Changes in serum thyroxine levels during smolting in captive and wild Atlantic salmon, Salmo salar L.

Abstract

On a commercial fish farm in Western Scotland, smolting Atlantic salmon, Salmo salar L., showed no major changes in serum thyroxine (T4) levels in the spring. In contrast, serum T4 levels in smolting, wild salmon, trapped as they left the Girnock Burn, Aberdeenshire, varied markedly over this period. Serum T4 levels were correlated with the stream discharge rate pertaining as the fish left the stream. Levels in migrating, smolting fish were higher than those in fish captured above the fish‐trap by electro‐fishing. In a stream channel, serum T4 levels in smolting fish caught previously in the fish‐trap were rapidly elevated by exposure to increased water‐flow, whereas levels in fish previously captured above the trap remained substantially unchanged. Serum T4 levels in salmon parr leaving the stream in autumn were generally lower than those in smolting fish leaving in spring and were not demonstrably affected by stream discharge rate.