The PD/A CRSP also conducts research activities not described in workplans. These activities represent an additional effort from CRSP researchers and students, and are reported as Special Topic Research. During this reporting period the potential use of bull testes for sex-reversal of tilapia is being investigated at Auburn University. In addition, researchers are investigating the influence of the treatment environment (aquaria versus hapas) on the efficacy of 17a-methyltestosterone, a commonly used substance for sex-reversal.
Sex reversal of newly hatched tilapia generally is accomplished via oral administration of 17a-methyltestosterone (MT), which has been incorporated into a starter fish feed at 60 mg MT/kg feed (Popma and Green, 1990). While use of the 60 mg MT/kg feed dose consistently yields populations comprised of less than 5% females (i.e., > 95% males), this has not been shown to be the optimal dose. Other investigators have reported sex reversal of tilapia at dose rates less than 60 mg MT/kg feed (Guerrero, 1975; Tayamen and Shelton, 1978; McGeachin et al., 1987; Jo et al., 1988; Varadaraj and Pandian, 1989). However, results from some of these studies are inconsistent, and it is difficult to separate treatment environment effects. Thus, it is necessary to identify the optimal dose of MT for consistent, successful sex reversal in a variety of treatment environments.
Naturally occurring sources of testosterone may be an alternative to using a synthetic androgen, which also is an anabolic steroid, for tilapia sex reversal. Haylor and Pascual (1991) reported successful tilapia sex reversal using ram's testes as a source of dietary testosterone. Bull testes are a by-product of beef industry in the U.S., and are a potential source of dietary testosterone for tilapia sex reversal.
The objectives of this research were to determine the efficacy of sex reversal of different dosage rates of MT to fish treated in different environments, and to evaluate the potential of freeze-dried bull testes as a dietary source of testosterone for tilapia sex reversal.
Materials and Methods
Newly hatched Nile tilapia (Oreochromis niloticus) were stocked at 8 fry/L into 80-L glass aquaria located inside a hatchery building or into hapas (45-L volume) suspended in 20-m3 outdoor concrete tanks located at the Fisheries Research Unit, Alabama Agricultural Experiment Station, Auburn University, AL. Fry were stocked on 1 Aug 1995, and harvested after a 28-d treatment period. Subsamples of fry from each treatment unit were transferred to hapas suspended in 20-m3 outdoor concrete tanks for nursery rearing to approximately 5-g size. Once fingerlings attained an average weight of 5 grams they were sacrificed, and the gonads excised and gonadal sex determined according to the aceto-carmine squash method (Guerrero and Shelton, 1974).
Trout chow (42% protein) was the carrier for MT, which was incorporated into the feed at 0, 15, 30, 45 or 60 mg MT/kg of feed. The appropriate quantity of MT was dissolved in 500 mL of 95% ethanol/kg feed, and this solution was mixed with the powdered feed. Ethanol only was mixed with feed for the 0 mg MT/kg feed treatment. Ethanol was evaporated from the alcohol-feed mixture, and the dried feed was stored in refrigeration until used. Fry in each treatment were fed at 20% body weight during week 1; the daily ration was divided into four meals. Feed rate was decreased by 2.5%/wk during weeks 2-4. Feed rate was adjusted weekly based on results of weekly population samples.
Frozen bull testes were obtained from a meat packing plant in Montgomery, AL. Individual testes were skinned, sliced, freeze-dried and ground, and mixed with trout chow either in a 1:1 or 1:3 freeze-dried testes:trout chow ratio. Mixed feed was refrigerated until fed and feed was offered as described above.
Results and Discussion
The sex reversal period was completed and the nursery rearing phase initiated by the end of August 1995. Nursery rearing and gonadal sex determination are expected to be completed by December 1995. Data analyses will be performed upon completion of all data collection.
After the 28-d MT treatment period, fry total length ranged from 32.8-39.6 mm and 40.7-44.3 mm for fry treated in aquaria (indoors) and hapas (outdoors), respectively. Average respective final weight ranges were 0.7-1.0 and 1.2-1.9 g/fry. Fry survival in both environments was low and ranged from 16.7-27.7% and 25.7-43.6% in aquaria (indoors) and hapas (outdoors), respectively.
Fry fed feed containing bull testes were 55.6 and 59.7 mm total length for 1:1 and 1:3 ratio feeds, respectively, following the 28-d treatment period. Mean final weights were 2.0 and 0.7 g/fry for 1:1 and 1:3 ratio feeds, respectively, which undoubtedly reflected the difference in respective survival during treatment (28.3% versus 69.2%).
Results of these trials should indicate the effects of treatment environment on efficacy of tilapia sex reversal using MT. In addition, information on dose rate response should help narrow the range of dose rates that need to be examined to determine the optimal dose rate. Information on the efficacy of using freeze-dried bull testes as a dietary ingredient for sex reversal of tilapia may offer an alternative to using dietary MT.
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Guerrero, R. D., and W. L. Shelton. 1974. An aceto-carmine squash method for sexing juvenile fishes. Progressive Fish Culturist 36:56.
Haylor, G. S., and A. B. Pascual. 1991. Effect of using ran testis in a fry diet for Oreochromis niloticus (L.) on growth, survival and resultant phenotypic sex ratio. Aquaculture and Fisheries Management 22: 265-268.
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