient to initiate and to cause maximum induction of most of the urea cycle enzymes activitty. These results further confirm the capacity of transition from ammoniotelism to ureotelism in this unique freshwater air-breathing teleost to tolerate a very high ambient ammonia.

Ammonia and urea metabolism in relation to gill function and acid-base balance in a marine elasmobranch, the spiny dogfish (Squalus acanthias)

Nitrogenous waste excretion in resting dogfish occurred largely (>90 %) as urea-nitrogen (urea-N) efflux across the gills, with a very small urea efflux via the kidney. Ammonia excretion, almost entirely at the gills, accounted for less than 3 % of total nitrogen excretion. Given the extremely high blood urea levels (approximately 640 mmol-N l-1) 'retained' for osmoregulation, and blood ammonia levels (approximately 80 µmol-N l-1) comparable to those of teleosts, the gills of resting dogfish were exceptionally impermeable to both urea and ammonia. Experiments investigated the origins of these low permeabilities and the responses of urea-N and ammonia-N excretion and acid­base status to 6 h infusions with iso-osmotic solutions of NaCl (control), NH4Cl, NaHCO3, urea and its analogues thiourea and acetamide. NaCl had no effects, whereas NH4Cl loading caused intense acidosis and marked elevation of acidic equivalent, ammonia-N and urea-N excretion rates, the latter despite unchanged blood levels of urea-N. Apparent branchial ammonia permeability increased greatly. Acidosis resulted from both stimulated urea production and branchial NH3 loss, the former making the larger contribution. NaHCO3 loading caused intense alkalosis, a marked elevation of basic equivalent excretion and a moderate stimulation of urea-N excretion. Blood urea-N levels were again unchanged. Infusion of urea itself raised blood urea-N levels, but initially reduced branchial urea-N excretion. Acetamide and thiourea infusions both moderately elevated branchial urea-N excretion. We suggest that the low ammonia permeability may arise metabolically from an ammonia scavenging system in the gills, that a 'back-transport' mechanism in the gills may contribute to the low urea permeability, and that the dissociation between blood urea-N levels and excretion rates may reflect urea production at extrahepatic sites. These studies demonstrate that urea synthesis in the dogfish is linked more to nitrogen availability than to acid­base status.

Comparison of a questionnaire to estimate dietary sodium and protein intake to measured urinary sodium and urea-nitrogen excretion

Daily urinary excretion rates of sodium (Na) and of urea-N should accurately reflect daily dietary Na and protein intake in the steady state. Self-administered dietary questionnaires are often used to assess Na and protein intake but their accuracy in relation to measured urine composition is not known. We compared estimates of dietary Na and protein intake based on a self-administered questionnaire, analyzed using Nutritionist-IV software, to the measured mean daily urine Na and urea-N excretion rates in 3 24-hour urines collected by each of 94 healthy adults eating their usual home diets. We studied 50 women and 44 men, ages 20–70 years, BMI 25.2 ± 3.7 SD kg/m2. The urines were accurately collected as judged by their creatinine content in relation to body weight (r=0.65) and to predicted individual daily creatinine excretion based on sex, age and weight using the Cockcroft & Gault formulas (r=0.80). Measured urinary Na excretion, mean 156 and range 47 to 322 mmol/day, was greater than and not significantly related to estimated Na intake based on the questionnaire, mean 114 and range 28 to 239 mmol/day; r=0.14. Similarly, dietary protein intake; estimated as ((Urea-N,g/day)+(Weight,kg∗0.032))∗ (6.25), mean 78 and range 36 to 136 g/day, was less than and not closely related to protein intake based on the questionnaire, mean 95 and range 27 to 206 g/day; r=0.20. Both daily Na (r=0.55) and daily protein intake based on urinary urea-N and weight (r=0.76) were related to daily urinary creatinine excretion. Additionally, urinary potassium (K) excretion, mean 67 and range 24 to 125 mmol/day, was less (as expected because of fecal K excretion) but more closely related to K intake estimated by the questionnaire, mean 94 and range 37 to 213 mmol.day; r=0.42. However, urinary K excretion was not so closely related to daily urinary creatinine excretion, r=0.45, as were Na and urea-N. We conclude: 1) the dietary questionnaire underestimates dietary Na intake, presumably because the questionnaire does not take account of intersubject variation in NaCl added during cooking; 2) the dietary questionnaire overestimates protein intake but the basis for this overestimation is not apparent; 3) daily urinary K excretion in accurately collected urines may provide a useful estimate of dietary K intake.

A rapid method for determination of hepatic amino nitrogen to urea nitrogen conversion ('the Functional Hepatic Nitrogen Clearance').

The Functional Hepatic Nitrogen Clearance (FHNC) is a measure of the functional liver mass as to conversion of amino-N to urea-N. FHNC is the slope of the linear regression of multiple samples (10-20) of urea-N synthesis rates (UNSR) on blood alpha-amino-N concentrations (alpha-AN) during infusion of amino acids. UNSR is measured as urinary urea-N excretion rate corrected for accumulation in total body water (TBW) and loss in gut. A simplified method which estimates FHNC from only two samples of UNSR and alpha-AN was developed. Urine was collected in two hourly intervals: before infusion of alanine, and from 2 to 3 h after start of alanine infusion. Blood-urea-N and alpha-amino-N was measured at the beginning and at the end of each urine sampling interval. TBW was estimated from a nomogram, and gut loss of urea was assigned a fixed value (14%). The two-sample FHNC was calculated as delta UNSR (mmol h-1)/delta mean alpha-AN (mmol l-1). Linear regression analysis of the two-sample estimates of FHNC on the 'true' multiple-sample values of FHNC in an independent population of control and cirrhotic subjects showed the two-sample estimates to be closely related with values of the multiple-sample method, the regression equation being: two-sample FHNC = -0.24 + 0.99 x multiple-sample FHNC, r2 = 0.98. A close relationship was also obtained when cirrhotic patients were considered alone: two-sample FHNC = 0.01 + 0.94 x multiple-sample FHNC, r2 = 0.98.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of Dietary Protein and Lipid on Nitrogen and Energy Losses in Lake Trout,Salvelinus namaycush

