Removal of the red tide dinoflagellate Cochlodinium polykrikoides using chemical disinfectants

For decades, red tide control has been recognized as necessary for mitigating financial damage to fish farms. Chemical disinfectants, frequently used for water disinfection, can reduce the risk of red tides on inland fish farms. This study systematically evaluated four different chemical disinfectants (ozone (O₃), permanganate (MnO₄^-), sodium hypochlorite (NaOCl), and hydrogen peroxide (H₂O₂)) for their potential use in inland fish farms to control red tides by investigating their (i) inactivation efficacy regarding C. polykrikoides, (ii) total residual oxidant and byproduct formation, and (iii) toxicity to fish. The inactivation efficacy of C. polykrikoides cells by chemical disinfectants from highest to lowest followed the order of O₃ > MnO₄^- > NaOCl > H₂O₂ for different cell density conditions and disinfectant doses. The O₃ and NaOCl treatments generated bromate as an oxidation byproduct by reacting with bromide ions in seawater. The acute toxicity tests of the disinfectants for juvenile red sea bream (Pagrus major) showed that 72-h LC₅₀ values were 1.35 (estimated), 0.39, 1.32, and 102.61 mg/L for O₃, MnO₄^-, NaOCl, and H₂O₂, respectively. Considering the inactivation efficacy, exposure time of residual oxidants, byproduct formation, and toxicity toward fish, H₂O₂ is suggested as the most practical disinfectant for controlling red tides in inland fish farms.