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

Tilapia may be infected with:

• Nematodes / roundworms, such as Contracaecum sp.[12], Rhabdochona esseniae [13], Gnathostoma spinigerum, Gnathostoma doloresi and Gnathostoma hispidum.[14] Human gnathostomiasis infection is related to ingestion of "ceviche".[15]
• Cestodes (aka tapeworms), such as Amirthalingamia sp. and Cyclustera sp.[16]
• Trematodes / flukes (encysted metacercariae)[17], such as Clonorchis sinensis (human liver trematode)[18], Stictodora tridactyla (in brackish-water fish)[19], Ribeiroia marini [20], Clinostomum sp.[21], Haplorchis pumilio, Haplorchis taichui, Haplorchis yokogawai, Centrocestus formosanus, Stellantchas musfalcatus, Echinochasmus japonicus [22], Heterophyes heterophyes, Heterophyes aequalis, Pygidiopsis genata, Phagicola sp., Stictodora sp.[23] and Prohemostomumn vivax.[24] These trematodes loose their viability after 48 hours at -5°C.[25] The conditions of gas supersaturation (gas pressure > 111.2% saturation; N supersaturation, O2 undersaturation) may result in heavy monogenetic trematode infections in tilapia.[26] 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%) [27] 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.[28] In Vietnam, the overall FZT prevalence in tilapia from wastewater ponds was 2.0%, but much higher in tilapia from farm ponds. [29]
• Monogenea (very small flatworms), such as Neobenedenia melleni [30][31], Enterogyrus cichlidarum (associated with overcrowding [32] and infection intensity increases with (tilapia) host size[33]), Gyrodactylus malalai (ectoparasite)[34] and of the genus Cichlidogyrus [35], such as Cichlidogyrus tilapiae and Cichlidogyrus sclerosus. The highest parasite number was recorded in reservoir-dwelling, and lowest in stream-dwelling tilapia.[36] Gyrodactylus niloticus infection may particularly become lethal (in tilapia) when followed by exposure to the bacterial pathogen Streptococcus iniae.[37]
• Palaeacanthocephala, such as Polyacanthorhynchus kenyensis [38] In Kenya, infection rates in wild Tilapia zillii ranged from 4.1 to 77.7%.[39]

Tilapia are relatively resistant to parasitic infections.[40] The prevalence of heterophyid encysted metacercariae infection decreases as fish size increases.[41] Tilapia may produce an induced humoral immune response against Cichlidogyrus sp.[42] 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.[43] Tilapia may also develop resistance to Neobenedenia sp. (Monogenea) infection.[44]

Freshwater mollusks in tilapia ponds, such as Melania tuberculata, Melanoides turricula, Pomacea flagellata, Haitia cubensis and Anodontiles luteola, may host various parasites that are harmful to tilapia.[45] The Biomphalaria sp. snails are often found in tilapia ponds (fresh water reservoirs are their natural habitat[46]) and are intermediates for Schistosoma mansoni.[47] Biomphalaria cf. havanensis is an intermediate host for Diplostomum ompactum, causing higher levels of infections in cultured tilapia than wild tilapia.[48] The snail Bulinus truncatus is an intermediate host for Clinostomum tilapiae, Euclinostomum heterostomum, Bolbophorus levantinus and Neascus-type metacercariae. The snail Melanoides tuberculata is an intermediate host for Centrocestus sp. and Haplorchis sp. The snail Melanopsis costata is an intermediate host for Pygidiopsis genata, Phagicola longa.[49] The snail Thiara granifera is commonly infected with Haplorchis pumilio.[50] Dogs, cats and pigs may also be intermediate hosts for metacercariae.[51]