Abstract One of the main problems to be solved in the commercial production of Nile tilapia is the use of large volumes of steroids during the sex reversion process. The precise evaluation of the time after hatching to start the steroid treatment could help reduce its use in the short-term. The main objective of this work was to evaluate the sex proportion and the growth obtained in a mixed population of Nile tilapia fed fluoxymesterone at different days (3, 6, 9, 12, and 15) after hatching. A one-factor experiment was designed, with the day, after hatching, that the fluoxymesterone (5 mg) began to be supplied as factor (3, 6, 9, 12 and 15). Initial stocking density was 0.4 larvae L-1. All groups were evaluated by triplicate. Steroid treatment finished at day 25 after larvae stocking. Squash technique was used to determine sex proportion at the end of the experiment (45 days). Growth evaluation was performed based on wet weight and total length obtained from each biometry. The results showed the highest percentage of males (92%) in the fish fed fluoxymesterone from day three after hatching. No anabolic effect on growth was observed in any steroid-treated fish. A negative effect on growth was observed after withdrawal of the steroid in the fish that were fed fluoxymesterone for the longest time. These results show that it is necessary to supply the fluoxymesterone prior to the onset of sex differentiation (5-6 days after hatching) to achieve a permanent reversion of the sex differentiation process.

Sex-reversal by exogenous hormones is the most common technique used to generate monosex populations of Nile tilapia (Oreochromis niloticus). However, this technique has provoked a negative perception in recent years. Because of this, alternative techniques have been developed, including the production of YY males. Although the final product (for sale) is not administered hormones, the first part of this technique still requires the feminization of XY fry by use of estrogens, including some of a synthetic nature, such as diethylstilbestrol (DES), an estrogen that has shown particularly excellent results in related species. The aim of this study was to evaluate the effect of increasing concentrations of DES (100, 200, 300, and 400 mg kg-1) during the fry stage on the sex proportion, growth and gonadosomatic index (GSI) of Nile tilapia. The 400 mg kg-1 concentration was the one that produced the highest proportion of females (91 %). However, increasing the concentration of DES provided through diet does not guarantee a 100 % feminization rate. Additionally, the growth, survival and GSI, showed a significant decrease (p < 0.05) in all groups fed with DES compared to the control group. It is possible that the anabolic effect of DES observed in other species is not present in Nile tilapia.

La reversión sexual a través de hormonas exógenas es la técnica más usada para obtener poblaciones monosexo de tilapia del Nilo (Oreochromis niloticus). Sin embargo, esta técnica ha generado una percepción negativa en los últimos años. Debido a lo anterior, se han desarrollado técnicas alternativas incluyendo la producción de machos YY. Aunque con esta técnica el producto final (para vender) no recibe hormona, la primera parte de esta técnica aun requiere la feminización de alevines XY a través de estrógenos, incluyendo algunos de naturaleza sintética como el dietilestilbestrol (DES), el cual ha mostrado excelentes resultados en especies relacionadas. El objetivo del presente trabajo fue evaluar el efecto de concentraciones crecientes de DES (100, 200, 300, 400 mg kg-1) durante el periodo de alevín en la proporción de sexos, crecimiento e índice gonadosomático (IGS) de la tilapia del Nilo. La concentración de 400 mg kg-1 fue la que arrojó la mayor proporción de hembras (91 %). Sin embargo, incrementar la concentración de DES proporcionada a través de la dieta no garantiza una feminización del 100 %. Adicionalmente, el crecimiento, la supervivencia y el IGS, mostraron un descenso significativo (p < 0.05) en todos los grupos alimentados con DES, en comparación con el grupo control. Es posible que el efecto anabólico del DES observado en otras especies no este presente en la tilapia del Nilo.

