Fish size at vaccination influence the development of side-effects in Atlantic salmon (Salmo Salar L.)

Vaccination of Atlantic salmon effectively reduces the risk of diseases, but also results in the development of side-effects. The aim of the present study was to elucidate the effect of fish size at vaccination on the development of intraabdominal lesions. Individual tagged Atlantic salmon smolts (n=1191) in the size range between 20 and 175 g, were vaccinated i.p. on 4 November 1998. In May 1999 they were transferred to indoor seawater tanks and reared under simulated natural photoperiod until 12 July 2000. Development of growth, side-effects and antibody levels were followed. At slaughter, intra-abdominal lesion score was lowest in the fish that were largest at vaccination. Fish smaller than 75 g at vaccination had 60% incidence of high lesion score at slaughter, while larger fish had 24% high lesion score. The largest fish at vaccination had the highest growth rate throughout the experiment and were still largest at slaughter. Lesion score did not affect the weight at the termination of the experiment in the size groups that were smaller than 75 g at vaccination. However, fish that were between 75 and 100 g at vaccination and had developed high lesion score (n = 15) were significantly smaller than the corresponding fish with low lesion score. Antibody levels were not different between size groups, or between high and low lesion score fish. The experiment shows that the risk for developing adverse side-effects can be substantially reduced by increasing fish size at vaccination. (c) 2007 Elsevier B.V. All rights reserved.