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 (ectoparasites) [15], metacercariae [16] (such as Haplorchis pumilio, Haplorchis taichui, Haplorchis yokogawai, Centrocestus formosanus, Stellantchas musfalcatus, and Echinochasmus japonicus [17], which loose their viability after 48 hours at -5°C)[18] and monogeneans of the genus Cichlidogyrus [19], such as Cichlidogyrus tilapiae and Cichlidogyrus sclerosus. The highest parasite number was recorded in reservoir-dwelling, and lowest in stream-dwelling tilapia.[20]

Tilapia are relatively resistant to parasitic infections.[21] The prevalence of heterophyid encysted metacercariae infection decreases as fish size increases.[22] Tilapia may produce an induced humoral immune response against Cichlidogyrus sp.[23] Gyrodactylus numbers fluctuate independently of temperature. In anticipation of immune defenses reaching the fish surface through mucus, Gyrodactylids (Monogenea) worms progressively move away from fins with high mucus cell density to those with low density.[24] Tilapia may also develop resistance to Neobenedenia sp. (Monogenea) infection.[25]

Freshwater mollusks in tilapia ponds may host various parasites that are harmful to tilapia, such as Melania tuberculata, Melanoides turricula, Pomacea flagellata, Haitia cubensis and Anodontiles luteola.[26] The snail Biomphalaria cf. havanensis is a intermediate host of Diplostomum ompactum, causing higher levels of infections in cultured tilapia than wild tilapia.[27] Dogs, cats and pigs may be intyermediate hosts for metacercariae.[28]