The effect of feeding thyroid hormones on saltwater tolerance and growth rate of Atlantic salmon

Two experiments, involving the addition of thyroid hormones to the diet of salmon parr (Salmo salar) were carried out. The prime purpose of these experiments was to study the effect of feeding thyroid hormones on saltwater tolerance. Saltwater tolerance is interrelated with size and therefore growth rate was investigated. In experiment I three different thyroid preparations were used: thyroid extract, 3,5,3′-triiodo-L-thyronine (T3), and thyroxine (T4). Each thyroid preparation was fed to two different sizes of fish with two different concentrations in the diet. T3 gave the best overall results, both for saltwater tolerance and growth rate. No interaction was found between fish size and concentration of thyroid hormone in the diet. In experiment II three different concentrations of T3 in the diet were fed to salmon parr of two different ages. Feeding T3 improved saltwater tolerance and growth rate of 7-month-old fish but had a negative effect upon growth for 19-month-old fish. For the 7-month-old fish 10 mg T3/kg dry food gave the best effect on saltwater tolerance, while 1 mg T3/kg dry food was enough to improve growth. It is concluded that it is possible to induce smoltification in underyearling Atlantic salmon out of season by feeding thyroid hormones.