Total residual oxidants in Atlantic salmon post-smolt performance, health and welfare in brackish water

Atlantic salmon Salmo salar production in recirculating aquaculture systems (RAS) is gaining momentum with numerous new facilities being built to produce smolts, post-smolts or market-size fish. Ozone is a strong oxidant and its use in RAS has shown to improve water quality and fish health. Specifically, ozone application in RAS inactivates pathogens and reduces organic matter, chemical oxygen demand, dissolved organic carbon, nitrite and water colour. However, its use in aquaculture is nowadays restricted to freshwater systems. In brackish and seawater systems, ozonation by-products, hereafter referred as total residual oxidants - TRO, are generated. Bromide (Br-) is present in seawater and is not toxic. But it reacts with ozone to form hypobromous acid (HOBr) and hypobromite ion/Bromine (BrO-). Bromine (BrO-) is extremely toxic, LC50 of 68 ìg/L for rainbow trout. It is thus expected that ozone use in brackish and seawater RAS will generate and accumulate bromine to toxic levels. However, no information is available for Atlantic salmon post-smolts. This study objective is to determine the effect of total residual oxidants in Atlantic salmon post-smolt (body weight 100 - 300 g, N = 825) performance, health and welfare in brackish water. Fish will be exposure to 5 different TRO concentrations from 0 – 60 ìg/L of TRO during 8 weeks. The results from this study will be important to the design and management of ozonation in brackish and seawater RAS in Norway and worldwide.