Juvenile freshwater prawns Mucrobruchium rosenkrgii (mean wet weight = 0.75, 1986 or 0.17g, 1985) were stocked into 0.06–0.07 ha earthen ponds at densities ranging from 39,536 to 118,608/ha during two separate growing seasons. After growout periods ranging from 135 to 142 days, survival was from 54.3% to 89.9% (x = 77.0%). Mean prawn weight at harvest ranged from 15.0 to 44.3g and decreased with increasing stocking density. The larger stocking weight of prawns was associated with harvest weights that were 16.8 to 39.6% greater than those achieved with the smaller stocking weight at comparable stocking densities. Stocking of juveniles of the proper size and the effective management of the social structure of M. rosenkrgii appear to be critical to the success of intensive pond culture in temperate climates where the length of the growing season is restricted.

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.

Spontaneous outbreaks of bacterial gill disease (BGD) occurred in 17-44-g rainbow trout Oncorhynchus mykiss after they were placed in a water recirculation system. In each of five groups stocked from September 1991 through July 1992, BGD occurred within 6-8 d after stocking. In each instance, BGD was followed by a secondary amoeba infestation. The spontaneous BGD outbreaks did not occur among previously stocked groups that had recovered from earlier BGD disease outbreaks. Examination of gill tissue by Gram stain and indirect fluorescent antibody technique showed increased numbers of filamentous bacteria associated with BGD after the rainbow trout were stocked into the system. Bacterial numbers decreased after a 1-h treatment with chloramine-T at concentrations of 9-15 mg/L but increased within 2 d after treatment Although the chloramine-T treatments controlled mortality related to BGD, the amoeba infestation persisted. Histological examination of gills showed some focal hyperplasia before the rainbow trout were placed into the recirculation system, but hyperplasia became more extensive and lamellar fusion and mild telangiectasis developed within a week after placement in the system. The density at which the fingerling rainbow trout were stocked and the suspended solids present in tank water may have contributed to the BGD outbreaks. Exposure of juvenile rainbow trout to tank water from the recirculation system before they were placed into the system did not afford them protection against BGD after stocking.

Types of chemical feeding effectors are listed, and locations and functions of decapod chemoreceptors are discussed. Low molecular weight substances found most stimulatory in behavioral, physiological, and electrophysiological studies on decapods are tabulated. Most of these chemicals have not been widely tested, and some are prohibitively expensive for use in feeds. The most generally stimulatory single substances are L‐glutamic acid, glycine, and taurine. Mixtures of substances can be more effective than single substances; e.g., the use of betaine plus amino acids shows some promise.

Post‐larval and juvenile Macrobrachium rosenbergii were exposed for 72 h at 29 C to four pH levels (8.5, 9.0, 9.5, 10.0) and four concentrations of un‐ionized ammonia‐nitrogen (0, 1, 2, and 3 mg/L NHj‐N). Results indicated potentiation between NH3 and high pH. Juveniles were more tolerant of high pH and NH3 than post‐larvae. For post‐larvae, estimates of 72 h LC50 for pH were 9.43, 9.21, and 8.71 at 0, 1, and 2 mg/L NH3‐N, respectively; 72 h LC50 estimates for NH3‐N were 2.18 and 1.45 mg/L at pH levels of 8.5 and 9.0, respectively. For juveniles, estimates of 72 h LC50 for pH were 9.91, 9.56, 9.04, and 8.76 at 0, 1, 2, and 3 mg/L NH3‐N, respectively; 72 h LC50 estimates for NH3‐N were 2.02 and 0.54 mg/L at pH 9.0 and 9.5, respectively. In pond culture of M. rosenbergii, high pH levels can cause mortality at stocking. The 72 h data can be used as an indication of safe stocking levels of pH and ammonia. These data suggest that post‐larvae should not be exposed to pH > 9.0 nor to NH3‐N > 1 mg/L in the pH range 8.5–9.0 and juveniles should not be exposed to pH > 9.5 nor to NH3‐N > 0 mg/L at pH 9.5, > 1 mg/L at pH 9.0, or > 2 mg/L at pH 8.5.