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 There are extensive data on the toxicity of zinc to fish laboratory conditions supported to some extent by field data on fish kills there are virtually no field observations to indicate the concentrations of zinc that are not inimical to fish populations or fisheries, mainly because the analytical data are too meagre, information on water hardness perhaps the most important factor affecting the toxicity of zinc to aquatic animals is missing details of the status of the fish pcmùation are not given. For these reasons only tentative criteria can be suggested which may have to be revised when more adeauate field data becone availale.

The toxicity of solutions containing zinc is mainly attributable to the zinc ion and perhaps also to particulate zinc present as the basic carbonate or the hydroxide held in suspension. It is modified water being reduced in particular by an increase in hardness and also temperature, salinity suspended solids increased by a decrease in dissolved oxygen. The effect of pH however, is uncertain.

The acute toxicity of zinc in the presence of other heavy metals and other common pollutants seems to be largely simply additive but there is no evidence that the chronic toxicity of different in a mixture is also additive. The effect of zinc is modified, end can be reduced, by acclimation and by the age of the fish.

A low but significant mortality has been found among rainbow trout exposed continuously for 4 months to constant concentrations of 0.2 of the 5~d LC50 and among rudd exposed for 8.5 months to 0.3 of the 7~d LC50.

Laboratory studies of avoidance reactions have shown that Atlantic salmon and rainbow trout may avoid concentrations of zinc in soft water which are 0.14-0.01 of the 7-d LC50. Avoidance reactions have algo been observed at 0.35-0.43 of the 7-d LC50 by migrating Atlantic salmon in a river polluted with copper and zinc. Carp and goldfish show avoidance of 0.3—0.45 of lethal concentrations under laboratory conditions.

Field observations show that brown trout populations were present when the concentrations of zinc were less than 0.06 of the 2-d to rainbow trout or when the annual 50 and 95 percentiles were up to 0.05 and 0.19 of the LC50 respectively; coarse fish were present when the corresponding percentiles were 0.02 and 0.11 of the 5-d LC50 to roach.

Because concentrations zinc in freshwaters fluctuate, being distributed approximately log-normally over a period of a year tentative criteria are expressed as annual 50— and 95—percentile values.

Pending the availability of more information it is tentatively recommended that for the maintenance of thriving populations of fish the annual 95 percentile of concentration of zinc should be no greater than 0.1 of the appropriate 7—d TC50 at 15⁰ C; thus the criteria in ration of zinc would depend upon water hardness and type of fish as shown in Table I.

The concentration of 0.03 mg Zn/l for salmonids in very soft water may be too severe if brown trout only are present, since this species appears to survive successfully at higher concentrations; in such cases a 95—percentiIe concentration of 0.2 of the LC50 (0.06 mg/l) may be more appropriate. For the minnow might be more appropriate to set a more at stringent standard but further data would be desirable to support and explain the existing laboratory findings.

The values of the corresponding annual 50 percentiles would be approximately 0.25 of the proposed 95 percentiles unless the distribution was much wider in range has hitherto been found or was not log-normal, resulting in a larger ratio between the 95 percentile and higher percentiles, in which cases more stringent standards would be appropriate. Where other poisons are present and dissolved oxygen concentrations are below the air-saturation value, allowance should also be made for their contribution to the toxicity.

Table Maximum annual 95—percenti1e concentrations of zinc (mg Zn/l)

___________________                                               

|Water hardness  |      Coarse fish      |      Salmonids    |

|(mg/l CaCO₃)    |      except minnow |                         |

|          10          |      0.3                  |      0.03            |  

|          50          |      0.7                  |      0.2              |

|          100        |      1.0                  |      0.3              |

|          500        |      2.0                  |      0.5              |