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Phenolic wastes contain monohydric phenols, including phenol, the three cresol isomers, and the Eid. xylenol isomers, together with other substances. They may adversely affect freshwater fisheries by their direct toxicity to fish and fishfood organisms, by their high oxygen demand resulting in oxygen depletion of the receiving water, and by the production of undesirable flavours in the edible flesh of fish. Laboratory tests show that the toXicity of phenol isincreacedby decrease in dissolvedoxygen concentration, increase in salinity, and decreeze in Lempela.ture. Salmonids and newly hatched fish are more sensitivo than coarse fish eed aemits respecively. Cresols, xylenols, and phenols are of similar to.cicity, and the bo:.1:icity of miKtures of phenols is apparently additive, although the toxicity of phenolic wastes may be greater than expected from chemical analyses, since these may noL be cqua1ly 3ensitive to all phenols and may neglect the contribution from other poisons. Because of difficulties caused by inadequate chemical analysis there are few field Observations which can be used to reinforce laboratory findings. For this reason, and also because of gaps in our knowledge of the effect of temperature on toxicity, only tentative criteria can be established, which may have to be modified in the light of further experience. These criteria are expressed as maximum concentrations which should not be exceeded but it should be appreciated that because of the natural fluctuation in the water quality found in rivers over a period of time, the average concentration will be lower to an extent depending on local circumstances. Salmonid fish. To ensure longterm survival of salmonids in the presence of phenolic wastes, the concentration of phenol, cresols or xylenols should not exceed 1.0 mg/1, either singly or collectively. Where 2,5xylenol is the main constituent, the concentration should not exceed 0.5 mg/l. Where the temperature is lower than 500, concentrations may have to be halved to ensure the survival of fish. Coarse fish. Since laboratory data show that coarse fish are more resistant than salmonids to phenols, the concentration of phenol, cresol or xylenol should not exceed 2.0 mg/1, either singly or collectively, provided that,exidation of this concentration does not produce an adverso reduction in the dissolved,oxygen concentration of the water. In the absence of data on the effect of temperature on the toxicity of phenols to coarse fish it is proposed that the reduction in concentration of 50% adopted for salmonids at temperatures below 5°C, should also apply to these species. Where other poisons are present in addition to phenols duo allowance must be made for their contribution to the toxicity, particularly in the case of free chlorine. Commercial fisheries, There is no laboratory evidence to indicate that levels of phenol and cresols that are safe for fish cause their flesh to become tainted, but xylenols should not exceed 0.5 mg/l. Other phenolic substances, particularly the chlorophenols, are known to cause taint at very low concentrations; strictly these are outside the scope of this report, but the information that has been considered suggests that they should be excluded from waters supporting commercial fisheries.