The effects of ozonation on performance, health and welfare of rainbow trout Oncorhynchus mykiss in low-exchange water recirculation aquaculture systems

A controlled four-month trial was conducted to compare the effects of ozonation (oxidation-reduction potential setpoint = 250 mV) versus no ozonation on rainbow trout Oncorhynchus mykiss performance, health, and welfare in replicated WRAS operated at low exchange rates (0.26% of the total recirculating flow) and high feed loadings (3.9 kg/day per m3/day makeup flow). Rainbow trout at 74 ± 2 g (mean ± standard error) in size were randomly stocked into six replicated 9.5 m3 WRAS (1000 fish per system). Ozonation of the three treatment WRAS began two months post-stocking following biofilter acclimation when fish were 295 ± 1 g. Fish were maintained at densities between 40 kg/m3 and 80 kg/m3 throughout the study at water temperatures of approximately 13-17 °C. By study's end, percentage survival for both groups was high (>98%). Fish in ozonated systems weighed significantly (p < 0.05) more at study's end than fish from the non-ozonated systems (1161 ± 6 g vs. 993 ± 12 g, respectively). Histopathological evaluation revealed increased levels of gill epithelial hyperplasia and hypertrophy, as well as hepatic lipidosis, in fish from ozonated systems; however, all lesions were subclinical. Fin condition was good overall in both groups, although fish from non-ozonated WRAS had better (p < 0.05) dorsal fin indices than fish in the ozonated cohort. The major blood chemistry finding was higher (p < 0.05) urea nitrogen in fish from non-ozonated systems (15.33 ± 0.90 mg/dL, vs. 6.800 ± 0.470 mg/dL in fish from ozonated WRAS). Overall, the results of this study indicate that raising rainbow trout to market size in ozonated WRAS improves fish performance without significantly impacting their health and welfare.

► Rainbow trout health and welfare were assessed in ozonated and non-ozonated WRAS. ► Fish grew to market size faster in ozonated WRAS. ► Subclinical lesions, including gill epithelia hyperplasia, were noted in the ozone group. ► Fin condition was comparable between ozone and no ozone groups. ► Overall, fish health and welfare did not appear to be compromised through use of ozone.