Tilapia

Diet

Tilapia may survive on algae or duckweed alone, but combined feeding results in higher growth rates. Superior growth is achieved by partial replacement with fishfeed or insects / worms.

Masculinization

Estrogen synthesis is crucial for ovarian differentiation, and transcription of the aromatase gene can be proposed as a key step in that process in fish. Treatments with an aromatase inhibitor (ATD, 1,4,6- androstatriene-3-17-dione) result in 75.3% masculinization of an all-female (XX) population in tilapia (dosage 150 mg/kg of food). The effectiveness of the aromatase inhibition by ATD is demonstrated by the marked decrease of the gonadal aromatase activity in treated animals versus control.[1]

Aromatase activity as a key factor in sexual differentiation in Oreochromis niloticus.[2] The most sensitive time to aromatase inhibitors lies in the first week (between 7 and 14 days post hatch). Treatment with the aromatase inhibitor Fadrozole (nonsteroidal) showed a dose-dependent increase in the percentage of males from 0 to 200 mg per kg. At higher doses (200 to 500 mg / kg), the percentage of males remained more or less constant (92.5-96.0%).[3]

The masculinizing actions of 17alpha-methyltestosterone (MT) are most potent at up to day 20 of age.[4]

Treating female tilapia Oreochromis niloticus with methyltestosterone (at a dose of 50 mcg/g diet) resulted in 100% masculinization.[5]

Treatment with tamoxifen and letrozole (200mg/kg feed) to fingerlings of O. niloticus for 60 days brought about 98.5% masculinization. Treatment with 17α methyltestosterone (35 mg/kg feed) to fingerlings of O. mossambicus after 8 days post hatch for 60 days obtained 100% sex reversed males with excellent growth.[6]

During the restricted developmental period temperature is of great influence. The critical period for elevated-temperature-induced masculinization lays between days 10 and 15 post-hatch.[7] Higher temperatures (during 5 days) before they are 5 days old induces deformities. Masculinization is induced at elevated temperatures (28 to 32C) during 5 days after 10 days old.[8]

Feminization

The critical period for low-temperature-induced feminization lays between days 5 and 10 post-hatch.[9] The period of maximal feminizing action of 17beta-estradiol (E(2)) upon sex ratio is before 10 days posthatching in tilapia (Oreochromis mossambicus).[10]

During the restricted developmental period temperature is of great influence. Higher temperatures (during 5 days) before they are 5 days old induces deformities. Gonadal feminization is induced at lower temperatures (20C) for 5 days before 10 days old.[11]

Parasites

In Thailand, fish-borne zoonotic trematode (FZT) metacercarial infections were found in Nile tilapia from cage (2.5%) and pond aquaculture systems (10%) and in wild caught fish.(53%) [12] In China, tilapia from nursery and grow-out ponds were sampled from monoculture, polyculture and integrated aquaculture systems, revealing a 1.5% prevalence of FZT infections (Heterophyidae and Echinostomatidae); lower than in wild caught fish. Integrated systems using animal manure and latrine wastes as fertilizer did not show a higher prevalence.[13] In Vietnam, the overall FZT prevalence in tilapia from wastewater ponds was 2.0%, but much higher in tilapia from farm ponds. The FZT species recovered from infected fish included both liver (Clonorchis sinensis) and intestinal flukes (Haplorchis taichui, Haplorchis pumilio, and Centrocestus formosanus).[14]

Tilapia may also be infected with Gyrodactylus malalai [15] and monogeneans of the genus Cichlidogyrus.[16]