Data from: The fish pituitary directly responds to daylength and drives seasonality:

Seasonal timing is important for many critical life history events of
organisms, and annual changes in daylength provide a reliable seasonal
cue. In birds and mammals, photoperiod-driven seasonality is caused by
changes in pituitary thyroid stimulating hormone (TSH), brain deiodinase
(DIO) and triiodothyronine (T3), but it is unclear if a similar mechanism
exists in fish. We used anadromous Atlantic salmon that migrate downstream
and enter the ocean in spring to examine photoperiod signaling in fish.
Manipulations of diurnal and circannual photoperiod cycles indicate that
pituitary tshβb and brain dio2b transcription are long day signals that
precede endocrine and physiological changes necessary for seawater entry.
Exposure to long days caused increased dio2b transcription and T3 levels
throughout the brain, and intracerebral T3 treatment resulted in elevated
levels of plasma GH that drives migration physiology. Significant ambient
light levels reach the pituitary, and transcription of non-visual opsins
were found in the pituitary. The isolated (in vitro) pituitary directly
responds to increased daylength with elevated tshβb transcription, the
first time this has been shown in any animal. The results indicate that
photoperiod-driven seasonality in fish is driven by direct light
stimulation of the pituitary, expanding the known mechanisms of
photoperiod responses in vertebrates.