Significance of Hypochlorous Acid in Free Residual Chlorine to the Avoidance Response of Spotted Bass (Micropterus punctulatus) and Rosyface Shiner (Notropis rubellus)

Abstract

The spotted bass (Micropterus punctulatus) and rosyface shiner (Notropis rubellus), when exposed to successively doubling doses of total residual chlorine (TRC) from 0.025 to 0.80 mg/ℓ, generally avoided higher concentrations when acclimated and tested at five temperatures from 6 to 30 °C. Both species avoided 0.05 mg/ℓ TRC when acclimated and tested at 6 °C and avoided 0.20 mg/ℓ TRC at 24 and 30 °C. Variations in avoidance of the free residual chlorine (FRC) ranged from 0.017 to 0.091 mg/ℓ (0.05 to 0.20 mg/ℓ TRC) between 6 and 30 °C. After the initial avoidance response, the rosyface shiner continued to avoid the remaining higher residual levels, while spotted bass showed less avoidance or none at all. The same behavioral responses occurred when each species was exposed to a single concentration (0.05 or 0.10 mg/ℓ TRC) for three successive 10-min observations.The avoidance response was most closely associated with the calculated hypochlorous acid (HOCl) fraction of the TRC, which declined (relative to the percent of TRC) as the pH and acclimation temperature increased. Avoidance to HOCl occurred between 0.012 mg/ℓ (6 °C) and 0.031 mg/ℓ (30 °C) when TRC was between 0.05 and 0.20 mg/ℓ, respectively. Avoidances did not occur at the 12 and 18 °C acclimation temperatures until the HOCl content comprising the 0.10 and 0.20 mg/ℓ TRC doses surpassed the initial HOCl threshold level (0.012–0.014 mg/ℓ) calculated for 0.05 mg/ℓ TRC at 6 °C. The HOCl content may be the most important constituent within the TRC to influence the avoidance response of fish in unpolluted aquatic systems. Key words: avoidance response, hypochlorous acid, free residual chlorine, Micropterus punctulatus, Notropis rubellus