Low temperature limits photoperiod control of smolting in atlantic salmon through endocrine mechanisms.
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 degrees C or ambient temperature (2 degrees C from January to April followed by seasonal increase) under simulated natural day length. At 10 degrees 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 degrees C. Plasma growth hormone (GH), insulin-like growth factor, and thyroxine increased within 7 days of increased day length at 10 degrees C and remained elevated for 5-9 wk; the same photoperiod treatment at 2 degrees C resulted in much smaller increases of shorter duration. Plasma cortisol increased transiently 3 and 5 wk after LD 16:8 at 10 degrees C and ambient temperature, respectively. Plasma thyroxine was consistently higher at ambient temperature than at 10 degrees C. Plasma triiodothyronine was initially higher at 10 degrees 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.
McCormick
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Conte Anadromous Fish Research Center, Biological Resources Division, US Geological Survey, Turners Falls, Massachusetts 01376, USA.
Moriyama
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Björnsson
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D000222:Adaptation, Physiological; D000818:Animals; D001522:Behavior, Animal / Q000502:physiology; D003080:Cold Temperature; D005880:Gills / Q000201:enzymology; D013006:Growth Hormone / Q000097:blood; D006728:Hormones / Q000097:blood*; D006854:Hydrocortisone / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D017440:Photoperiod; D020317:Salmo salar / Q000502:physiology*; D012621:Seasons; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
D006728:Hormones; D014284:Triiodothyronine; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D000254:Sodium-Potassium-Exchanging ATPase; D013974:Thyroxine; D006854:Hydrocortisone