Identification of Optimized Quantity of Methyl-Testosterone Incorporated Feed for Sex Reversal of Nile Tilapia

Abstract

Tilapia is likely to be the most important aquaculture fish species in the 21 century (Fitzsimmons, 2000). Tilapia has certain favourable characteristics for aquaculture such as more tolerant to adverse environmental conditions; survive at low dissolve oxygen, relatively fast growth and efficient food conversion (Yi et al., 1996; Penna Mendoza, 2005). One of the main impediments in tilapia grow out production at commercial scale is its rapid reproduction rate. It attains sexual maturity at an early age and reproduces after every 4-6 weeks in the ponds and start to reproduce in culture ponds resulting a low production. This can be controlled by stocking ponds with all male tilapia produced with mono-sex production technique. Tilapia has sexual growth dimorphism in which males grow faster and show more standard size than females (Mair and Little, 1991). Manual process by visual examination, hybridization, gene manipulation and masculinisation via steroid hormone are commonly used techniques for production of mono-sex tilapia.

Use of the hormone17α-Methyl Testosterone (MT) to induce sex reversal in farmed tilapias has become a common practice in many parts of the world. MT is a simple and reliable way to produce all-male tilapia stocks, which consistently grow to a larger/more uniform size than mixed sex or all-female stocks. MT treatment could still be perceived as hazardous to tilapia consumers if MT is applied at higher than the recommended dosages, or is used for longer periods. Moreover, there is no evidence that higher dosages or longer treatment periods improve the sex reversal effectiveness of MT. In fact research findings indicate that MT is actually less effective if the recommended dosage and duration of treatment are exceeded (Yoshikawa and Oguri, 1978). Hence, identification of minimal quantity of methyl testosterone incorporated feed for sex reversal of tilapia is more critical for the fish farmers. It helps to minimize the cost that needs to bear for hormone and feed. Lower usage of hormone treated feed will lower the accumulation it into the natural environment as well. The study was carried out at the Aquaculture Development Center (AQDC) in Polonnaruwa. The selected sample size was 3750 tilapia individuals on fry stage and twelve tanks were used for the study. All tanks were cleaned through the scrubbing of algae and allowed to dry for three days. The water was filtered through sand and gravel filter media and pumped into the tanks. The outlet pipes were sealed with plastic caps. Tilapia fry was taken from the Polonnaruwa Aquaculture Development Center (AQDC) from the same spawning. Fry were quarantined for 2 to 3 days. The quarantined fry with average body weight of 0.02 ± 0.001 g were randomly stocked in twelve tanks at the rate of 250 fry per tank. Each tank was covered with small mesh net material to block the entrance of predators. In order to take 60 mgMT/kg, 0.5g of testosterone powder was measured using an electronic analytical balance (Model: FA1004N) with 0.1 mg accuracy and dissolved in one litter of 95% “Methyl Alcohol” solution. 120 ml was taken and spread over 1 kg of powder form commercial tropical fish feed (FF 00) and kept at room temperature for one day to evaporate the excess methanol solution. Fifty individuals from each tank were weighed after blotting with paper towel. Based on data obtained mean body weight was calculated. Under treatment 01, all tilapia fry was given 3% of their mean body weight. Respectively, 6% for treatment 02, 9% for treatment 03 and finally 12% for treatment 04 were continued up to two weeks.

After two weeks, fifty individuals were randomly taken from each tank and body weight was measured and mean body weight for each tank was calculated separately. Using same methodology individual weights was measured at every other week until the end of research period and mean body was calculated. The amount of hormone incorporated feed for the individual and total amount of hormone utilized were calculated for each treatment according to the mean body weight and fed with those new values.

After four weeks, tilapia fingerlings were fed with usual commercial tropical fish feed (FF 01) for another three weeks and at the eighth week, the amount of male, female was recorded by observing the genital papilla of the advance fingerlings using methylene blue. The results were analysed using Minitab 16 statistical software. Survival rates and percentage male production were analysed using analysis of variance (ANOVA) in order to determine the differences among treatments at 95% significance level.