Dogmas and controversies in the handling of nitrogenous wastes: The effect of feeding and fasting on the excretion of ammonia, urea and other nitrogenous waste products in rainbow trout

Abstract

SUMMARY Ammonia and urea are the primary forms of nitrogen excretion in teleost fish. There exists, however, a discrepancy between the sum of ammonia plus urea nitrogen and total nitrogen, indicating that `unknown' nitrogen end products may play an important role in nitrogen metabolism. The current study analysed a wide range of nitrogen end products in both fed and fasted juvenile rainbow trout. Ammonia-N (53–68%) and urea-N (6–10%) were confirmed as the most important forms of nitrogenous waste, but an interesting finding was the considerable excretion of nitrogen as amino acids(4–10%), via the gills, and as protein (3–11%), probably via the body mucus. Use of anal sutures delineated an important role for the gastrointestinal tract in the production of ammonia-N and urea-N in fed fish, but amino acid-N and protein-N output by this route were both negligible. Alternative nitrogen products – trimethylamine,trimethylamine oxide, uric acid, and nitrite + nitrate – were not excreted in detectable quantities. Creatine-N and creatinine-N outputs were detected but contributed only a small fraction to total nitrogen excretion(<1.4%). Despite the wide scope of nitrogenous end products investigated, a considerable proportion (12–20%) of nitrogen excretion remains unknown. Possible alternative end products and methodological considerations are proposed to explain this phenomenon. The findings described above were used to recalculate the nitrogen quotient(NQ=ṀN/ṀO2)on trout that had been either fasted or fed various daily rations (1%, 3% or 5% dry food per unit wet body mass per day). Feeding increased oxygen consumption (ṀO2)and total-N excretion (ṀN). The NQ is often used as a measure of protein utilisation in aerobic metabolism and assumes that all protein (and amino acid) fuels are converted by oxidation to nitrogenous waste products that are excreted. However, the results showed that calculation of the NQ based on total nitrogen excretion may overestimate protein utilisation in aerobic metabolism because of significant excretion of N in the form of proteins and amino acids, whereas the use of summed ammonia-N and urea-N excretion probably underestimates the contribution of protein towards aerobic metabolism. These errors increase as ration increases, because the discrepancy between total-N excretion and ammonia-N + urea-N excretion increases.

Revisiting the effects of crowding and feeding in the gulf toadfish, Opsanus beta: the role of Rhesus glycoproteins in nitrogen metabolism and excretion

“…In keeping with this hypothesis, the relative mRNA expression and presumably protein of all Rh isoforms in the intestine was significantly higher in fed than in fasted fish. These findings are consistent with a previous study by Kajimura and co-workers, who demonstrated that significant quantities of ammonia are lost through gastrointestinal routes in fed rainbow trout (Kajimura et al, 2004).…”

“…In addition to playing a prominent role in digestion and nutrient absorption, the gastrointestinal tract is an important site of ammonia transport and a major contributor to post-prandial ammonia release in fish (Brown and Cameron, 1991;Kajimura et al, 2004). At present, information about gastrointestinal Rh proteins comes from a single mammalian study by Handlogten and colleagues (Handlogten et al, 2005), who demonstrated that both RhBG and RhCG mRNA and protein were prominently expressed in the proximal regions of the stomach and intestinal tract, with a trend for decreasing expression along the duodenum, jejenum, ileum and colon (Handlogten et al, 2005).…”

Waste loading into a regulated stream from land-based trout farms

“…Axler et al 1997). Ammonia nitrogen can form 53 to 69% of total nitrogen wastes in the effluent of rainbow trout farms (Kajimura et al 2004), but the ratio may increase up to 79% in some in stances (Dalsgaard & Pedersen 2011). The ratios of TAN/TN in effluents in our study were unexpectedly lower than the literature values, suggesting that nitrification of ammonia and temporal variations in the samplings most likely played a significant role, as reported previously (2008), and Aubin et al (2011).…”

“…The ratios of TAN/TN in effluents in our study were unexpectedly lower than the literature values, suggesting that nitrification of ammonia and temporal variations in the samplings most likely played a significant role, as reported previously (2008), and Aubin et al (2011). Although urea, amino acids, and nitrogen excretion via the gills and/or skin and mucus may comprise a considerable amount of the soluble fraction of organic nitrogen (Kajimura et al 2004), both soluble and particulate fractions may reach up to 36% of TN (Foy & Rosell 1991). Because we did not determine these fractions separately, our TON values are indirectly consistent with the range of the TON/ TN ratio published by Foy & Rosell (1991).…”

Wastewater treatment for land-based aquaculture: improvements and value-adding alternatives in model systems from Australia

“…2). This simple approach is used by more than 70% of Aus- Kajimura et al (2004) and Dalsgaard & Pedersen (2011). Graphics are from the Integration & Application Network (ian.umces.edu/symbols/).…”