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Chlorine as hypochlorous acid and chloramines is toxic to aquatic life. Coarse fish species, invertebrate organisms and plants are generally more resistant to chlorine than salmonids, although the prodotion of young Daphnia is reduced at a concentration of 0.0035 mg/1 (0.0004-0.0014 mg HOCl/l).

From the limited data available, it seems that concentrations of chlorine greater than 0.008 mg HOCl/l could be harmful or lethal within 4 days to both salmonid and coarse fish, whereas 0.004 mg HOCl/l has been endured 1,7 sensitivz species of fish for 5 days although with some decline in the normal activity.

The toxicity of chlorine to fish is increased by a reduction in the dissolved oxygen, and little changed by increase in salinity up to 50 percent sea water, but there are few or no data available on the effect of temperature, pH, and water hardness.

Avoidance behaviour of rainbow trout under experimenal conditions has been slight at a concentration of chlorine of 0.001 mg/l (0.0002-0.0008 mg HOCl/l) and marked at 0.001-0.008 mg HOCl/l.

Chlorine is too reactive to persist for long in most streams and there are the upper limit for fish survival might be set closer to the lethal levels than might otherwise be the case, especially as avoidance behaviour is likely to afford additional protection to the fish from regions of high concentration. Consequently it is tentatively suggested that an acceptable upper limit would be 0.004 mg HOCl/l, because it should result in little or no mortality of fish, perhaps have only a minor effect on fish behaviour and not be high enough to be damaging to the majority of fish food organisms. The amount of total chlorine to this concentration varies according to the temoerature and pH value 02 corresponding the water, as shown in the following table:

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Temperature    |                    pH                     |

°C                   |    6     |    7    |    8    |    9    |

  5                   | 0.004 | 0.005 | 0.011 | 0.075 |

25                   | 0.004 | 0.005 | 0.016 | 0.121 |

Since chlorine may react with thiocyanide to produce tethal concentrations of hydrogen cyanid and/or cyanogen chlorine and concentrations of chlorine as low as 0.001 mg/l in the presence of phenols are likely to produce taints in the flesh of fish an upper limit of less than 0.004 mg HOCl/l might be necessary in the presence of these other poisons.