The effect of triploidy on vaccine side-effects in Atlantic salmon

• No effect of ploidy on vaccine side effects in yearlings
• Vaccinated under-yearling triploids had higher abdominal adhesion scores than diploids.
• Vaccinated under-yearling triploids had reduced male growth compared to females.
• Fewer male triploids sexually matured in comparison to diploids.

Vaccination against bacterial diseases is an important aspect of commercial salmon production, but is known to have a number of unwanted side-effects including reduced growth, abdominal adhesions, and vertebral deformities. Our study objective was to compare these side-effects between diploid and sterile triploid salmon, the use of which would prevent genetic interactions between escaped farmed and wild salmon. A full factorial cross of vaccinated and unvaccinated, diploid and triploid, yearling (1 +) and under-yearling (0 +) mixed sex salmon were reared in triplicate in freshwater before common rearing in sea cages for 12–15 months. To further examine how ploidy may influence welfare with respect to vertebral deformities, a set of wild caught salmon were examined. Unvaccinated fish were larger (9% at termination) than vaccinated fish, but there was no effect of vaccination on skeletal deformities. In general, triploids grew as well or better in freshwater, but poorer in seawater compared to diploids, resulting in diploids being 12% larger than triploids at termination irrespective of smolt regime. In addition, triploids had a higher prevalence of fish with a least one deformed vertebra in 1 + smolts compared to diploids (34 vs 8%, respectively). However, vertebral compressions and fusions were observed in both ploidy, but were not apparent in the wild caught adult salmon control fish. Ploidy effects on the response to vaccination were limited to triploids having significantly higher vaccine-induced abdominal adhesion scores in 0 + smolts compared to diploids. Triploidy also prevented males becoming larger than females in vaccinated 0 + smolts in contrast to all other ploidy, vaccination, smolt regime combinations where males became the largest (8–12% at termination). Finally, notable levels of sexual maturation occurred in the male 0 + smolts, but fewer triploids matured as post-smolts or grilse in comparison to diploids. Based on our results, for best welfare practices the timing of vaccination may need to be altered when producing 0 + triploid smolts.