Endocrine and gonadal changes during the annual reproductive cycle of the freshwater teleost,Stizostedion vitreum.

The annual reproductive cycle of walleye (Stizostedion vitreum) was characterized by documenting changes in gonadal development and serum levels of estradiol-17ß (E2), testosterone (T), 17α,20ß-dihydroxy-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 development with exogenous vitellogenesis beginning in autumn. Oocyte diameters increased rapidly from ∼ 200 µm in October to ∼ 1,000 µm in November, and reached a maximum of 1,500 µm just prior to spawning. Changes in gonadosomatic indices (GSIs) paralleled changes in oocyte diameters. Serum E2 levels in females increased rapidly 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 ovarian growth. Subsequently, E2 levels decreased from December through 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 prior to spawning. We detected 11-KT in the serum of some females at concentrations up to 5.6 ng ml(-1), but no seasonal pattern was apparent. In this study (unlike our results in a related study) 17,20-P was not detected. In males, differentiation of spermatogonia began in late August, and by January the testes were filled (> 95% of germ cells) with spermatozoa. Mature spermatozoa could be expressed from males from January through April. GSIs ranged from 0.2% (post-spawn) to 3.2% (pre-spawn). 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) I prior to spawning. Levels of 11-KT in males remained low (< 10 ng ml(-1), from post-spawning through January, then increased significantly by March and peaked just prior to spawning at 39.7 ng ml(-1). Our results indicate that vitellogenesis and spermatogenesis are complete or nearly so, in walleye by early winter, and suggest that it may be possible to induce spawning in this species several months prior to the normal spawning season by subjecting fish to relatively simple environmental and hormonal treatments.

Effects of dietary tryptophan levels on growth, feed/gain, carcass composition and liver glutamate dehydrogenase activity in rainbow trout (Salmo gairdneri).

1. The tryptophan requirement of rainbow trout (initial body wt, 13 g) was estimated by feeding diets containing varied levels of tryptophan from 0.06 to 0.5% of diet for 6 weeks. 2. The estimated tryptophan requirement was 0.20-0.25 (0.57-0.71)% of diet (dietary proteins). 3. Nitrogen retention increased and feed/gain decreased with dietary tryptophan levels up to 0.14%, but no further effect was observed at levels above 0.14%. 4. Carcass protein content gradually increased and lipid and ash contents decreased with increasing dietary tryptophan levels. 5. Dietary tryptophan levels did not affect hepatosomatic index or liver glutamate dehydrogenase activity.