The effects of pulse exposures to road salt at various stages of early development in rainbow trout (Oncorhynchus mykiss)

Salmonids spawn in freshwater streams including those in urban areas that are impacted by human activities. In the Vancouver region of British Columbia, Canada, the extensive use of road salt (primarily NaCl) is associated with frequent 24-h "pulses" of salt in streams, some of which may exceed the provincial acute guideline for maximum chloride concentrations (600 mg L^-1 Cl^-) by up to 11-fold. For some salmonids, road salting coincides with critical developmental stages, as many species spawn between October and January. We explored the concentration-dependent effects of a 24-h salt pulse (600-9600 mg L^-1 Cl^-) on salmonid development using rainbow trout (Oncorhynchus mykiss). Salt pulses were imposed at one of three developmental time points: <1 h post-fertilization, the eyed-stage or 7 days post-hatch. Significant mortality occurred only in the <1 h post-fertilization treatment, at 2400, 4800 and 9600 mg L^-1 Cl^-, all environmentally relevant salt concentrations. Significant differences in whole-embryo ion concentrations at the end of the salt exposure and at the eyed-stage (17 days post-salt exposure) indicated lasting ionoregulatory effects on embryos. Co-exposure to CaCO₃ during the salt pulse, at a level that increased dissolved Ca²⁺ by 2-to 3-fold in the ion poor Vancouver water, greatly reduced mortality and altered whole-embryo ion levels. These findings support the need for site-specific water quality guidelines, as toxicity varies with water's ionic composition. This research also highlights the need for improved road salting practices to reduce salt contamination and its potential adverse effects on developing salmonids.