Photoperiodic history determines the reproductive response of rainbow trout to changes in daylength

The mechanisms underlying the photoperiodic entrainment of the endogenous circannual rhythm of maturation in the rainbow trout were investigated by subjecting December-spawning fish to abrupt changes in daylength which varied in their timing or magnitude. These protocols advanced spawning by up to 4 months. Maturation occurred in sequence in fish maintained on 18L:6D from January and February, and in fish exposed to 18L:6D from December, January and February, followed by 6L:18D in May, indicating that the abrupt increases in daylength were effective entraining cues. `Long' photoperiods of between 12 and 22 h applied in January, followed by shorter photoperiods of between 3.5 and 13.5 h from May, were equally effective in advancing maturation. Maturation was also advanced, though to a lesser extent, in fish maintained on photoperiods of 8.5 or 10 h from January, followed by a photoperiod of 1.5 h from May. In contrast, maturation was delayed in fish maintained under a constant 8.5-h photoperiod from January, and these fish also exhibited a desynchronization of spawning times characteristic of endogenous circannual rhythms in free-run. Collectively, these results indicate that photoperiodic history determines the reproductive response of rainbow trout to changes in daylength. Accepted: 7 August 1998