Production of monosex male black crappie, Pomoxis nigromaculatus, populations by multiple androgen immersion

Monosex populations can solve the problem of stunted black crappie populations in small impoundments and can increase the potential of black crappie as an aquaculture species. In this study, we investigated the effective mode and duration of exogenous androgen administration to produce monosex male black crappie populations. We conducted two experiments using the synthetic androgens, 17alpha-methyltestosterone (MT) and trenbolone acetate (TBA). In the first experiment, the same age (45 days old), two different size fry with mean total lengths ( +/-S.D., N = 50) of 20.1 +/- 1.4 and 26.1 +/- 2.0 mm were either fed 60 mg MT/kg diet for 45 days or immersed in a 1 mg MT/l solution on 10 occasions for 5 h/day every 3-5 days between 45 and 86 days post-hatch (dph). In the second experiment, a different cohort of fry was immersed in a 1 mg TBA/ 1 solution for 5 h/day every 3-5 days either on seven occasions between 40 and 66 dph or on 10 occasions between 45 and 86 dph. Mean total lengths ( +/-S.D., N = 50) of fry at 40 and 45 dph were 20.3 +/- 0.9 and 21.6 +/- 1.2 mm, respectively. Both modes of MT administration were ineffective for altering sex ratios in larger black crappie fry. Oral administration of MT to smaller fry resulted in 23% intersex fish. MT administration via short periodic baths of smaller fry produced 96% male populations. TBA was as potent as MT to induce masculinization in black crappie: both immersion treatments produced all male populations. Results of the present study indicated that initial size of fry as well as initial age should be considered when selecting appropriate size black crappie fry for hormone treatments. Monosex male populations in black crappie can effectively be produced through a series of (seven or less) short (5 h) immersions of young fry (40 - 45 days old and 20 - 21 mm) in a 1 mg/l aqueous solution of MT or TBA. Besides high effectiveness, immersion method will be the preferred mode of androgen administration to black crappie because it reduces variation in hormone uptake associated with the oral administration of steroids and provides more flexibility in feeding during the period of gonadal differentiation. (C) 2004 Elsevier B.V. All rights reserved.