Photoperiodic control of reproduction and patterns of melatonin secretion in the rainbow trout, Oncorhynchus mykiss

Abstract

Reproduction In salmonids is an annual event with spawning confined to a brief (typically 6 week) period each year. The reproductive cycle appears to be controlled by on endogenous circannual rhythm or •clock' which, under natural conditions, is entrained by the seasonal changes in daylength. This thesis Investigates the mechanisms by which photoperiod entrains the circannual clock which, it is proposed, controls maturation In the rainbow trout, Oncorhynchus mykiss, Abrupt changes m photoperiod can either advance or delay spawning and the timing of changes m serum calcium, oestradol-178 and testosterone which accompany maturation in the female rainbow trout. These effects can be interpreted as corrective phase advances or phase delays of a circannual dock. 'Long' photoperiods of between 12 and 22 hours applied in January, followed by shorter photoperiods of between 3.5 and 13.5 hours from May were equally effective tor the advancement of maturation In December-spawning female rainbow trout. Maturation was also advanced, though to a lesser extent, in fish which remained on typical winter photoperiods (8.5 or 10 hours), provided they received a decrease to an even shorter photoperiod prior to the summer solstice. In contrast, maturation was delayed in fish maintained under a constant winter photoperiod (8.5 hours), and these fish also exhibited a desynchronization of spawning times characteristic of endogenous circannual rhythms in free-run. Collectively, these results indicate that direction of change of daylength is the feature of the photoperiodic signal responsible for the entrainment of the endogenous circannual clock; the same photoperiod may be interpreted as 'long' or 'short’ providing it is longer or shorter than that to which the fish have been previously exposed. The concept of a rigid critical' daylength for reproductive function it therefore untenable in the rainbow trout. The liming of the increase to a 'long' photoperiod was also on important determinant of spawning lime; maturation occurred in sequence In December-spawning female rainbow trout maintained on constant 'long' days from January and February, and in fish exposed to 'long' days from December, January and February, followed by 'short' days in May. Maturation can also be advanced or delayed by exposing rainbow trout to short (S2 months) periods of continuous light at different phases of the reproductive cycle. These effects can be described in the form of a partial phase-response curve. The proportion of fish responding to short periods of continuous light was dependent on both the duration of the light period, and. most importantly, its position in relation to the phase of the reproductive cycle. A high proportion (285%) of fish responded with an advance in spawning time only when the period of exposure to continuous light occurred close to the preceding natural breeding season. The minimum period of exposure capable of advancing maturation in a majority (280%) of rainbow trout was 1 month. In 3 consecutive experiments over 90% of female rainbow trout exposed to continuous light for 2 months from January to March spawned again in a 6-wook period In July and August, approximately 5 months in advance of their natural spawning period. Exposure of rainbow trout to short periods of continuous light therefore provides a simple, cheap and predictable method for the production of out- of-season eggs on commercial fish farms. Patterns of melatonin secretion In the rainbow trout accurately reflected the prevailing photoperiod, with levels elevated for the duration of darkness under both long (16L:8D or 18L:6D) and short (8L:16D or 6L:18D) daylengths. Distinct diurnal rhythms in circulating melatonin were also detected in the Atlantic salmon. Salmo salar and Nile tilapia. nranr.hrnmi« niinticiis. Melatonin production in the rainbow trout is not under endogenous circadian control; changes in melatonin levels always coincided with the light to dark or dark to light transitions, and the melatonin rhythm did not persist in constant darkness. Additionally, the melatonin rhythm Immediately re adjusted to the new photoperiod when rainbow trout were transferred from long (t8L:6D) to short (6L:18D) days. Those results indicate that melatonin production in the rainbow trout is a direct response to darkness. Although the seasonally-changing patterns of melatonin secretion clearly provide the rainbow trout with accurate information on both daily and calendar lime the results of experiments designed to lest the hypothesis that melatonin conveys photic Information to the reproductive axis were inconclusive.