Improved field trial methodology for quantifying vaccination side-effects in farmed Atlantic salmon (Salmo salar L.)

A controlled field trial was conducted on a commercial Atlantic salmon farm employing an improved field trial methodology for quantifying the effects of vaccination on harvest weight. Vaccinated and unvaccinated passive integrated transponder (PIT) tagged fish were hosted in a commercial production from vaccination till harvest for individual fish data recording. A standardized harvest weight (SHW), based on the thermal growth coefficient (TGC) was utilised in order to handle sorting and differences in growth periods. Visual analog scales (VAS) were used to score vaccine-induced adhesions, melanin on abdominal organs and melanin on abdominal wall. Weight at vaccination, sex and presence of spinal deformity were recorded in order to control for potential biasing effects. The results were analyzed using a multiple linear regression model, with the individual fish as the statistical unit. It was shown that vaccination caused a 506 g (95% CI: 243, 768) reduction in SHW, male fish were 1211 g (95% CI: 950, 1473) heavier than females, each gram in vaccination weight gave 36.2 g (95% CI: 11.1, 61.2) extra at harvest and deformed fish were 1177 g (95% CI: 465, 1889) smaller than normal fish. None of the scoring variables for vaccine-induced lesions contributed with additional explanation of SHW when added to the final model, nor was weight at vaccination associated with the adhesion score. The biological cost of vaccination even in fish with mild abdominal lesions was the most striking finding of the study. The improved field trial methodology used, controlling for confounding and the use of individual fish data in multivariable models, will give both more accurate and valid estimates in quantifying the adverse effects of vaccination.