Influence of orally administered <scp>L</scp>-thyroxine or 3,5,3′-triiodo-<scp>L</scp>-thyronine on growth, food consumption, and food conversion of underyearling coho salmon (<i>Oncorhynchus kisutch</i>)

Abstract

Between August and November, groups of underyearling coho salmon, held at 10 °C and on a natural photoperiod, were fed twice daily to satiation with Oregon moist pellets supplemented with 0, 20, 100. or 500 ppm of L-thyroxine (T 4 ) or 3,5,3′-triiodo-L-thyronine (T 3 ).The T 3 significantly increased weight (20 and 100 ppm) and length (all doses), depressed condition factor (100 and 500 ppm), stimulated food consumption (20 ppm), improved food conversion (all doses), decreased thyroid follicle epithelial height (500 ppm), and influenced pituitary basophil and somatotrop number, size, and granulation.The T 4 influenced neither growth nor food conversion, but did increase food consumption (20 ppm) and condition factor (500 ppm). and decreased thyroid follicle epithelial height (500 ppm).No breakdown of either added T 4 or T 3 was observed during prolonged food storage.It is concluded that T 3 unlike T 4 , can be administered in the diet of coho held at 10 °C to enhance growth and improve food conversion.