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Frequently Asked Questions

What does GMT® stand for?

How can I obtain GMT®?

Are GMT® a product of genetic engineering?

What is a "supermale"?

Can I produce GMT® in my own hatchery?

Do I need any special facilities to produce or grow GMT®?

Can I use GMT® as broodstock?

Do you have red GMT®?

How fast do GMT® grow?

What is the difference between GMT® and sex reversed tilapia?

Will GMT® grow well in brackish water?

 

Answers

What does GMT® stand for?
GMT® is a registered trademark and acronym for Genetically Male Tilapia and refers to progeny from YY "supermales".  When crossed to females these supermales produce only XY male progeny.  The properties of GMT® are derived from the fact that they represent populations made up of ONLY genetically male progeny.  Each individual GMT® is a normal genetic male (XY).  The superior properties of GMT® are a function of the whole population being genetically male, obviating the problems of  inferior growth of females, diversion of energy to reproduction and the production of unwanted fry and fingerlings.

How can I obtain GMT®?
GMT® can be purchased directly from Fishgen or from one of our agents located worldwide. Contact Fishgen for a quotation stating your location and the approximate number of GMT® you would like to purchase. It would also be useful if you could provide brief details of your production system.

Are GMT® a product of genetic engineering?
Genetic engineering commonly involves techniques that manipulate genes in ways that bypass normal sexual reproduction, such as the introduction of novel genes through transgenesis.  The products of genetic engineering are often referred to as Genetically Modified Organisms (GMOs). GMT® are produced in a simple breeding programme combining sex reversal and progeny testing which does not involve any genetic engineering techniques.  GMT® are thus little different to fish produced from normal crosses or standard selection programmes and are not considered as GMOs according to legal standard definitions.

What is a supermale?
A supermale is a male which has the novel YY genotype, possessing two male sex determining chromosomes instead of one, as in the normal, naturally occurring male genotype (XY).  Supermales themselves, are not hormonally or otherwise treated in any way. In the Fishgen technology, supermales (YY) are crossed to normal females (XX) to produce GMT® (XY).  Supermales have been produced in a number of fish species but are normally inviable in higher organisms such as humans. 

Can I produce GMT® in my own hatchery?
You can produce GMT® in your own hatchery by purchasing GMT® producing broodstock from Fishgen.  We supply GMT® producing broodstock as sets (each set includes 1 YY male and 3 females).  Contact Fishgen for a quotation for GMT® producing broodstock indicating your location and the number of broodstock sets you might require (or alternatively, your targeted fingerling production).  GMT® producing broodstock are not available in some locations due to commercial restrictions.

Do I need any special facilities to produce or grow GMT®?
No, as normal genetic males GMT® require no special treatment or facilities.  GMT® producing broodstock can be used successfully in a wide range of commercial hatchery systems and likewise the benefits of GMT® can be seen in all grow-out systems. We do, however, recommend that all efforts are taken to prevent contamination of YY males with any normal males in hatchery systems.  Similarly, efforts should be made to ensure that  females don't contaminate GMT® in grow-out systems.  This can be achieved by draining systems before stocking and by careful screening of inlet and outlets.  We recommend the tagging of YY males in hatcheries although this is not required. 

Can I use GMT® as broodstock?
As stated above, GMT® are normal fertile genetic males and can be used as broodstock.  However, use of GMT® as male broodstock, crossed with normal females, will produce normal  mixed sex progeny so there is little advantage to this.  GMT® themselves can only be produced as progeny of Fishgen's YY supermales.

Do you have red GMT®?
Fishgen has been developing a GMT® in a red strain of pure O. niloticus.  This red GMT® should be available by the second half of 2002. 

How fast do GMT® grow?
The short answer to this question is that it depends on the conditions in which you grow them.  We cannot accurately predict the growth rate of GMT® in any particular system but we do know, based on extensive growth trials,  that GMT® have outperformed comparable tilapia in a wide range of production systems. 

What is the difference between GMT® and sex reversed tilapia?
As stated above, GMT® are normal male tilapia and are not GMOs.  Sex reversed tilapia (SRT) are all-male progeny, produced through direct oral administration of male hormone during the period of sexual differentiation.  Approximately 50% of SRT remain as genetic females although phenotypically male. The advantages of GMT® over SRT include: 

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no special facilities or treatments are needed within the hatchery system reducing labour and consumable costs; 

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more consistent sex ratio as there is no need to maintain stringent optimum treatment protocols necessary for production of good SRT; 

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a safer hatchery environment as no hormones are used;

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better consumer acceptance as some markets may be cautious over the consumption of hormone treated fish;

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GMT® grow faster than SRT of the same strain even with the same sex ratio, presumably due to slower growth of the genetic females in the SRT.

Will GMT® grow well in brackish water?
At present GMT® are available only in the Nile tilapia, O. niloticus.  Nile tilapia, whilst not the most saline tolerant of the tilapias, will tolerate and grow well in brackish water.  Effects on growth and some problems with disease are sometimes seen when salinity is consistently above 22ppt.   However, GMT® growers in Mexico have seen impressive performance of GMT® even at salinities above 25 ppt.

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This page was last updated on 25 August 2005