Abstract Conditions for the feminization of undifferentiated XY fry of Nile tilapia (Oreochromis niloticus) fed on a diet supplemented with estradiol-17β (E2) were investigated in order to produce functional phenotypical females (genetic males). Different E2 concentrations (60 and 120 mg kg-1) were tested over a period of 30 days in a closed recirculating system. Fry were fed ad libitum at 1-h intervals. The effects on growth, gonadal development and body composition were evaluated at sexual maturity. Sex-reversed females were produced in both E2 treatments. The proportion of males was significantly lower (p < 0.05) in both E2 treatments compared to the control group. Fish with atypical papilla and normal ovaries were produced in both E2 concentrations. These fish were considered sex-reversed females. Growth was significantly higher (p < 0.05) in the males compared to both the females and the sex-reversed ones in all groups analyzed. No testicular tissue was observed in any of the ovaries of the sex-reversed females analyzed. The percentage of fat in muscle increased and that of protein decreased significantly (p < 0.05) in the sex-reversed females compared to that observed in the control group. Identified sex-reversed females will be used in the production of YY males through mating with normal males, as part of a developing breeding program to produce a male genetic population in the southern region of Mexico.

Resumen Se investigaron las condiciones para la feminización de alevines XY de la tilapia del Nilo (Oreochromis niloticus) a través de una dieta suplementada con estradiol-17β (E2) con la finalidad de producir hembras fenotípicas funcionales (machos a nivel genético). Se pusieron a prueba dos concentraciones de E2 (60 y 120 mg kg-1) durante 30 días en un sistema de recirculación cerrado. Los alevines fueron alimentados ad libitum a intervalos de 1-h. Los efectos sobre el crecimiento, el desarrollo gonadal y la composición corporal fueron evaluados a la madurez sexual. Se observó una disminución significativa (p < 0.05) en la proporción de machos en los grupos tratados con E2, en comparación con el grupo control. Se detectaron peces con papilla genital atípica y ovarios normales en ambas concentraciones de E2. Estos peces se consideraron hembras revertidas. El crecimiento fue significativamente mayor (p < 0.05) en los machos, en comparación con las hembras normales y las hembras revertidas en todos los grupos analizados. No se observó tejido testicular en ninguno de los ovarios de las hembras revertidas analizadas. El porcentaje de grasa en músculo aumentó, mientras que el de proteína disminuyó significativamente (p < 0.05) en las hembras revertidas, en comparación con lo observado en el grupo control. Las hembras revertidas identificadas se utilizarán en la producción de machos YY mediante su cruza con machos normales, como parte de un programa de cría para la producción de poblaciones compuestas al 100% por machos genéticos en la región sur de México.

Successful YY technology depends on the production of XY females. However, their identification is complicated because they are indistinguishable from normal females. Atypical fish could offer an alternative for a more rapid and precise identification. Progeny of atypical fish was evaluated in order to produce YY-males. In total, nine atypical fish and 18 normal males were selected. The fish were placed in 8 m³ concrete tanks at a 2:1 sex ratio. The produced fry were collected and reared at 28 ± 1°C in 85 L aquaria. Juveniles were placed in net cages for 30 days and finally in 8 m³ concrete tanks until the age of 120 days. Progeny test was achieved evaluating differences between sexes in the genital papilla structure. Six of the nine atypical fish selected showed the 3:1 sex ratio (male-female) expected for sex-reversed females. A significantly higher proportion of males than the expected 3:1 distribution were observed in two atypical fish. This boost in male proportion could be attributed to a parental effect interacting with the water temperature during the fry stage. Use of atypical fish could help reduce the time and effort spent to identify XY females during the initial stage of YY technology.

La aplicación exitosa de la tecnología YY depende de la producción de hembras XY. Sin embargo, su identificación es complicada, ya que son indistinguibles de las hembras normales. Los peces atípicos podrían ofrecer una alternativa para una más rápida y precisa identificación. Se evaluó la progenie de nueve peces atípicos con la finalidad de producir machos YY. Los alevines obtenidos se criaron a 28 ± 1°C en acuarios de 85 L. Los juveniles fueron colocados en jaulas flotantes por 30 días y, finalmente, en estanques de 8 m³ hasta los 120 días de edad. La prueba de progenie se realizó evaluando las diferencias entre sexos en la estructura de la papila genital. Seis de los nueve peces atípicos seleccionados mostraron la proporción de sexos 3:1 (macho-hembra) esperada para hembras revertidas. Se observó una proporción de machos significativamente mayor a la distribución 3:1 esperada en dos peces atípicos. Este aumento en la proporción de machos puede ser atribuido a la interacción del efecto parental con la temperatura del agua durante la etapa de alevín. El uso de peces atípicos podría reducir el tiempo y esfuerzo empleados en la identificación de hembras XY durante la etapa inicial de la tecnología YY.