INCREASED DAYLENGTH STIMULATES PLASMA GROWTH-HORMONE AND GILL NA-SALAR)(, K+ ATPASE IN ATLANTIC SALMON (SALMO)

Atlantic salmon juveniles reared at constant temperature (9-10 degrees C) were exposed to four photoperiod treatments and sampled every 2 we eks fi-om January through May. Fish reared under normal photoperiod ex hibited eight- and three fold increases in plasma growth hormone and g ill Na+, K+-ATPase activity, respectively, between January and April. Fish exposed to abrupt increases in daylength (LD 15:9) in February or March responded with earlier increases in plasma growth hormone and g ill Na+, K+-ATPase activity, and earlier decreases in condition factor relative to fish in the normal photoperiod group. Fish maintained und er short daylength (LD 9:15) from January to May exhibited delayed and muted increases in plasma growth hormone and gill Na+, K+-ATPase acti vity. Plasma thyroxine exhibited a 2.5-fold increase from February to late March in the normal photoperiod group, was generally lower in the LD 9:15 group, but exhibited no obvious response to abrupt increases in daylength. There was an increase in plasma 3,5,3'-triiodo-L-thyroni ne with time in all groups (43-80%) but no significant response to pho toperiod. Plasma levels of somatostatin-25 were highest in the LD 9:15 group, but there was no detectable response to increased daylength in any of the photoperiod treatments. The results indicate that plasma g rowth hormone is responsive to increased daylength and may be causally related to subsequent increases in gill Na+, K+-ATPase.