Vaccination and elevated dietary phosphorus reduces the incidence of early sexual maturation in Atlantic salmon (Salmo salar L.)

This study investigated the effect of ip injection of an oil-adjuvant vaccine and dietary phosphorus (P) level on the incidence of sexual maturation and occurrence of skeletal deformities in Atlantic salmon. Triplicate groups (12 × 12 m sea cages) of vaccinated (V) and unvaccinated (UV) Atlantic salmon underyearling smolts were fed diets with 0.6% (0.6P) or 0.9% available phosphorus (0.9P) from seawater transfer in mid-October and 3 months onwards, and then a commercial diet for a further 11 months. Both vaccination and 0.9% available phosphorus significantly increased the proportion of immature fish. V significantly reduced the incidence of mature females, while 0.9P significantly reduced the incidence of mature males. In addition, there were always relatively fewer mature V than UV males within each of the six sea-cages (three per diet). At the end of the period on different test diets, fish fed the 0.6P diet displayed significantly lower mechanical stiffness of the vertebral bodies than fish fed the 0.9P diet, but the mean values were both relatively high, which indicated that both groups enjoyed good mineral status. There was no effect of vaccination on stiffness, and there were no dietary or vaccination effects on the occurrence of externally detectable skeletal deformities (vertebral column, operculum, jaw).This is the first study to show that dietary phosphorus and vaccination can influence sexual maturation in fish. The results show that early seawater diets for underyearling Atlantic salmon smolts must be sufficient in phosphorus to prevent unwanted sexual maturation during on-growth in seawater.

► Vaccination and elevated dietary phosphorus reduced the incidence of maturation.
► No effect of diet P or vaccination on skeletal deformities in the present study.
► Vaccination may affect maturation via appetite reduction.
► How phosphorus affects maturation is uncertain.