Efficacy of chloramine-T to control mortality in freshwater-reared salmonids diagnosed with bacterial gill disease

Bacterial gill disease (BGD), caused by Flavobacterium branchiophilum and other species of yellow-pigmented, filamentous bacteria, is a common and potentially catastrophic disease of hatchery (freshwater)-reared fish. Chloramine-T (Chl-T) is a biocide proven effective for controlling mortality in freshwater-reared fish diagnosed with BGD. However, Chl-T is not approved by the U.S. Food and Drug Administration for such use. To generate data in support of a U.S. approval, we evaluated the effectiveness of Chl-T (administered at 12 mg/L of static bath water for 60 min/d on three alternate days) to control mortality caused by BGD in freshwater-reared chum salmon Oncorhynchus keta, Apache trout O. gilae apache, and rainbow trout O. mykiss. For each species, the mean percent total mortality in Chl-T-treated tanks (N = 3) was significantly less than that in control tanks (N = 3): chum salmon = 8.9% versus 99.7%, Apache trout = 39.2% versus 97.9%, and rainbow trout = 5.7% versus 25.8%. Because the Chl-T treatment regimen was efficacious for each species, we conclude that our findings support the approval of Chl-T for use in the USA to control mortality in freshwater-reared salmonids diagnosed with BGD.