Vaccination improves survival of Baltic salmon ( Salmo salar ) smolts in delayed release sea ranching (net-pen period)

Baltic salmon (Salmo s&r) of the Finnish Iijoki stock were hatched and reared in freshwater in a salmon hatchery on the Danish island of Bornholm in the Baltic sea. Salmon parr were divided in three groups each comprising 22 000 fish. One group was vaccinated by intraperitoneal injection with a non-mineral oil-adjuvanted vaccine consisting of formalin killed Aeromonas salmonicida, Yersinia ruckeri and Vibrio anguillarum (serotype 0 1 and 02). A second group was vaccinated by 1 h bathing 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 Sea (salinity 8 ppt), 500 m from the north-eastern coast of Bornholm, where they were studied for four months until tagging and release for restocking purposes. Mortality during this period in the ip vaccinated group was minimal (0.02%, RPS (relative per cent of survival) 99.80) and significantly lower compared to 10.13% mortality in the control group and 2.51% mortality (RPS 75.2) in the bath vaccinated group. Specific 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 humoral antibody response to the various bacteria was significantly elevated in the injection vaccinated group showing 4-5 fold titre increases three and four months after immunization. In contrast, no increase of titres was seen in the bath vaccinated and untreated groups. _ Corresponding author. Tel.: + 45 35282700; fax: + 45 352827 11; e-mail [email protected] 0044.8486/97/$17,00