Reproductive steroids during maturation in a primitive teleost, the Pacific herring (Clupea harengus pallasi)

Concentrations of reproductive steroids were measured in the plasma of captive Pacific herring, Clupea harengus pallasi, (1) prior to ovulation and milt production, (2) during the periovulatory and newly milt-producing period, (3) during a "ripe" holding period after ovulation and during milt production, and (4) before and after spawning. 17alpha,20beta-Dihydroxyprogesterone (17,20beta-P), despite being present only in low concentrations in the unconjugated (free) form (<10 ng/ml), is likely to be the maturation inducing steroid in females and was associated with the initiation of milt production in males since its levels are elevated coincidentally with these events. Glucuronated 17,20beta-P, free 17alpha-hydroxyprogesterone (17-P), and free and glucuronated 3alpha, 17alpha-dihydroxy-5beta-pregnan-20-one (3alpha,17-P-5beta) were present in high concentrations (140-250 ng/ml) in periovulatory females and newly milt-producing males. This steroid pattern suggests that the low levels of 17,20beta-P are due to glucuronation and competitive conversion of its precursor, 17-P, to free and glucuronated 3alpha, 17-P-5beta. Glucuronated testosterone was the principal steroid in preovulatory and premilt-producing fish (200-350 ng/ml), coincident with similar levels of glucuronated 11-ketotestosterone in males. After ovulation females did not spawn synchronously until 2 months later, which may be partially due to reduced environmental cues in the captive situation, while male fish released milt sporadically throughout the ripe holding period. Steroidal indicators of readiness to spawn in females or males were not detected. Rather, levels of all steroids gradually decreased in ripe holding fish (<30 ng/ml) to reach even lower levels (<1 ng/ml) after spawning. We suggest that "runniness" of gametes is a distinctive characteristic of females that are ready to spawn, but that this may result from relaxation of sphincter muscles rather than being an additional maturational step.