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

Authors: Weber, Gregory - Greg; Leeds, Timothy - Tim

Submitted to: Aquaculture Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/13/2022
Publication Date: 1/20/2022
Citation: Weber, G.M., Leeds, T.D. 2022. Effects of duration and timing of immersion in 17alpha-methyltestosterone on sex reversal of female rainbow trout. Aquaculture Reports. 23: 101014. https://doi.org/10.1016/j.aqrep.2022.101014.
DOI: https://doi.org/10.1016/j.aqrep.2022.101014

Interpretive Summary: Juvenile female rainbow trout can be masculinized by exposure to a steroid (17alpha-methyltestosterone). Use of these fish as sires to produce all-female progeny is beneficial for rainbow trout aquaculture. The traditional method of masculinization involves adding steroid to the feed and is very effective, but drawbacks include containment of unused steroid and most fish must be euthanized to collect sperm from their testes. We previously developed and reported an immersion-based approach that was equally as effective, and has additional benefits of being able to fully contain unused steroid and produces a greater proportion of males from which sperm can be collected non-lethally. The current report describes a series of studies in which the immersion-based protocol was further refined. The data show that the total time that juvenile female rainbow trout are exposed to the steroid can be reduced by approximately 40% without affecting efficacy when tested in a large-scale breeding program, and suggest similar results can be obtained if the exposure time is reduced by up to 70%. The reduction in exposure time is advantageous because it presents less risk to animal welfare and results in a significant reduction in labor requirements.

Technical Abstract: Sex reversal of female rainbow trout (Oncorhynchus mykiss) by oral administration of 17alpha-methyltestosterone (17MT) is well established and shown to consistently result in nearly 100% of the treated animals being XX males. Drawbacks to this protocol include the difficulty in preventing the steroid from reaching the environment when used as a feed additive and a high proportion of males fail to develop fully functional sperm ducts and therefore the males must be euthanized to harvest sperm from the testes. Recent experiments demonstrate immersion of female rainbow trout in 17MT at 400 ug/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 with about half the fish forming functional sperm ducts. The immersion protocol should increase efficiency of hatchery operations by reducing harvesting of immature males and allowing reuse of functional males, as well as simplify preventing the steroid from reaching the environment. The 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% of treated fish able to release milt when pressure was applied to the abdomen. The effect of reducing the duration of the immersions and the timing of the second immersion was evaluated. Immersion for 1 h was as effective as 2 h, and there was a small but significant decrease in effectiveness with 0.5 h. The effectiveness of the protocol is less consistent if the timing of the second immersion or the weekly immersions start later than three weeks 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. The reduction in the duration of the immersions to 1 h is a significant reduction in time required for the procedure and reduces risk by reducing the time the larvae or fry are maintained in static water. In addition, administering the second immersion at three weeks post hatching yields consistently high efficacy.