Study of the effect of cadmium on the early life stages of brown trout (Salmo trutta) at different levels of water hardness
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The effect of water hardness on the chronic toxicity of cadmium to early life stages of brown trout (Salmo trutta) has been studied in accordance with OECD Guideline 210; Fish Early Life Stage Test. High-hardness waters were prepared by adding calcium to natural soft water achieving a nominal hardness level of 40 mg CaCO3/l. The experiment consisted of a factorial design comprising three levels of CaCO3 (2.7, 12.8 and 42.7 mg/l) and six levels of cadmium (0, 0.1, 0.32, 1, 3.2, 10 μg/l), including a control group of the natural background water. Eggs of brown trout were fertilised in their respective exposure water and exposed for approximately 120 days in flow-through chambers. In addition to cumulated mortality, hatching success, time-to-hatching and length and weight of larvae at four sampling points (newly fertilised eggs, eyeing stage, hatching and start feeding stage) were observed. Furthermore, body concentrations of calcium and cadmium were measured at the same four stages. The body cadmium concentrations changed a lot over the ontogeny, starting very high and reducing over time, but the body cadmium concentration always remained at least 10 times higher than the control group, and it was always highest in the low-calcium water concentration treatment. The biological effects were not very pronounced for the life stages under study. Even though it was documented significant interaction effects between calcium and cadmium concentrations on cumulated mortality (p<0.0001), the mortality in general was very low (0.5–12%), and the high-concentration cadmium treatments were not always producing a higher mortality rate than the control. The clearest effects measured were for size- and weight at start feeding. Clearly, the hatching-to-start-feeding growth rate was highly affected by high cadmium dosages, and mostly so at low CaCO3 concentrations. There was also found a significant (p < 0.0001) additive effect of cadmium on hatching trajectories where high levels of cadmium gave a delayed hatching probability. This effect involved a delayed hatching time of less than 10 degreedays. The results are summarised in a NOEC/LOEC table that concludes that LOEC for size and weight at start feeding is 0.95, 3.2 and 3.2 μg/l of water cadmium at water CaCO3 concentrations of 2.7, 12.8 and 42.7 mg/l, respectively. The estimated hardness slope of 0.42 would require extremely low CaCO3 concentrations in order to surpass the prevailing PNEC value (0.08 Cd μg/l). Estimated EC10 values for weight at start-feeding (95% CI) were 0.34 (0.25,0.47), 0.92 (0.38,2.22) and 6.16 (1.92,19.8) μg/l at the same CaCO3 concentrations, yielding a water hardness slope of 1.03. For cumulated mortality and hatching trajectory LOEC is set at 10 μg/l of cadmium for all water hardness levels involved in this study. The results are discussed in light of previous findings confirming that the most profound effects of cadmium on fish relate to post-hatching life stages.