Vitellogenin levels in male and female rainbow trout (Salmo gairdneri Richardson) at various stages of the reproductive cycle.

The immunoassayable vitellogenin (VTG) in plasma from male rainbow trout had the same molecular weight as authentic VTG from female fish. The VTG level in male trout was low (usually nanograms, occasionally up to a few micrograms, per ml) and did not correlate with the stage of sexual maturity. The plasma VTG level of female trout that were two years from first spawning was 200-fold higher than males of the same strain and age. The plasma VTG level of female rainbow trout rose approximately a million-fold during the two or three years required to attain sexual maturity.