PLASMA-LEVELS OF SEX STEROIDS DURING SEXUAL-MATURATION OF SNAPPER, PAGRUS-AURATUS (SPARIDAE), CAUGHT FROM THE WILD

Snapper, Pagrus auratus, were caught by baited hook from the Hauraki Gulf, northeast New Zealand, from April (autumn) 1990 to January (summer) 1991. Fish were blood-sampled immediately upon being taken aboard the vessel. From April to August, all female fish caught were sexually immature, the first ovulated fish was caught in late September and, thereafter, they occurred through to December. Male fish caught from April to July were sexually immature, the first spermiating fish was caught in October and they occurred through to January. In females, plasma estradiol (E2) increased from low levels in sexually immature fish, to peak (0.93 ng ml-1) in fish undergoing ovarian recrudescence and ovulation. Plasma testosterone (T) levels followed a similar pattern to E2, with a maximum mean of 0.63 ng ml-1. There was evidence that levels of both of these steroids changed according to time of day during the spawning season. Levels of 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) were low (mean level 0.3 ng ml-1) and did not change significantly with either month or time of day. In males, plasma T level increased from a minimum in sexually immature fish, to peak at 1.1 ng ml-1 in fish undergoing spermiation. Plasma 11-ketotestosterone levels followed a very similar pattern to T levels, but the range of values was greater (max. 2.8 ng ml-1). Levels of both androgens varied according to time of day in spermiating fish. Plasma levels of 17,20beta-P changed significantly with month, and were highest (0.63 ng ml-1) during the period of spermiation, but no change with time of day was apparent. Plasma levels of 17,20beta-P sulphate were determined in selected fish, and were low (< 1 ng ml-1) at all times. The absence of changes in plasma 17,20beta-P level during sexual maturation in females is at variance with its suggested role in final oocyte maturation in snapper.