The effects of photoperiod manipulation on growth and reproduction in rainbow trout (Oncorhynchus mykiss)
Photoperiod is an important signal involved in the timing and co-ordination of many processes such as growth and reproduction in salmonids. Both growth and reproduction appear to be controlled by endogenous rhythms, which under natural conditions, is entrained by the seasonal changes in daylength, that is accurately reflected by the diel pattern of melatonin. This thesis investigates the influence of photoperiod on growth and reproduction in rainbow trout (Oncorhynchus mykiss), and examines the effect on plasma insulin-like growth factor-I (IGF-I). These studies aim to further our scientific understanding of the endocrine mechanisms controlling growth and reproduction and transfer this knowledge to commercial trout farming practices. Studies at fry (<5g) and fingerling (>25g) stages investigated the effect of exposure to constant long-days (LD18:6) and constant light (LL) on growth performance. In all experiments, there was a significant growth enhancing effect following exposure to extended light regimes relative to those under ambient or constant short-day photoperiods. Growth enhancement appeared to be caused through direct photo-stimulation or a phaseshift in an underlying endogenous rhythm dependent on the timing of photoperiod application. Measurement of plasma IGF-I accurately reflected growth rate during juvenile development. Furthermore, a clear autumnal increase in IGF-I was observed that was apparently up-regulated by long-day photoperiods. Additionally, it was shown that melatonin implants significantly reduced growth rate below that of unimplanted controls irrespective of photoperiod, but did not reduce circulating IGF-I levels suggesting that elevated melatonin levels masked the perception of daylength but did not act directly on the somatotropic axis to control growth. As size or one of its correlates (growth rate, energy balance or nutritional status) is regarded as an important determinant of the ability to undergo puberty, the interaction of natural (SNP) and advancing (ADV: long-short day) photoperiods with growth, IGF-I and reproduction was investigated in virgin female broodstock. Under SNP 63% of the population attained maturity while only 29% spawned in the ADV regime. Under SNP both size and growth rate in late spring/early summer appeared to determine whether an individual may initiate maturation. Conversely, under ADV, condition factor appeared to be a better predictor. A complete seasonal relationship between plasma IGF-I, daylength and temperature was demonstrated for the first time in rainbow trout under natural conditions, and provides direct evidence for the relationship between maturation and IGF-I. IGF-I levels showed a negative correlation with testosterone as fish initiated maturation. Furthermore, IGF-I levels accurately reflected growth rate prior to the initiation of vitellogenesis, suggesting that IGF-I may provide an endocrine signal between the somatotropic and reproductive axes that nutritional status, growth rate and/or size is sufficient to initiate maturation. In addition, maturing individuals under both photoperiods typically expressed higher circulating IGF-I levels than those that remained immature and may reflect a greater opportunity for IGF-I to act on the pituitary to stimulate gonadotropin production. In this sense, the observation of elevated levels for 3 months under SNP compared to only 1 month under ADV may reflect a reduction in the window of opportunity to initiate maturation under advancing photoperiods and hence explain the reduction in fish spawning. Finally, using a commercially available ELISA kit provided evidence of a possible detection method for measurement of a leptin-like peptide as absolute levels differed significantly between mature and immature fish. This provides preliminary evidence for a possible involvement of a leptin-like peptide in rainbow trout reproduction and energy homeostasis. Given the above evidence of a growth enhancing effect of photoperiod in covered systems, the transfer of constant light (LL) regimes to uncovered commercial trout farming practices was also examined. The application of LL regimes during either fry grow-out in tanks using floating lights or ongrowing to harvest size during winter in cages using submersible lights was investigated. In all trials conducted, the exposure to LL in autumn significantly enhanced winter growth rate and feeding efficiency by up to 30% and 25% respectively relative to those maintained under ambient light. Furthermore, the effect of light intensity was clearly shown to be an important requirement in successfully applying photoperiod regimes onto the ambient photoperiod in order to enhance commercial production in "uncovered systems". These studies clearly indicate the importance of photoperiod in influencing growth and reproduction in rainbow trout. Furthermore, the use of plasma IGF-I as an indicator of growth provides a practical tool for studying growth-photoperiod-reproduction interactions in this species. However, further studies are necessary to further our understanding of the endocrine pathways governing physiological mechanisms, especially growth and reproduction. Overall, this work has provided important information to improve both scientific understanding and commercial development although it is clear that substantial research is still required.