A review of the published literature on effects of ammonia on fish indicates that un-ionized ammonia alone is probably not the cause of gill hyperplasia, indicative of, or previously attributed to, chronic ammonia poisoning. The maximum safe concentration of un-ionized ammonia is unknown, but in many cases it is not close to the 0.0125 mg/L value commonly accepted by fish culturists.There is confusion concerning the sublethal, chronic effects of ammonia exposure on fish. In this review I attempt to point out important contributions, as well as contradictions, in the literature. The safe or acceptable levels of ammonia suggested for fish culture are at best questionable, and at worst misleading, for three reasons. First, the commonly accepted maximum safe concentration of ammonia, based on gill histology (hyperplasia of epithelium), has recently been directly and repeatedly contradicted by published empirical data. There is great variation, diurnally and hourly, in ammonia excretion rates due to differences in diets and feeding regimes, and different methods of predicting ammonia levels, based on diets and feeding, produce answers that vary by several fold. Thus a predicted ammonia production rate, calculated from literature examples for an unstudied rearing system, may not be near the actual value. And third, evidence based on studies of acute and chronic ammonia effects indicates that the effects of total metabolites cannot be predicted on the basis of only the concentrations of un-ionized ammonia. Researchers have used various methods inreporting the form and unit of measurement of ammonia. In this review un-ionized ammonia is represented as NH3, the ionized form as NH4 +, and that sum referred to as ammonia. Concentrations are reported in terms of nitrogen, and reviewed data not in this form were converted by multiplying by the appropriate factor (NH3-N = 0.8235 NH•). Allowable Concentration Guidelines Colt and Armstrong (1981) reviewed effects of nitrogen compounds on aquatic animals and noted that sublethal effects were often reported exclusively as the effects of un-ionized Prog. Fish-Cult. 47(3), July, 1985 135 weight of fish that would produce an NH•-N concentration of 0.0125 mg/L. Westers (1981)proposed such a computation and developed a practical loading formula. However, use of the NH•-N concentration as a basis for the formula is in question, primarily because recent evidence discounts much of the work that attributes gill damage to NHs. Also, there is significant variation in reported levels of ammonia production. Moreover, other by-products of metabolism, including fecal solids, bacterial solids, and yet undiscovered toxic metabolites, cannot be ignored (Colt 1978) as factors that may contribute to tissue damage attributed to ammonia. Contradictions in Reported EffectsSmart (1981) reviewed the "safe" levels of ammonia and noted some contradictions. Smart compared the EPA criteria (Willingham et al. 1979) with the finding of Schulze-Wiehenbrauck (1976), that NH3-N concentrations of up to 0.13 mg/L we...

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Phototaxis of walleyes Stizostedion vitreum vitreum was examined during the first 11 weeks of larva and juvenile development. Fish were subjected to six light intensities ranging from 2 to 34 lux, plus 7,800 lux as an extreme. The number of fish positioned in each light intensity was used to determine the phototactic response. Larvae and juveniles 1 to 8 weeks old (9 to 32 mm total length) were attracted to the highest light intensity (7,800 lux), and juveniles older than 8 weeks (32 to 40 mm long) aggregated at the lowest intensities (2 and 4 lux). The change from positive to negative phototaxis was more closely related to fish size than to age. Received July 16, 1982 Accepted February 8, 1983

Parr of Atlantic salmon (Salmo salar) were reared to final densities of 0.53‐4.29 lb/ft2 of rearing container bottom area under conditions of constant temperature, water quality, and diet. Growth and survival were not affected by density. After 171 d, the condition of pectoral and dorsal fins had not changed from that initially recorded and did not differ among treatments. We conclude that rearing density alone does not affect growth, survival, or fin condition of Atlantic salmon reared at densities likely to occur at production hatcheries.