Susceptibility to furunculosis, cold water vibriosis, and infectious pancreatic necrosis (IPN) in post-smolt Atlantic salmon ( Salmo salar L.) as a function of smolt status by seawater transfer

Abstract

A possible correlation between smolt quality and post-smolt mortalities in Atlantic salmon provoked by the infectious agents Aeromonas salmonicida, Vibrio salmonicida, or infectious pancreatic necrosis virus (IPNV) was investigated. Smolt groups of different qualities were produced by shifting the photoperiod from 8 h light per day to continuous light, followed by elevation of the water temperature from 4 to 7 °C, 10, 6, and 2 weeks prior to seawater transfer of the smolt groups A (‘early smolts’), B (‘optimal smolts’), and C (‘late smolts’), respectively. All smolt groups were vaccinated at the same time with an oil-emulsified injected vaccine protective against A. salmonicida and V. salmonicida several weeks before the start of smoltification. One group of fish (smoltified like group B ‘optimal smolts’) was left unvaccinated. At the time of seawater transfer, the three smolt groups were shown to be at different stages in the smoltification process based on the ‘chloride ion test’, but all were seawater tolerant, as judged by survival and growth in seawater. Immediately after transfer to seawater, fish from all smolt groups were challenged with IPNV or A. salmonicida. After being reared for 29 weeks in a seacage, fish were also challenged with V. salmonicida. No significant differences in mortalities between the vaccinated smolt groups A, B, and C were found after challenge with A. Salmonicida, V. salmonicida or IPNV, and differences in IPNV titres were also non-significant. Protection against A. salmonicida and V. salmonicida was clearly obtained with the oil-emulsified injected vaccine. In addition, protection against IPNV (18.8% mortality in the unvaccinated, compared to 0% in the vaccinated group) occurred, possibly as a result of a stimulation of non-specific immunological defence mechanisms by certain components in the vaccine. The results indicate that timing of seawater transfer in relation to the smoltification process is not critical for the survival, growth, and disease resistance in the post-smolt, provided that the fish have been vaccinated with an injected oil-emulsified vaccine and have an acceptable seawater tolerance at transfer.