The use of 17α-methyltestosterone and 3,5,3′-triiodo-L-thyronine for sterilizing and accelerating the growth of zero-age coho salmon smolts ( Oncorhynchus kisutch)

Abstract

Coho salmon alevins were immersed in a 200-μg/l solution of 17α-methyltestosterone (MT) upon hatching and 2 weeks later. MT was included in the diet at a concentration of 10 mg/kg to complete sterilization. The fry were reared at 10 °C under a 30-day delayed photoperiod. After 12 weeks of feeding with MT (phase 1), it was withdrawn from the diet and 3,5,3′-triiodo-L-thyronine (T 3) was added to the feed at 4 mg/kg (phase 2 ). Ten weeks later, the fish were transferred to 11 °C salt water (phase 3) and fed supplementary MT at levels of 0.2, 1.0 and 5.0 mg/kg of diet in order to test its efficacy as a growth promoter. Sterile fish weighed 4.9% more than controls at the end of phase 1, but 7% less at the end of phase 2. Fish given T 3 weighed 14% more than controls at the end of phase 2. Supplemental MT did not affect growth performance in phase 3. At termination, the technique had produced sterile zero-age smolts with 97% survival, but with 33% lower weight than controls and 43% lower food conversion efficiency. Thyroid follicle epithelial cell height and skin thickness increased with the initial MT treatment, but both effects gradually reverted after hormone withdrawal. Proximate body composition differed significantly at the end of phase 1 with a 30% lower lipid content in sterile fish compared with controls, but lipid was 23% higher in sterile fish than in controls at the end of phase 2 regardless of T 3 level. T 3 treatment resulted in an increased moisture content in both sterile and control fish at the end of phase 2, but did not significantly alter body lipid content. Sterile fish had higher lipid than controls at the end of phase 3 (18% more) but there was no effect of supplemental MT. Hormone treatments had a minor effect on protein and ash content during the experiment.