Effects of water hardness on toxicological responses to chronic waterborne silver exposure in early life stages of rainbow trout (Oncorhynchus mykiss).

Rainbow trout embryos and larvae were exposed to 0, 0.1, and 1 microg/L total silver (as AgNO3) in water of three different hardnesses (soft water [2 mg/L as CaCO3], moderately hard water [150 mg/L], and hard water [400 mg/L]) in a flow-through system from fertilization to swim-up (64 d). The objective of the study was to investigate the effects of water hardness on chronic silver toxicity. In the absence of silver, elevating hardness had a positive effect on early life stage survival and development, significantly decreasing mortality and accelerating time to 50% swim-up. Following hatch, exposure to 1 microg/L Ag significantly increased mortality relative to exposure to 0 microg/L Ag. No significant effects of silver on time to 50% hatch were observed; however, time to 50% swim-up was delayed, and 50% swim-up was not achieved over the course of the experiment during some exposures to 1 microg/L Ag. These results suggest that the current Canadian Water Quality Guideline (http://www.ccme.ca/assets/pdf/e1_062.pdf) of 0.1 microg/L Ag is sufficient in preventing mortality and altered development in early life stages of rainbow trout. Increasing water hardness from 2 to 150 or 400 mg/L was modestly protective against the mortality and delays in time to 50% swim-up associated with exposure to 1 microg/L Ag. The 150- and 400-mg/L hardnesses were equally protective against mortality, but 150-mg/L was more protective than 400-mg/L hardness against the delays in time to 50% swim-up. Overall, the protective effects of hardness on chronic silver toxicity in early life stages of rainbow trout are modest but similar to the protection afforded to acute silver toxicity in juvenile and adult rainbow trout.