ENDOCRINE AND GONADAL CHANGES DURING THE ANNUAL REPRODUCTIVE-CYCLE OFTHE FRESH-WATER TELEOST, STIZOSTEDION-VITREUM

The annual reproductive cycle of walleye (Stizostedion vitreum) was ch aracterized by documenting changes in gonadal development and serum le vels of estradiol-17 beta (E(2)), testosterone (T), 17 alpha,20 beta-d ihydroxy-4-pregnen-3-one (17,20-P), and 11-ketotestosterone (11-KT) in wild fish captured from upper midwestern lakes and rivers throughout the year. Fish from the populations used in this study spawn annually in early-to mid-April. Walleye showed group synchronous ovarian develo pment with exogenous vitellogenesis beginning in autumn. Oocyte diamet ers increased rapidly from similar to 200 mu m in October to similar t o 1,000 mu m in November, and reached a maximum of 1,500 mu m just pri or to spawning. Changes in gonadosomatic indices (GSIs) paralleled cha nges in oocyte diameters. Serum E(2) levels in females increased rapid ly from low values in October (< 0.1 ng ml(-1)) to peak levels of 3.7 ng ml(-1) in November, coinciding with the period of the most rapid ov arian growth. Subsequently, E(2) levels decreased from December throug h spawning. Serum T levels exhibited a bimodal pattern, increasing to 1.6 ng ml(-1) in November, and peaking again at 3.3 ng ml(-1) just pri or to spawning. We detected 11-KT in the serum of some females at conc entrations up to 5.6 ng ml(-1) but no seasonal pattern was apparent. I n this study (unlike our results in a related study) 17,20-P was not d etected. In males, differentiation of spermatogonia began in late Augu st, and by January the testes were filled (> 95% of germ cells) with s permatozoa. Mature spermatozoa could be expressed from males from Janu ary through April. GSIs ranged from 0.2% (post-spawn) to 3.2% (pre-spa wn). Serum T levels rose from undetectable levels in post-spawn males to 1.6 ng ml(-1) by November, remained elevated throughout the winter, and peaked at 2.8 ng ml(-1) prior to spawning. Levels of 11-KT in mal es remained low(< 10 ng ml(-1)) from post-spawning through January, th en increased significantly by March and peaked just prior to spawning at 39.7 ng ml(-1). Our results indicate that vitellogenesis and sperma togenesis are complete or nearly so, in walleye by early winter, and s uggest that it may be possible to induce spawning in this species seve ral months prior to the normal spawning season by subjecting fish to r elatively simple environmental and hormonal treatments.