Effects of duration and timing of immersion in 17α-methyltestosterone on sex reversal of female rainbow trout

Sex reversal of female rainbow trout (Oncorhynchus mykiss) by oral administration of 17α-methyltestosterone (17MT) is well established for production of XX males. Drawbacks to this protocol include a high proportion of males fail to develop fully functional sperm ducts and must be euthanized to harvest sperm, and difficulty in preventing the steroid from reaching the environment. Recent experiments demonstrate immersion of female rainbow trout in 17MT at 400 µg/L for 2 h at one week post hatching, and six weekly immersions starting at the onset of first feeding, was also effective for inducing sex reversal and should increase efficiency of hatchery operations by reducing harvesting of immature males and allowing reuse of functional males, as well as simplifying steroid containment. This experimental protocol was evaluated in a broodstock population of 501 treated animals and shown to deliver similar results with 95% developing testes and a minimum of 45% able to release milt. Treatment was uniformly effective for fish first immersed at 4–7 days post hatching. The effects of reducing the duration of the immersions and the timing of the second immersion were evaluated. Immersion for 1 h was as effective as 2 h. The treatment was consistently effective when the second immersion was applied by 21 days post hatching. Furthermore, there was an apparent family effect on fish that failed to develop gonads independent of treatment, that can impact interpretations of efficacy. Administering the first immersion at 4–7 days post hatching and the second immersion by three weeks post hatching achieves a reliably high rate of sex reversal and a reduction in the duration of the immersions to 1 h is a significant decrease in time required for the procedure and lessens risk by reducing how long the larvae or fry are maintained in static water.