A series of experiments suggests that photoperiod entrains the clock. When salmon were exposed only to artificial light on a light-dark schedule that either expanded or compressed the natural seasonal cycle of daylight into periods greater than or less than 12 months, the spawning time was delayed or advanced, respectively. Again, this suggests that an internal biological timing mechanism somehow keeps track of each photoperiod and compares it with the previous one. In mammals that breed only during certain seasons, photoperiod is perceived by the eyes and relayed via a complex neural pathway to the brain's pineal gland. The pineal gland then transmits this information into a daily rhythm of melatonin secretion that controls the timing of reproduction.
Fish have a similar daily melatonin rhythm that is changed by photoperiod.
Aside from its scientific interest, an understanding of the mechanism underlying spawning in salmon has commercial value. Because of the circannual rhythm of spawning, the availability of fish of market size is restricted to certain times of year. Photoperiod techniques, which involve artificial manipulation of day length, are currently used by a number of fish farms to modify the spawning times of their fish stock and hence the supply of young fish to industry. If scientists can determine how the biological clock controls reproduction, better ways of raising fish commercially might be developed.