Effects of L-thyroxine and ovine growth hormone on smoltification of amago salmon (Oncorhynchus rhodurus).

In a study of the hormonal control of salmon smoltification, L-thyroxine (T4; 200 ppm in diet), ovine growth hormone (oGH; 2 micrograms/g intramuscular injection weekly), or a combination of these hormones was administered to underyearling amago salmon parr for 72 days. Administration of both T4 + oGH remarkably stimulated growth of the fish. All hormone treatments caused body silvering of the fish. The body silvering was the greatest in the group treated with T4 + oGH. Both T4 + oGH and oGH increased survival rate of the fish in 32% seawater, and kept plasma osmolarity and plasma sodium concentration lower than those of T4-treated and control groups in 27% seawater. Significant elevation of gill Na+, K+-ATPase activity was found in T4 + oGH-treated fish. On the other hand, T4 by itself did not affect the seawater tolerance of the fish. These findings suggest that GH or synergistic action of GH and T4 plays an important role in seawater adaptation during smoltification of amago salmon.