Considerations for Consistently Applying Flow-Through Chloramine-T Treatments to Hatchery Raceways

Chloramine-T (CLT) was recently approved for use in the United States by the U.S. Food and Drug Administration (FDA) to control mortality in selected freshwater-reared finfishes diagnosed with bacterial gill disease or external columnaris disease. In support of this approval, we conducted a study to determine if a target dose of 12 mg/L CLT could be delivered for 60 min via a “charged,” flow-through treatment protocol. The study was conducted in two production-size, linear-design, plug flow raceways devoid of fish. Each raceway was dosed twice, resulting in four replicate trials (N = 4). During each trial, CLT was added under static conditions to establish a target concentration of 12 mg/L. Inflow water was then resumed, and additional CLT stock solution was metered into the raceway for the 60-min treatment period. Water samples were collected from a matrix of 27 sampling locations (3 positions along raceway length × 3 positions across raceway width × 3 depths) for colorimetric determination of CLT concentrations at 0 min (after charging but before resuming water inflow), 30 min, and 60 min. Chloramine-T doses delivered (data from all sampling locations and times pooled) did not vary from trial to trial. Median CLT doses delivered were almost always less than 12 mg/L; however, all had corresponding 95% confidence intervals within 9–15 mg/L. Overall, the results of our study demonstrated that the treatment method can be used to deliver a target dose of CLT for 60 min in production-size raceways in a manner that was found acceptable to the FDA.