Abstract
This study investigates the effect of different smolt production strategies on vertebral morphology (radiology), composition (mineral content) and mechanical strength (load-deformation testing) in Atlantic salmon (Salmo salar). Rapid-growing underyearling (0+) smolt were compared with slower-growing yearling (1+) smolt and a reference group of wild smolt (w). The underyearling and yearling smolt were transferred to seawater in October 2002 and May 2003, respectively. The underyearling smolt were reared under continuous light and the yearling smolt under natural light during the first twelve weeks in seawater, at ambient temperatures. Thus, the underyearling smolt hit seawater at 13[no-break space][deg]C and were reared at 10-13[no-break space][deg]C during the early seawater phase, whereas the yearling smolt hit seawater at 7[no-break space][deg]C and were reared at 7-10[no-break space][deg]C during the early seawater phase. All groups displayed increased longitudinal growth (up to 9% increase in relative length) of the caudal vertebrae during parr-smolt transformation. However, at transfer to seawater, the underyearling smolt had significantly lower vertebral mineral content (0+ 44%, 1+ 47%, w 50%) and higher incidence of deformed vertebrae (0+ 1.5%, 1+ 0%, w 0%), and at twelve weeks after transfer to seawater significantly lower vertebral mineral content (0+ 36%, 1+ 41%, w 43%), yield-load (0+ 6492[no-break space]g, 1+ 8797[no-break space]g, w 9150[no-break space]g) and stiffness (0+ 7578[no-break space]g/mm, 1+ 15,161[no-break space]g/mm, w 20,523[no-break space]g/mm), and significantly higher incidence of deformed vertebrae (0+ 2.5%, 1+ 0.3%, w 0%). There was a significant correlation between the mineral content and mechanical properties of the vertebrae. The underyearling smolt had significantly elevated plasma concentrations of total Ca, and P and Ca2+ during the parr-smolt transformation and in the early seawater phase.The results show that underyearling smolt may have an increased risk of developing vertebral deformities. It is possible that this risk can be reduced by postponing the start of the short-day treatment. This will reduce the temperature during smoltification, the temperature and daylength during the early seawater phase, and increase the age at smoltification.
