Chronic toxicity thresholds for fish early life stages exposed to ²²⁶Ra in water

In 2021, a consensus was established to generate toxicity data for ²²⁶Ra in water with the objective to develop a water quality guideline that would effectively allow policy makers to set discharge limits for ²²⁶Ra that are protective for aquatic ecosystems. In the present study, fish early life stage (ELS) toxicity data for guideline derivation included three fish species, namely white sucker (Catostomus commersonii), rainbow trout (Oncorhynchus mykiss), and fathead minnow (Pimephales promelas). To evaluate ELS toxicity, embryos from all fish species were exposed at fertilization (O. mykiss, C. commersonii) or 4 h post-fertilization (P. promelas) to a series of environmentally relevant activities (0-62.5 Bq/L ²²⁶Ra, nominal) in very soft (10-13 mg/L CaCO₃; O. mykiss and C. commersonii) or hard (160-180 mg/L CaCO₃; P. promelas) reconstituted water. Toxicity thresholds for mortality, development (time to eyed stage, hatch, and swim-up), morphometrics (length, dry mass, condition), and deformities were estimated at swim-up. Among all fishes, traditional statistical approaches produced no observed effect concentrations (NOECs) of > 62.5 Bq/L for all endpoints, suggesting that the current discharge limit (0.37 Bq/L) is protective as set in the Canadian Metal and Diamond Mining Effluent Regulations (MDMER). Conversely, a non-traditional mixed model approach, which included time to swim-up in degree days as a biological endpoint, produced lowest observed effect concentrations (LOECs) between 0.1 and 2.5 Bq/L for O. mykiss and P. promelas body length, respectively. These data indicate that current discharge limits may not be protective to all freshwater fishes.