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Absrracr.-Ectoparasite infections can cause death or a decline in the general health of farm-raised finfish. This paper reports the findings from two studies conducted to evaluate the efficacy of hydrogen peroxide as a therapeutant for the control of infections of Amyloodinium sp. on cultured Pacific threadfin Polydactylus sexjilis (locally called "moi"). Threadfin with amyloodiniasis collected from a commercial farm were used in both trials. Prior 0

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Ectoparasite infections can cause death or a decline in the general health of farm‐raised finfish. This paper reports the findings from two studies conducted to evaluate the efficacy of hydrogen peroxide as a therapeu‐tant for the control of infections of Amyloodinium sp. on cultured Pacific threadfin Polydactylus sexfilis (locally called “moi”). Threadfin with amyloodiniasis collected from a commercial farm were used in both trials. Prior to the trials, and following hydrogen peroxide treatment, the extent of infection was determined by a gill biopsy procedure. An initial trial was conducted in the laboratory to assess the response of juvenile threadfin and Amyloodinium sp. trophonts to hydrogen peroxide exposure at four dosages: 0, 75, 150, or 300 mg/L for 30 min. In a trial on a commercial farm, a hydrogen peroxide treatment at 75 mg/L for 30 min was applied to juvenile threadfin in a grow‐out tank. In both trials, hydrogen peroxide was immediately flushed from the culture system with sea‐water after the 30 min exposure period. In the laboratory trial, treatment with 300 mg/L hydrogen peroxide resulted in 100% mortality of the exposed group of fish. However, single treatments with hydrogen peroxide at concentrations of 75 or 150 mg/L eliminated Amyloodinium sp. trophonts without causing loss of fish. In the field trial, a single treatment with 75 mg/L hydrogen peroxide greatly reduced levels of Amyloodinium infestation, and a second treatment 6 d later reduced Amyloodinium trophonts to a nondetectable level. These findings suggest that hydrogen peroxide is a suitable chemical for the treatment of amyloodiniasis of cultured, juvenile Pacific threadfin.

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