S. Moriyama S. D. McCormick 2000 <p><span>We have examined the interaction of photoperiod and temperature in regulating the parr-smolt transformation and its endocrine control. Atlantic salmon juveniles were reared at a constant temperature of 10°C or ambient temperature (2°C from January to April followed by seasonal increase) under simulated natural day length. At 10°C, an increase in day length [16 h of light and 8 h of darkness (LD 16:8)] in February accelerated increases in gill Na</span>⁺<span>-K</span>⁺<span>-ATPase activity, whereas fish at ambient temperature did not respond to increased day length. Increases in gill Na</span>⁺<span>-K</span>⁺<span>-ATPase activity under both photoperiods occurred later at ambient temperature than at 10°C. Plasma growth hormone (GH), insulin-like growth factor, and thyroxine increased within 7 days of increased day length at 10°C and remained elevated for 5–9 wk; the same photoperiod treatment at 2°C resulted in much smaller increases of shorter duration. Plasma cortisol increased transiently 3 and 5 wk after LD 16:8 at 10°C and ambient temperature, respectively. Plasma thyroxine was consistently higher at ambient temperature than at 10°C. Plasma triiodothyronine was initially higher at 10°C than at ambient temperature, and there was no response to LD 16:8 under either temperature regimen. There was a strong correlation between gill Na</span>⁺<span>-K</span>⁺<span>-ATPase activity and plasma GH; correlations were weaker with other hormones. The results provide evidence that low temperature limits the physiological response to increased day length and that GH, insulin-like growth factor I, cortisol, and thyroid hormones mediate the environmental control of the parr-smolt transformation.</span></p> application/pdf 10.1152/ajpregu.2000.278.5.R1352 en American Physiological Society Low temperature limits photoperiod control of smolting in atlantic salmon through endocrine mechanisms article