Gonad histology and plasma steroid profiles in wild New Zealand freshwatereels (Anguilla dieffenbachii and A-australis) before and at the onset of the natural spawning migration. I. Females

To determine steroid profiles in immature and maturing female eels from the wild, non-migratory and migratory New Zealand longfinned (Anguilla dieffen bachii) and shortfinned (A. australis) eels were caught and blood and ovari an samples collected. Plasma steroid levels were determined and related to the developmental stage of the ovary. Ovaries of non-migrants contained oog onia and previtellogenic oocytes. Vitellogenic oocytes were never observed in these groups, but instead were very common among migrants (up to 88% of oocytes). Concentrations of both androgens (androstenedione (AD), testoster one (T)) and estradiol-17 beta (E-2) were higher iri migrants than in non-m igrants. Among migrants, T levels were higher in shortfins (2.27 +/- 0.14 n g ml(-1)) than in longfins (0.82 +/- 0.10 ng ml(-1)), whereas E-2 levels we re higher in longfins (mean 2.46 ng ml(-1)) than in shortfins. Levels of se x steroids were generally low in non-migrants. In contrast, plasma levels o f 17-hydroxyprogesterone were significantly higher in non-migrants than in migrants. Similarly, cortisol levels were higher in non-migrating than in m igrating shortfinned, but not longfinned, females. 17,20 beta-Dihydroxy-4-p regnen-3-one, the putative maturation inducing steroid in anguillids, was n ear minimum-detectable levels for all animals examined. Surprisingly, very high levels of 11-ketotestosterone (KT) were found in migrants, averaging n early 3 ng ml(-1) in longfins and over 20 ng ml(-1) in shortfins. The ident ity of KT and several 5-reduced androgens was confirmed using gas chromatog raphy - mass spectrometry. The function of KT in females is not known, but we suggest that this steroid hormone may play a role in preparing maturing animals for their spawning migration.