Daily Pattern of Nitrogen Excretion and Oxygen Consumption of Sockeye Salmon (Oncorhynchus nerka) under Controlled Conditions

Abstract

Measurements of the rate of NH4 and urea excretion of fingerling sockeye salmon (O. nerka) in freshwater were made at 2-3-h intervals throughout the day (average weight = 29 g; temperature = 15.degree. C). One group of fish was fed a maintenance ration while another group was starved for 22 days. Ammonia excretion rose to a sharp peak of 35 mg N/kg per h, 4-4 1/2 h after the start of feeding (at 0830) and fell to a baseline level of 8.2 mg N/kg per h between 0200-0800. Urea excretion remained relatively steady at a mean rate of 2.2 mg N/kg per h throughout the day, showing no diurnal response to feeding. Starved fish showed a N excretion rate close to that for both the steady state of urea excretion and the baseline rate of ammonia excretion of the fed fish. O2 consumption rose to a peak of 370 mg O2/kg per h just before and during a 1-h feeding period, decreasing thereafter to a low of 170 mg O2/kg per h at 0300 h. For the starved fish this diurnal metabolic fluctuation continued from the start in a variable and diminishing form; N excretion showed no such response. The results are discussed in relation to hatchery observations. For nonstressed salmon at 15.degree. C NH4 is the chief excretory product of exogenous N metabolism.