Urea Synthesis/excretion and Ammonia Toxicity in the Gulf Toadfish (opsanus Beta): Basic and Applied Aspects

This study investigated the acute effects of un-ionized ammonia (NH3-N) on11 g fingerlings of Nile tilapia Oreochromis niloticus at two temperatures(23 and 33C); and at 28C with two sizes of fish, 3 and 45 g. In addition,sublethal effects of un-ionized ammonia on growth of Nile tilapia at 28Cwere evaluated. All tests were conducted in flow-through bioassay system. In acute toxicity tests at 23 and 33C, the 96-h LC50's (±SD) were 2.25 ±0.07 and 2.51 ± 0.16 mg/L NH3-N, respectively. There was not a significanteffect between the two temperatures on acute toxicity of NH3-N in these tests(P 0.05). Estimates of the 96-h LC50's (±SD) at 28C were 1.36 ± 0.45mg/L NH3-N for the small fish and 2.65 ± 0.09 mg/L NH3-N for the largefish. In these tests, there was a significant difference between the two sizesof fish tested (P 0.05). In 35-day study of the effects of sublethalconcentrations of NH3-N at 28 C, there was a linear decrease in fish weightgain with increasing un-ionized ammonia concentrations (r = 0.90, P0.001). The concentrations of un-ionized ammonia that cause no reductionin growth, 50% reduction in growth, and 100% reduction in growth were0.06, 0.73, and 1.46 mg/L NH3-N, respectively. These data suggest thatNile tilapia has a tolerance to un-ionized ammonia similar to that of tilapiaspecies, somewhat greater than that of channel catfish, and greater than thatof many other warmwater fish and salmonids.