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

Abstract

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.