Risk factors for spinal deformities in Atlantic salmon, Salmo salar L

Risk factors for spinal deformities or short tails in Atlantic salmon, Salmo salar L., were examined in a cohort study in Hordaland County of Norway, in which 177 slaughter groups consisting of 5000 salmon were followed from sea water introduction in April 1994 until slaughter in February 1996. If the smolt weight increased from 0.05 to 0.1 kg, the risk of spinal deformities decreased (relative risk, RR = 0.2). An increased period from smolt vaccination to seawater introduction of from 2 to 5 months was associated with an increased risk (RR = 1.6). Salmon reared in fjord sites were at higher risk for spinal deformities (RR = 1.9) than those reared in oceanic sites, possibly due to the larger variation in salinity and temperatures in the fjord sites. Rapidly growing salmon were at lower risk of spinal deformities at slaughter (RR = 0.2), and they were likely to be slaughtered earlier, which could explain the observed lower risk for salmon slaughtered in June (RR = 0.4). Moreover, the findings concerning egg suppliers could indicate a possible role of genetic factors in the aetiology of spinal deformities.