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

Abstract

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.