VACCINATION IMPROVES SURVIVAL OF BALTIC SALMON (SALMO-SALAR) SMOLTS IN DELAYED RELEASE SEA RANCHING (NET-PEN PERIOD)

Baltic salmon (Salmo salar) of the Finnish Iijoki stock were hatched a nd reared in freshwater in a salmon hatchery on the Danish island of B ornholm in the Baltic sea. Salmon parr were divided in three groups ea ch comprising 22 000 fish. One group was vaccinated by intraperitoneal injection with a non-mineral oil-adjuvanted vaccine consisting of for malin killed Aeromonas salmonicida, Yersinia ruckeri and Vibrio anguil larum (serotype O1 and O2). A second group was vaccinated by 1 h bathi ng in a corresponding vaccine without adjuvant. A third group was left as untreated control. Subsequently, presmolt groups were transferred to three identical net-pens located next to each other in the Baltic S ea (salinity 8 ppt), 500 m from the north-eastern coast of Bornholm, w here they were studied for four months until tagging and release for r estocking purposes. Mortality during this period in the ip vaccinated group was minimal (0.02%, RPS (relative per cent of survival) 99.80) a nd significantly lower compared to 10.13% mortality in the control gro up and 2.51% mortality (RPS 75.2) in the bath vaccinated group. Specif ic disease outbreaks were not observed during the four months. Growth was significantly enhanced in the injection vaccinated group compared to both the unvaccinated control and the bath vaccinated group. The hu moral antibody response to the various bacteria was significantly elev ated in the injection vaccinated group showing 4-5 fold titre increase s three and four months after immunization. In contrast, no increase o f titres was seen in the bath vaccinated and untreated groups. Marked cellular reactions in the abdominal cavity of injected fish were regis tered. A total of 3000 fish have been tagged and released to evaluate the effect of vaccination on the recapture rate. The implications of i mmunoprophylactic measures in the restocking programme with Baltic sal mon are discussed. (C) 1997 Elsevier Science B.V.