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Higher‐energy fish feeds can reduce waste discharges and might also improve water quality in recirculating fish‐culture systems. A higher‐energy diet, Zeigler Salmon High Energy feed (HE; 45% protein, 20% fat, 17.4 MJ digestible energy kg‐1) and a lower‐energy diet, Zeigler Hi‐Fat Trout Grower (LE; 38% protein, 12% fat, 14.6 MJ digestible energy kg ‐1) were fed ad libitum at different times to rainbow trout, Oncorhynchus mykiss (Walbaum), in a semi‐closed recirculating culture system by means of demand feeders. The system contained two 10‐m3 fish‐culture tanks, each with a downstream microscreen (80 μn) filter. Feeding rates per day and per unit biomass were not significantly different between diets. In general, use of HE was associated with higher levels of total ammonia nitrogen (TAN) and NO2‐N, lower BOD5 and total suspended solids (TSS), and lower effluent releases of suspended solids per unit feed, NO3‐N per unit feed, and dissolved phosphorus per unit feed. Although total effluent P per unit feed or P fed did not differ significantly between diets, HE had significantly more of the total effluent P in the settleable solids, 85% vs. 76%. Differences in water quality in the system were probably not of great importance with respect to fish health.

Carp pox, a putative viral disease exotic to North America, occurred in golden ide 1 yr after the fish were imported into the United States from the Federal Republic of Germany. The raised, white, plaque-like lesions, which occurred on about 5% of the fish, healed spontaneously and caused no mortality. Electron micrographs showed herpesvirus-like particles associated with lesion specimens; however, no infectious viruses were detected in tests with seven warmwater fish cell lines.

Four fish chemotherapeutants, formalin, benzalkonium chloride, chloramine-T, and hydrogen peroxide were evaluated for their effect on the nitrification efficiency of fluidized-sand biofilters. The chemotherapeutants were added at conventional concentrations to two small-scale (2,200 L) coldwater recirculating rainbow trout (Oncorhynchus mykiss) culture systems each containing six fluidizedsand biofilters operating in parallel. Nitrification efficiency of biofilters was calculated before and after chemotherapeutant treatments by determining ammonia removal efficiency at ambient conditions, and also when challenged with a sudden increase of ammonium chloride at a concentration four times that of the ambient total ammonia-nitrogen (TAN). Two formalin treatments in recycle bath mode at 167 and 300 ppm were conducted with only the 300 ppm treatment having a significant negative effect on biofilter nitrification efficiency. Four single benzalkonium chloride treatments of one and 2 ppm were conducted; two static bath treatments and two recycle bath treatments. Of these four tests, only the recycle bath treatments caused biofilter nitrification efficiency to be significantly impaired. Two multiple treatments with benzalkonium chloride were conducted: one static bath treatment and one recycle.bath treatment. These treatments caused ammonia removal efficiency to decrease by 18% in the static bath treatment and by 63% in the recycle bath treatment. Of these two tests, only the recycle bath treatment caused a significant impairment of nitrification. Single static bath and recycle bath treatments with 9 ppm of chloramine-T both resulted in significant impairment of nitrification, as did a 12 ppm multiple static bath treatment. A single static bath treatment with 100International Journal of Recirculating Aquaculture, vol. l, no. 1 61 ppm of hydrogen peroxide caused almost total failure of nitrification within 24 h of treatment but biofilters were able to remove 23% of TAN within 48 h of treatment.