Comparison of the marine survival of atlantic salmon, Salmo solar L. smolts vaccinated and nonvaccinated against vibriosis

Abstract

The effect of vaccination against vibriosis on the marine survival of one-year-old smolts and two size-classes of two-year-old smolts of the short-migrating River Neva stock of Atlantic salmon ( Salmo salar L.) was studied through paired releases of Carlin-tagged fish in the Bothnian Sea (Baltic Sea) in 1991 and 1992. Vibriosis was supposed to contribute to the poor survival of post-smolts in the Bothnian Sea compared to the Gulf of Finland. Vaccinated groups were immunized with a single intraperitoneal injection the previous autumn with a commercial vaccine containing formalin-killed Vibrio anguillarum and V. ordalii bacteria incorporating aluminium sulphate as adjuvant. The experiment was designed in advance to detect a departure of 20–25% from the initial 50:50 stocking ratio of vaccinated and nonvaccinated fish at a 5% risk level and 90% power. The difference was substantial and could thus explain a significant proportion of the differences in stocking results between the Gulf of Finland and the Bothnian Sea. Tag recoveries were collected from the date of release until the end of 1995. In 1991, a total of 4000 two-year-old smolts were released, half of them vaccinated, with 588 (29.6%) tags returned from the vaccinated and 551 (27.6%) from the nonvaccinated groups. For two-year-old smolts in the 1992 year class, the corresponding numbers were 4200 released, with 347 (16.6%) and 327 (16.4%) recaptured, respectively. For one-year-old smolts in 1992, only 10 recoveries (0.5%) were obtained from both the vaccinated and nonvaccinated groups from the 4000 released, excluding the statistical analysis. For both year classes of two-year-olds, the distribution of recaptures over the years was homogeneous between vaccinated and nonvaccinated groups at a 5% risk level. The observed recapture ratios from the total number of recoveries did not differ statistically significantly ( G-test for goodness of fit) from the original stocking ratio of vaccinated and nonvaccinated fish at a 5% risk level. This indicates that vaccination did not affect survival statistically significantly.