Relationships between environmental changes, maturity, growth rate and plasma insulin-like growth factor-I (IGF-I) in female rainbow trout.

Size reflecting growth rate, energy balance or nutritional status is regarded as an important determinant of the ability of trout to undergo puberty. The relationship of a change in photoperiod, either natural (SNP) or advancing (ADV), with growth, IGF-I and reproduction was investigated in virgin female rainbow trout. Under SNP 63% of the population attained maturity while only 29% spawned 6 months in advance in the ADV regime. Under SNP both size and growth rate in late spring-early summer appeared to determine whether an individual may initiate reproduction while condition factor appeared to be a better predictor in the ADV regime. A complete seasonal relationship between plasma IGF-I, daylength and temperature was demonstrated under natural conditions, and provides direct evidence for the relationship between reproduction and IGF-I. Conversely, trout maintained under ADV exhibited a significantly different plasma IGF-I profile relative to those under a natural photoperiod. Furthermore, IGF-I levels accurately reflected growth rate prior to elevations in sex steroids, suggesting that IGF-I may provide an endocrine signal between the somatotropic and reproductive axes that growth rate and/or size is sufficient to initiate gonad development. In addition, maturing individuals under SNP 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. We observed elevated levels in maturing fish for 3 months under SNP compared to only 1 month under ADV were observed. This may reflect a reduction in the window of opportunity to initiate reproduction under advancing photoperiods and hence explain the reduction in fish successfully recruited.

Photoperiod influences growth rate and plasma insulin-like growth factor-I levels in juvenile rainbow trout, Oncorhynchus mykiss.

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.