Influence of nutritional status on the daily patterns of nitrogen excretion in the carp (Cyprinus carpio L.) and the rainbow trout (Salmo gairdneri R.).

Nitrogen (ammonia and/or urea) excretion in carp and rainbow trout kept under different feeding regimes was continuously monitored over 24-hr cycles. The daily nitrogen excretion patterns resulting from these feeding regimes were studied over several days after a change from one made to another. Constant levels of endogenous nitrogen excretion were reached about one week after the start of a fasting period; almost one week was needed for the daily nitrogen excretion pattern to stabilize after the feeding rhythm was changed. Overall daily nitrogen excretion rates were directly related to nitrogen consumption in carp as well as in trout. The rate of ammonia excretion increased immediately after each meal; the maximum rate occurred at different intervals, depending on the amount of nitrogen intake as well as on the time-lapse after a particular feeding regime was initiated. Contrary to data on sockeye salmon, post-digestive nitrogen excretion rate was distinctly different from the ENE rates observed during fasting in carp and rainbow trout.

Renal regulation of acid-base balance in a freshwater fish (1).

Intra-arterial injection of a fixed acid load caused only a short-lived (less than 2h) disturbance of blood pH but a long lived (2-3 days) elevation of urinary acid excretion in freshwater trout (Salmo gairdneri). The renal response comprised an immediate increase in acid output in the form of titratable acidity minus bicarbonate, and a slower rise in acid output in the form of ammonia. The total elevation in urinary acid efflux over 72h was such that no other mechanism besides renal function is needed to explain the ultimate compensation of this experimental acid-base disturbance.