Several experiments were conducted to investigate the effect of water chemistry during water hardening on survival of eggs of Atlantic salmon (Salmo salar), rainbow trout (Salmo gairdneri), and. brook trout (Salvelinus fontinalis). Results of these experiments showed that survival was very low when eggs were exposed to very hard water containing high concentrations of calcium (approximately 520 mg/L or greater) during the first few hours of water hardening. Such high concentrations of calcium were associated with gypsum (calcium sulfate) in the water supply. In contrast, survival of eggs significantly increased when they were initially water‐hardened (1–3 h) in softer water (Ca concentrations, 34–64 mg/L). Incubation of eggs in high‐calcium water after the initial water‐hardening period did not significantly affect survival. Results of another experiment on water hardening of rainbow trout eggs showed a significant increase in survival when eggs were initially water‐hardened in either low‐gypsum water or in high‐gypsum water softened by ion exchange to markedly reduce the calcium; but not the sulfate, content.
