Prolonged low-salt immersion effectively controls Flavobacterium columnare infection in Murray cod Maccullochella peelii peelii

A disease outbreak occurred in Murray cod Maccullochella peelii peelii in a recirculating aquaculture farm in Tianjin city, China, in 2019. Strain MRX-2019 was isolated and considered to be the etiological pathogen; it was identified as Flavobacterium columnare based on a 16S rDNA gene sequence analysis and physiological and biochemical tests. The effect of salinity on the growth of MRX-2019 was investigated in vitro. Salinity >4 parts per thousand (i.e. 6 parts per thousand) inhibited MRX-2019 growth, whereas 8 and 10 parts per thousand salinity killed it. The effect of 4 parts per thousand salinity on F. columnare was not significant (p > 0.05). When MRX-2019-infected Murray cod were treated with 4, 6, or 8 parts per thousand salinity, the mortality rate was reduced by 8.9, 67.76, or 75.56%, respectively, compared with that of the control. However, the mortality rate increased by 7.77% at 10 parts per thousand salinity. In this study, we found that maintaining the fish in freshwater with 6-8 parts per thousand salinity effectively reduced the mortality of these fish when infected with F. columnare. The findings provide an environmentally friendly control strategy for columnaris disease in Murray cod.