Vertebrae of the trunk and tail display different growth rates in response to photoperiod in Atlantic salmon, Salmo salar L., post-smolts

This study shows that exposure to continuous light induces regional differences in growth in the vertebral column of Atlantic salmon under-yearling post-smolts reared in seawater cages. Exposure to continuous light during the period from January to the summer solstice (June) generally produced significantly larger fish than those reared under natural light. Until April, however, the fish exposed to continuous light grew more slowly, as reflected in the vertebral column by decreased growth in the truncal vertebrae, while the growth of the tail vertebrae was similar in the two groups. From April to June the fish exposed to continuous light displayed the fastest growth, with a higher growth rate in the vertebrae of the trunk than among fish reared under natural light. The mineral rate and mechanical strength of the vertebrae at the summer solstice was significantly lower in fish exposed to continuous light. Our results suggest that the growth of different regions of the vertebral column of salmon may be regulated partly independently, and that mineralisation is also affected during fast growth.