Endocrine changes during natural spawning in the white sucker, Catostomus commersoni. I. Gonadotropin, growth hormone, and thyroid hormones.
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White suckers (Catostomus commersoni; Cypriniformes, Teleosteii) spawning in a small stream in central Alberta were captured during different stages of their spawning migrations in 1981 and 1982, blood was sampled, and the fish were examined to determine their reproductive condition. Blood samples were analyzed for gonadotropin (GtH), growth hormone (GH), triiodothyronine (T3), and thyroxine (T4) by radioimmunoassay. GtH levels in both sexes were lowest prior to the onset of spawning, increased significantly in spawning males, females in which germinal vesicle migration had begun, and ovulated females and then dropped significantly in spent fish of both sexes. GH was lowest in prespawning females, increased significantly at ovulation, and remained high in spent females. In contrast, GH levels in males were relatively constant throughout spawning. In both sexes, highest T4 levels were found in prespawning fish, and T4 decreased significantly in spent fish. Although a similar decline was seen in T3 in 1981, in 1982 there were no T3 changes associated with changes in reproductive condition. No significant diurnal variations were detected in the levels of GtH or T3; T4 levels appeared to vary on a diurnal basis in prespawning males only. Spawning activity in both sexes therefore appears to be associated with increases in GtH occurring at ovulation in females and at the initiation of spawning in males. GH levels may also be related to reproductive condition in females, but not in males. The relationship of thyroid hormone levels to reproductive condition is less clear, however, and these levels may reflect both endocrine and environmental influences on thyroid function.