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

We investigated the feminization of bluegill and black crappie by short periodic immersions in an aqueous estradiol‐17β (E₂) solution. Bluegill fry (13.8 mm) 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 20.1 mm) 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 1 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.