In mass culture tanks for the rotifer Brachionus plicatilis, the concentrations of inorganic. nitrogen compounds, pH, water temperature and specific gravity were measured daily at the Aqua-culture Research Laboratory of Nagasaki Prefectural Institute of Fisheries from April to July, 1984. The rotifer density decreased with an increased concentration of un-ionized ammonia nitrogen (NH₃-N). The highest concentration of NH₃-N observed was 3.6ppm. The rotifer densities above 150 rods./ml were observed, only when the pH ranged from 7.3-7.8. Acute toxicity test of NH₃-N to rotifers showed that 24h LC₅₀ value was 17.0ppm at 23°C. Chronic toxicity test showed that concentrations of NH₃-N at which the population growth and the reproduction reduced by 50% of the controls were 13.2 and 7.8ppm, respectively. The concentrations at which the intrinsic rate of population growth and the net reproduction rate were reduced by l% of those of the control, were adopted as the “no-effect” concentration of NH₃-N. They were estimated as 2.1 and 2.4ppm, respectively.
The results from mass culture and toxicity tests showed that higher concentration of NH₃-Nmight be one of restrictive factors affecting the increase of the rotifer.