INFLUENCE OF SUSTAINED EXERCISE AT 2 RATION LEVELS ON GROWTH AND THYROID-FUNCTION OF ALL-FEMALE CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) IN SEAWATER

Duplicate groups of seawater-adapted 70-g all-female chinook salmon (O ncorhynchus tshawytscha) were held in flowing seawater (24-29%; 8-12 d egrees C) corresponding to swimming speeds (SS) of 0.5, 1.0, or 1.5 bo dy 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 fivefol d during the study, but neither mean body weight, fork length, specifi c growth rate, condition factor, nor muscle morphometry was influenced by average SS. However, the SS of individually marked fish were negat ively 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 (T-4) concentration, but increased the plasma 3,5,3'-triiodo-L-thyronine (T -3) concentration and T-3/T-4 ratio. In part, this may reflect the gre ater food intake at higher SS. The reduced ration depressed all aspect s of growth and at 212 d tended to depress both plasma T-4 and T-3, bu t did not alter feed efficiency. We conclude that exercise over the ra nge of 0.5-1.5 bl/s does not enhance growth, but reduces feed efficien cy in seawater-adapted chinook salmon.