SEX REVERSAL OF RAINBOW TROUT BY IMMERSION OF FRY IN 17-ALPHA METHYLTESTOSTERONE

17-alpha methyltestosterone (MT) is currently used to sex reverse genetic female rainbow trout into phenotypic males, commonly referred to as neomales.  Neomales are primarily generated to propagate all-female lines.  The MT is most commonly administered orally, fed during the first 6-9 weeks after swim-up.    Oral administration is highly effective and uses little MT, but effluent water from the culture system may contain residual MT.  Furthermore, most of the neomales do not release milt due to malformed sperm ducts, thus necessitating the removal of the testes to obtain sperm.  Immersion of fry in a bath containing MT is also possible; however, reported treatment regimens have been less effective than oral administration.  Interest in making it easier to contain the MT used during sex reversal procedures, and interest in avoiding use of steroids as feed additives, has renewed interest in the use of immersion for neomale production.  

We have found that immersion of fry from an all-female line in 400 ug MT/L for 2 hours in a static bath at 1 week post-hatching and then once weekly for up to 9 weeks, starting at swim-up, is highly effective and results in >95% neomales. Furthermore, limited results suggest that up to half of the neomales are functional in that they can release milt.