Influence of Sustained Exercise at Two Ration Levels on Growth and Thyroid Function of All-Female Chinook Salmon (Oncorhynchus tshawytscha) in Seawater

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Abstract
Duplicate groups of seawater-adapted 70-g all-female chinook salmon (Oncorhynchus tshawytscha) were held in flowing seawater (24–29‰; 8–12 °C) corresponding to swimming speeds (SS) of 0.5, 1.0, or 1.5 body lengths (bl)/s for 212 d. Fish were fed either a maximum satiating ration of 75% of maximum ration at each SS. Fish grew four- to fivefold during the study, but neither mean body weight, fork length, specific growth rate, condition factor, nor muscle morphometry was influenced by average SS. However, the SS of individually marked fish were negatively correlated with specific growth rate. At both rations, higher SS increased the internal tissue-carcass ratio, hepatosomatic index, and total body protein. More food was ingested at higher SS resulting in reduced feed efficiency. SS did not alter the plasma L-thyroxine (T4) concentration, but increased the plasma 3,5,3′-triiodo-L-thyronine (T3) concentration and T3/T4ratio. In part, this may reflect the greater food intake at higher SS. The reduced ration depressed all aspects of growth and at 212 d tended to depress both plasma T4and T3, but did not alter feed efficiency. We conclude that exercise over the range of 0.5–1.5 bl/s does not enhance growth, but reduces feed efficiency in seawater-adapted chinook salmon.

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