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

A possible correlation between smelt quality and post-smelt mortalitie s in Atlantic salmon provoked by the infectious agents Aeromonas salmo nicida, Vibrio salmonicida, or infectious pancreatic necrosis virus (I PNV) was investigated. Smelt groups of different qualities were produc ed by shifting the photoperiod from 8 h light per day to continuous Li ght, followed by elevation of the water temperature from 4 to 7 degree s C, 10, 6, and 2 weeks prior to seawater transfer of the smelt groups A ('early smelts'), B ('optimal smelts'), and C ('late smelts'), resp ectively. All smelt groups were vaccinated at the same time with an oi l-emulsified injected vaccine protective against A. salmonicida and V. salmonicida several weeks before the start of smoltification. One gro up of fish (smoltified like group B 'optimal smelts') was left unvacci nated. At the time of seawater transfer, the three smelt groups were s hown 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 seawate r, fish from all smelt groups were challenged with IPNV or A. salmonic ida. After being reared for 29 weeks in a seacage, fish were also chal lenged with V. salmonicida. No significant differences in mortalities between the vaccinated smelt groups A, B, and C were found after chall enge with A. salmonicida, V. salmonicida or IPNV, and differences in I PNV titres were also non-significant. Protection against A. salmonicid a and V. salmonicida was clearly obtained with the oil-emulsified inje cted vaccine. In addition, protection against IPNV (18.8% mortality in the unvaccinated, compared to 0% in the vaccinated group) occurred, p ossibly as a result of a stimulation of non-specific immunological def ence mechanisms by certain components in the vaccine. The results indi cate that timing of seawater transfer in relation to the smoltificatio n process is not critical for the survival, growth, and disease resist ance in the post-smelt, provided that the fish have been vaccinated wi th an injected oil-emulsified vaccine and have an acceptable seawater tolerance at transfer. (C) 1997 Elsevier Science B.V.