ng g-1 of E2 and > 2,000 ng g-1 of TA. The nauplii treated with E2 contained 5,500 ng g-1 at 2 h, remained at that level until 6 h, and then reached a concentration of 7,000 ng g-1 at 12 h, which remained until 24 h. A similar pattern was observed when TA was used; however, at 24 h concentrations declined to 5,000 ng g-1. TA concentrations in Artemia nauplii enriched with highly unsaturated fatty acids (HUFA) ranged between 500 and 800 ng g-1. Despite these large differences in steroid concentrations between HUFA enriched and non-enriched Artemia, both methods resulted in significant sex inversion of fish fry. Both Nile tilapia and mojarra castarrica fry were masculinized (> 90% males) after being fed TA-enriched nauplii for 20 and 45 d, respectively. Tropical gars were also feminized (> 60% females) when fry were fed E2-enriched Artemia nauplii. It appears that bioencapsulation of steroids into Artemia nauplii is an efficient method for sex inversion of fish.
Introduction
In the tropics two approaches have been taken to utilize the large diversity of fish species for aquaculture purposes. The first is to use species with moderate growth rates that have high market values, and the second is the use of species with distinct sexual dimorphisms because there is a large demand for the attractively colored gender in aquarium stores. In southeastern Mexico, a growing number of fish producers have requested that alternative culture techniques, which involve native species, be developed. Since the early 1970s, the only species that has been available are Nile tilapia, (Oreochromis niloticus) and little effort has been devoted to develop options with local species. Worldwide, the administration of natural and synthetic steroids during early development of fish has been successfully used to induce sex inversion in several species (see reviews by Schreck, 1974; Hunter and Donaldson, 1983), and has become a common practice in the production of single sex populations to enhance productivity in the aquaculture industry. Protocols for masculinizing 47 species and feminizing 31 species have been developed (Pandian and Sheela, 1995). Among the steroids used, 17 -methyltestosterone (MT), trenbolone acetate (TA), and estradiol (E2) have proven effective for masculinizing (MT and TA) and feminizing (E2) fish.
At the Laboratory of Aquaculture at Universidad Ju‡rez Aut—noma de Tabasco (UJAT), the production of tropical gar (Atractosteus tropicus) fry has been successful over the last five years (M‡rquez-Couturier and Contreras-S‡nchez, 2003). A previous study demonstrated that females reach larger sizes than males (Contreras-S‡nchez and Alem‡n-Ramos, 1987), but little is known regarding the possible advantages of producing 100% female populations. The tropical gar is considered to be a delicacy in southern Mexico and is an integral part of local culture. It is analogous to the importance of salmon in the Pacific Northwest of the US. Gar are sold in every seafood restaurant in Tabasco, and fishing pressure has led to the listing of the species as "sus
ceptible" (Contreras-S‡nchez, 1990).
The mojarra castarrica (Cichlasoma urophthalmus) has been considered as one of the few native species that may perform as well as tilapia in aquacultural systems in terms of growth rates and achievable harvest size (Mart’nez-Palacios and Ross, 1994). As in tilapia, males grow faster and larger than females; however, little has been done concerning the production of 100% male populations, and the few studies that have been carried out resulted in less than 70% male populations (Hern‡ndez-Betancourt, 1988).
Possible alternatives to the problems encountered for sex inversion of new species of fish in aquaculture are the administration of steroids via bioencapsulation and immersions. The first technique is advantageous because larvae of the two species proposed for this study strongly prefer live food compared to artificial diets. This technique has been used for administering chemotherapeutics and nutrients to fish, shrimps, and prawns (Roque et al., 1998; Touraki et al., 1999) and is considered to be a good alternative method for incorporating hormones into manufactured feeds. The second alternative offers the possibility of feminizing or masculinizing the proposed species without the need of hormone-treated diets.
In order to determine the feasibility of sex inversion of native species of commercial importance, the bioencapsulation technique was validated using Nile tilapia (O. niloticus) as a model species. In this study we compared the most commonly used protocol for masculinization of tilapia (oral administration of MT) with either feeding of bioencapsulated TA or with immersions in TA. We also evaluated whether tropical gar fry could be feminized using bioencapsulated E2 or with immersions in this steroid.
Methods and Materials
Study A: Enrichment and Detection of E2 and TA in Artemia Nauplii.
This experiment was conducted at the Laboratory of Aquaculture, UJAT. Two procedures for Artemia enrichment were evaluated:
1) Bioencapsulation via immersion in steroid solutions and
2) Bioencapsulation via immersion in solutions containing HUFA as recommended by Stewart et al. (2001).
Procedure 1
Bioencapsulation via immersion in steroid solutions consisted of three treatments:
1) Artemia nauplii immersed in 2,500 mg L-1 E2
2) Artemia nauplii immersed in 2,500 mg L -1TA