Influence of Bovine Growth Hormone and L-Thyroxine on Growth, Muscle Composition, and Histological Structure of the Gonads, Thyroid, Pancreas, and Pituitary of Coho Salmon (<i>Oncorhynchus kisutch</i>)

Abstract

Groups of underyearling coho salmon (Oncorhynchus kisutch) were acclimated to 10 C well water and a photoperiod of 12 h L:12 h D. Excess ration (Oregon Moist Pellet) was presented daily. Doses of bovine growth hormone (5, 10, 20, 30, or 90 μg bGH/g body wt) and L-thyroxine (0.5, 5, or 30 μg T 4 /g) were administered over a period of 84 days (phase I) either by injection (via dorsal musculature or peritoneal cavity) or by hormone cholesterol implants into the muscle. Administration frequency of bGH and T 4 was such (range 2 times/wk-1 time/3 wk) that fish theoretically received either 10 or 30 μg bGH/g per wk or 1 or 10 μg T 4 /g per wk. Control fish received either alkaline saline (pH 9.5) or a cholesterol pellet. After cessation of treatment the fish were observed for an additional 84 days (phase II). During phase I, growth rates (weight) for bGH fish (2.0–2.4% per day) and for T 4 fish (0.97–1.1% per day) were significantly higher than those of control fish (0.42–0.59% per day). Among bGH fish, dorsal musculature injection (2 times/wk) was significantly more effective than intraperitoneal injection (1 time/2 wk).Increases in weight above control for bGH fish at 84 days ranged from 220 to 369%. Those for T 4 fish extended from 47 to 78%. In phase II, control fish growth rates were higher (0.61–0.67% per day) than those for bGH fish (0.47–0.57% per day) and T 4 fish (0.32–0.44% per day). Administration of bGH and T 4 (high dose) caused a progressive decline in condition factor of fish from the control range. This trend was stopped and reversed in phase II.At 84 days, generally no significant differences were detected among groups for percentages of muscle water. However, some groups had significantly higher (bGH) and others lower (T 4 ) percentages of muscle protein relative to those of control fish. Also, significant increases (T 4 ) and decreases (bGH) in muscle lipid percentages were found. Hormone treatment altered the histological structure of the ovary, thyroid, exocrine (T 4 ) and endocrine (bGH) pancreas, and somatotrop cells (T 4 ) of the pituitary. A poor growth response was noted for two groups of coho administered bGH after acclimation to sea water.