Influence of Oxygen on the Toxicity of Chloramine-T to Atlantic Salmon Smolts in Freshwater and Seawater

Chloramine-T is a widely used disinfectant for the treatment of gill diseases of fish in freshwater, and more recently attention has turned to its use in seawater. However, despite the wide use of chloramine-T, few studies have examined its acute toxicity to fish. In this study we examined the acute (within 12 h) toxicity of freshwater- and seawater-acclimated Atlantic salmon Salmo salar smolts under aerated (100% air saturation with O 2) and oxygen supersaturation conditions (200% air saturation with O 2). Chloramine-T was more acutely toxic to salmon in seawater than to those in freshwater, and oxygen supersaturation enhanced the toxicity in both sea- and freshwater. In aerated freshwater the median lethal times (LT50s) for chloramine-T concentrations of 50 and 25 mg/L were 166.8 and 474.3 min, respectively; in aerated seawater they were 119.1 and 297.3 min, respectively. However, in freshwater at 200% air saturation with oxygen, the 50 and 25 mg/L LT50s were 133.6 and 190.9 min, respectively; in seawater at 200% air oxygen saturation, the values were 85.3 and 158.9 min, respectively. Freshwater Atlantic salmon appear to be as sensitive as rainbow trout Oncorhynchus mykiss and more sensitive than channel catfish Ictalurus punctatus to chloramine-T toxicity. Seawater-acclimated salmon, however, appear to be more sensitive to chloramine-T than are trout and catfish. The primary mechanism of toxicity in both seawater and freshwater appears to be extensive oxidative necrosis of the gill filament and lamellar epithelium, causing acute osmoregulatory dysfunction.