Seasonal timing is important for many critical life history events including growth, migration and reproduction.  Changes in daylength (photoperiod) provide reliable seasonal cues that are widely used by animals to ensure appropriate timing of life history events.  Despite the critical role of photoperiod, we still lack a fundamental understanding of the mechanisms of photoperiodism in many taxa, including fish.

 

As part of their anadromous life history, Atlantic salmon undergo physiological changes in spring that prepare them for downstream migration and seawater entry.  This life history transition is known as smolting, is regulated by photoperiod and includes development of seawater tolerance that is critical for ocean survival. We have used Atlantic salmon as a model species to examine the mechanisms of the photoperiodic control of seasonal change in fish. 

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As in birds and mammals, exposure of juvenile salmon to increased daylength results in increased pituitary production of thyroid stimulating hormone (TSH), which stimulates brain the enzyme deiodinase (DIO2) to produce higher levels of the thyroid hormone T3.   Elevated T3 in the salmon brain result in the release of growth hormone from the pituitary, which in turn stimulates the development of salinity tolerance.  Significant ambient light levels can pass through the skull and reach the pituitary where non-visual opsins are present.  The isolated pituitary of salmon can directly respond to increased daylength with elevated TSH transcription, the first time this has been shown in any animal, thus expanding the known mechanisms of photoperiod responses in vertebrates.