Low temperature limits photoperiod control of smolting in Atlantic salmon through endocrine mechanisms

1 Conte Anadromous Fish Research Center, Biological Resources Division, US Geological Survey, Turners Falls, Massachusetts 01376; 2 Laboratory of Molecular Endocrinology, School of Fisheries, Kitasato University, Sanriku, Iwate, Japan; and 3 Fish Endocrinology Laboratory, Department of Zoology, Göteborg University, Göteborg, Sweden 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 + -K + -ATPase activity, whereas fish at ambient temperature did not respond to increased day length. Increases in gill Na + -K + -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 + -K + -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. growth hormone; insulin-like growth factor I; cortisol; thyroxine; osmoregulation; sodium-potassium-adenosinetriphosphatase; fish; anadromous; rhythm; development