Endocrine-mediated photostimulation of growth in Atlantic salmon

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Abstract
The response of the Atlantic salmon (Salmo salar) pituitary to photoperiod manipulation was investigated histologically, and the observations were related to gross morphometric changes. Salmon were placed under either a simulated natural photoperiod (NP), a photoperiod complementary to the natural photoperiod (RP) or constant daylight (CL) in December. The latter two regimens stimulated the pituitary, particularly the somatotrops, and enhanced growth. After 6 weeks, the CL fish were longer than all others (P < 0.01) and their pituitaries showed marked hypertrophy, with many somatotrops and other cell types in mitosis. The RP fish were also longer (P < 0.01) than the NP animals at that time but their pituitaries appeared less stimulated than those of CL fish. However, the stimulation in CL and RP fish was not maintained beyond 3 months. At that time NP pituitaries more closely resembled the stimulated condition shown earlier by the RP glands. Our results confirm that changes in day length elicit corresponding changes in pituitary cytology and growth rates, albeit without eliminating an apparently innate growth cycle. While increased day length in winter may stimulate the pituitary, such regimens appear to render the gland refractory and cause a phase-shift in the rhythm. It is proposed that suitable interruptions in photostimulation may prolong stimulation.

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