Directing gonadal differentiation in bluegill, Lepomis macrochirus (Rafinesque), and black crappie, Pomoxis nigromaculatus (Lesueur), by periodic estradiol-17(beta) immersions

We investigated the feminization of bluegill and black crappie by short periodic immersions in an aqueous estradiol-17(beta) (E2) solution. Bluegill fry (13.8mm) were immersed in a 1 mg L^-1 E2 solution every 5 days on three occasions over an 11-day period. In the positive control treatment, the same cohort of fry was fed 200 mg kg^-1 E2 diet for 45 days. Two different size (26.1 and 20.1mm) but the same age (45 days old) black crappie fry were immersed in the same concentration of E2 solution for 5 h a day every 3-5 days on 10 occasions between 45 and 86 days post hatch. Both modes of E2 administration were effective to induce feminization in bluegill. Feeding fry E2 impregnated diet for 45 days yielded 99.3% female populations from a 38.8% female experimental population. Only three short-time E2 immersions of fry over a much shorter treatment period also produced 76.9% female populations. E2 immersions had no effect on sex ratios of larger black crappie fry and produced variable levels of feminization (60-82% female) in smaller black crappie fry. Results of the present study showed that feminization in both species can be achieved by periodic immersions of fry in a I mg L^-1 E2 solution with no adverse effects on survival. Yet, further researches on timing and intensity of immersion treatments are still needed for optimization of the procedures.