Evidence for the role of growth hormone as a part of a 'light–pituitary axis' in growth and smoltification of Atlantic salmon (Salmo salar)

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Abstract
Atlantic salmon (Salmo salar) parr were subjected to one of two photoperiod regimens: a simulated natural photoperiod, or a reciprocal photoperiod (with a light-to-dark ratio opposite to that of the natural photoperiod). The animals were held at 10 °C from mid-December onward and were sampled at 3-week intervals.By February, salmon held under the reciprocal photoperiod were significantly longer, had lower condition factors, and looked and behaved more like smolts than did those under the natural photoperiod. A separate experiment showed that salinity tolerance increased at this time. In June the situation was reversed; the salmon kept under the natural photoperiod showed greatest growth and high salinity tolerance.Longer days in either regimen coincided with an increase in number and apparent activity of pituitary somatotrops of the animals. Adrenocorticotropin (ACTH), pars intermedia, and prolactin cells were also judged to be active under these conditions. A role for growth hormone and other peptides as components of a 'light–pituitary axis' is discussed in relation to the parr–smolt transformation of Atlantic salmon.

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