Nitrogen and energy loss in fecal and nonfecal excretions by lake trout, Salvelinus namaycush (171–268 g), varied directly with dietary protein (40 and 50% dry weight) and inversely with lipid (10 and 20%) among four test diets. Diets high in lipid increased dry matter digestibility resulting in reduced nitrogen and energy loss in feces. NH3-N constituted 74.0–93.0% of total N excreted in fasted (4–12 d) and fed trout, while urea-N constituted 4.6–17.8% of total N. Excretion of urea-N was high in fish fed the high-lipid diet, particularly when dietary protein was also high. Total N and NH3-N excretion rose to peak values between 4 and 12 h after feeding and thereafter declined to prefeeding levels by 24 h. Urea-N excretion rates varied significantly following ingestion of all but one (50% protein, 20% lipid) test diet, reaching peak values between 4 and 12 h. In fasted fish, total N and NH3-N excretions were significantly low and failed to show a daily pattern; the urea-N excretion rate was significantly high compared with fish fed diets 1, 2, and 3 and followed a daily pattern of change. On average, a larger proportion (13–25%) of food energy is lost through nitrogenous excretion than in the feces (8–22%).

Nitrogenous Excretion of Juvenile and Young Japanese Flounder.

Measurements of the rate of nitrogenous excretion were carried out on fed and starved Japanese flounder of 1.5 to 6.5g (juvenile), and 15 to 91g (young) in body weight at 20°C. Ammonia-N and urea-N excretion of the starved juvenile flounder were 0.76±0.20mg-N and 0.09±0.03mg-N/100g/h, and those of the young flounder were 0.23±0.05mg-N and 0.07±0.03mg-N/100g/h, respectively. Immediately after feeding, the rates of ammonia-N excretion per hour of both juvenile and young flounder became 2 to 3 times higher than those of the starved fish, and these elevated rates continued for more than 12h. In contrast to ammonia-N excretion, the rate of urea-N excretion per hour of the fed fish showed a major peak during the period of 6-12 or 12-24h after feeding. In juvenile and young Japanese flounder fed on a pelleted diet containing 7.5% nitrogen at average rations of 3.0 and 1.3%, respectively, it is estimated that 23 and 21% of the consumed nitrogen was excreted as ammonia-N, 3 and 4% as urea-N, respectively, and about 13% as faeces-N within 24h after feeding.

Effect of Feeding on Nitrogen Excretion of Japanese Flounder Paralichthys olivaceus.

Measurements of the rate of nitrogen excretion were carried out with fed and starved Japanese flounder of 163 to 575g in body weight at 20°C. Ammonia-N and urea-N excretion of the starved fish were 0.20±0.05mg-N/100g/h and 0.04±0.02mg-N/100g/h, respectively. Immediately after feeding, the rate of ammonia-N excretion per hour of the flounder became 3 to 4 times that of the starved fish. These elevated rates continued for 12, 30 and more than 36h, at 0.5, 1.0 and 1.5% rations, respectively. In contrast to ammonia-N excretion, the rate of urea-N excretion per hour of the fed fish showed a major peak during the period of 6-12 or 12-24h after feeding. In Japanese flounder fed on the pelleted diet containing 7.5% of nitrogen at average ration of 0.5%, it is estimated that 32% of the consumed nitrogen was excreted as ammonia-N, about 5% as urea-N and 8% as faeces-N within 24h after feeding.

Patterns of Nitrogen Excretion and Oxygen Consumption During Ontogenesis of Common Carp (Cyprinus carpio)

By monitoring nitrogen (ammonia and urea) excretion and oxygen consumption in a continuous manner during early development (from 18 to 350 degree days) of common carp, Cyprinus carpio, we noted high rates of ammonia-N excretion (> 100 and > 50 μgN∙h−1∙103 eggs−1) and of oxygen consumption (> 1 and > 2.5 mg∙h−1∙103 eggs−1) at hatching and at the onset of free-swimming stages, respectively. There was a diurnal rhythm in metabolic activity. The relative proportions of urea-N and ammonia-N varied during early development; the mean urea-N excretion rate amounted to 21.9 ± 8.7% of total nitrogen, and there was no significant difference between embryonic and postembryonic stages. Fasted free-swimming larvae exhibited increased metabolic activity just before the onset of massive mortality (> 75 μg N∙h−1∙103 eggs−1 and > 2 mg O2∙h−1∙103 eggs−1 of nitrogen excretion and oxygen consumption, respectively). Ammonia excretion rates increased with temperature, the Q10's being 2.36, 2.33, and 3.53 during embryonic, hatching, and free-swimming stages, respectively.Key words: carp, ontogenesis, nitrogen excretion, ammonia, urea, oxygen consumption, temperature, diurnal rhythm