Effect of zinc on the early life stages of brown trout (Salmo trutta) at different levels of water hardness
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- NIVA-rapport 
The effect of water hardness on the chronic toxicity of zinc to early life stages of brown trout has been studied. Separate tests were carried out in four dilutin waters. Two of these were natural lake waters from Lake Maridalsvann (hardness 8.6 mg CaCO3/l and Lake Store Sandungen (hardness 6.7 mg CaCO3/l). High-hardness waters were prepared by adding calcium to these waters to achieve a hardness level of 100 mg CaCO3/l. Eggs of brown trout were fertilised in the test waters and exposed for approximately 120 days in flow-trough channels. Hatching success, time to hatching and length and weight of larvae at termination of the test were observed. Signifcant delay of haching was observed at 100 µg/l nominal, added concentrations of Zn in both lake waters. The NOECs were 56 and 61 µgZn/l respectively. With increase of water hardness to 100 mg CaCO3/l, the effects of Zn was reduced in Lake Store Sandungen (NOEC=250 µg/l) but not in Lake Maridalsvann (NOEC=57 µg/l). The effects on length and weight of larvae on test termination were generally less prononced.