INFLUENCE OF RATION LEVEL AND SALINITY ON CIRCULATING THYROID-HORMONES IN JUVENILE ATLANTIC SALMON (SALMO-SALAR)

Following acute exposure to seawater (30 ppt), plasma thyroxine (T4) of Atlantic salmon (Salmo salar) smolts increased 80% in the first 6 hr, declined to initial levels after 24 hr, and remained stable for 18 days thereafter. In nonsmolts, plasma T4 did not rise immediately after exposure to seawater, fell slightly after 2 days, and remained low for 18 days. Plasma triiodothyronine (T3) of smolts and nonsmolts was not affected by acute exposure to seawater. To examine the effect of long-term adaptation to ration and salinity, Atlantic salmon smolts were acclimated to three salinities (0, 10, and 30 ppt) and four ration levels (0, 0.2, 0.8, and 1.6% wet weight per day) for 6 weeks. Plasma T4 increased with increasing ration level (P < 0.001) but was not significantly affected by salinity (P = 0.4). Plasma T3 also increased with increasing ration (P < 0.001) and was more strongly correlated with ration level (r = 0.85) and growth rate (r = 0.88) than was plasma T4 (r = 0.73 and 0.75, respectively). At low ration (0 and 0.2% per day), fish in 10 ppt had slightly but significantly lower plasma T3 than fish in 0 ppt. There was no effect of salinity on plasma T3 at the higher rations, nor did plasma T3 levels differ significantly in fish in 0 and 30 ppt at any ration. The results indicate that ration level is a more important influence on circulating levels of plasma thyroid hormones than is salinity. © 1990.