title: Photoperiod influences growth rate and plasma insulin-like growth factor-I levels in juvenile rainbow trout, Oncorhynchus mykiss.
abstract: The effect of different photoperiod regimes and the subsequent influence of melatonin on growth and insulin-like growth factor-I (IGF-I) were assessed in juvenile rainbow trout. In Experiment 1, triplicate groups of all female underyearling rainbow trout were exposed to one of three photoperiods; simulated natural photoperiod (SNP), constant short-days (LD 8:16), or constant long-days (LD 18:6) from June to December 2000 under ambient water temperatures. Fish exposed to LD 18:6 grew to a significantly heavier mean weight than the other treatments. Regression analysis showed a strong correlation between circulating plasma IGF-I, growth rate and temperature. Furthermore, it was apparent that fish exposed to LD 18:6 expressed significantly higher circulating levels of IGF-I. In a second experiment, duplicate groups of all female yearling trout were exposed to one of three photoperiods; SNP, LD 8:16, or constant light (LL), with sub groups receiving either a slow-release melatonin implant (18 mg), sham implant or left intact (control). LL increased growth rate in controls, reaching a significantly greater weight than SNP or LD 8:16 photoperiods but did not affect circulating IGF-I levels. Melatonin implants reduced growth rate in all photoperiod treatments below that of their respective controls but again did not affect circulating IGF-I levels. No differences in growth rate were found in implanted fish between photoperiods suggesting that a diel cycle of melatonin is necessary for the perception of daylength. These results would indicate that extended photoperiods (LD 18:6) may cause direct photostimulation of growth through up-regulation of IGF-I production. In contrast, in the absence of a changing diel melatonin signal, growth appeared to be maintained by a possible underlying endogenous rhythm, which was phase advanced under LL, as such plasma IGF-I levels simply reflected growth rate rather than photostimulation of the somatotropic axis. Overall, these findings indicate that measuring plasma IGF-I may be a useful tool for studying environmental influences on growth in rainbow trout.
authors.affiliation: Institute of Aquaculture, University of Stirling, Stirling, Scotland, FK9 4LA, UK. jft2@stir.ac.uk
authors.forename: J F
authors.initials: JF
authors.lastname: Taylor
authors.forename: H
authors.initials: H
authors.lastname: Migaud
authors.forename: M J R
authors.initials: MJ
authors.lastname: Porter
authors.forename: N R
authors.initials: NR
authors.lastname: Bromage
mesh_terms: D000465:Algorithms; D000818:Animals; D005260:Female; D006128:Growth / Q000502:physiology*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism* / Q000494:pharmacology; D008550:Melatonin / Q000097:blood / Q000494:pharmacology; D017686:Oncorhynchus mykiss / Q000502:physiology*; D017440:Photoperiod; D011863:Radioimmunoassay; D013696:Temperature; D015430:Weight Gain / Q000502:physiology
chemical_list: D007334:Insulin-Like Growth Factor I; D008550:Melatonin