Sex reversal of female rainbow trout by immersion in 17α-methyltestosterone

The use of oral administration of 17α-methyltestosterone (17MT) at 3 mg 17MT/kg of feed for 60 days following the onset of feeding to sex reverse female rainbow trout (Oncorhynchus mykiss) to produce XX males is well established. The treatment has shown to consistently result in nearly 100% of the treated animals being males. A primary drawback to this protocol is a high proportion of males are non-functional, in that milt is not released when pressure is applied to the abdomen, and therefore the males must be euthanized to harvest sperm from the testes. A second drawback to the protocol is the difficulty in preventing the steroid from reaching the environment when used as a feed additive. A protocol using immersion to administer 17MT for sex reversal was developed to address these issues. Immersion of female rainbow trout in 17MT at 400 μg/L for 2 h at one week post hatching, and weekly for five weeks starting at the onset of first feeding, for a total of seven immersions, resulted in nearly 100% sex reversal to male and about half the males being functional. The technique should increase efficiency of hatchery operations by allowing greater flexibility inherent in functional males, reduce the accidental harvesting of immature males, and simplify the capture of the steroid before reaching the environment.