Behavioral avoidance by fish of residual chlorine in power plant discharges

The behavior of fishes in and around power plant discharges has not been adequately explored despite the recent proliferation of electric generating stations. This study investigated the response of fish to heated and/or chlorinated waters in a field laboratory. Carp (Cyprinus carpio), channel catfish (Ictalurus punctatus) and golden shiners (Notemigonus crysoleucas) were tested at 6, 12, 18, 24 and 30-C acclimation temperatures in a steep concentration gradient. Residence time in the treated water was determined to be the most simple and sensitive index of avoidance behavior. TRC, CRC, and FRC avoidance thresholds varied between species and acclimation temperatures. HOCl avoidance thresholds were consistent between acclimation temperatures, suggesting that HOCl is the primary constituent of the TRC controlling avoidance behavior.

Trials were conducted at 12 and 24 C in which the treated water was heated to a preferred temperature, derived in a shallow horizontal temperature gradient. At low chlorine doses fish significantly preferred the warmer water, whereas at high chlorine doses, the fish were repelled by the treated water. The TRC, CRC, and FRC avoidance thresholds varied between fish species, but the HOCl avoidance thresholds were similar for all species at 24 C. The avoidance thresholds for the carp and golden shiners were well below 96hrLC₅₀ values, signifying that they can avoid dangerous solutions before they are adversely affected. The channel catfish avoidance thresholds are slightly greater than reported 96hrLC₅₀ values, indicating a potential for mortalities in power plant discharges. This mortality has not been observed in the field and alternate explanations are discussed.

Present interim criteria for chlorinated discharges may not adequately protect all warmwater fish. This study indicates that HOCl may greatly influence fish behavior and suggests that HOCl criteria be adopted for use in conjunction with current total residual chlorine criteria.