STRATEGIES FOR FISH DISEASE CONTROL AND PREVENTION
Location: Harry K. Dupree Stuttgart National Aquaculture Research Center
Title: Safety of feed treated with 17a-methyltestosterone (17MT) to larval Nile tilapia
Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 7, 2012
Publication Date: March 5, 2013
Citation: Straus, D.L., Bowker, J.D., Bowman, M.P., Carty, D., Mitchell, A.J., Farmer, B.D., Ledbetter, C.K. 2013. Safety of feed treated with 17a-methyltestosterone (17MT) to larval Nile tilapia. North American Journal of Aquaculture. 75:212-219.
Interpretive Summary: A synthetic steroid called 17 a-methyltestosterone (17MT) is frequently used to produce male (>90%) populations of tilapia. It is introduced in the diet for the first 28 days of life. The all-male population allows for greater feed conversion and larger, more marketable fish. We designed a study to test the safety of 17MT when included in the feed to tilapia at the recommended dose (9 mg 17MT/kg body weight/day), and at 3 and 5 times this dose. Fish fed aggressively regardless of the dose of 17MT in the feed, behavior was considered normal with no dose-related differences detected and no mortality was observed in fish that were treated with 3 times the recommended dose. We used a microscope to look for tissue changes in different organs of the fish including the gill, liver, kidney, brain, heart, muscle, skin, spleen, and intestine. Fish that were treated with 5 times the recommended dose had significant tissue damage, especially in the heart. We concluded that 17MT-medicated feed is safe for tilapia up to 3 times the recommended dose.
As a synthetic androgen, 17 a-methyltestosterone (17MT) is frequently used to redirect the course of sex differentiation by exposing the undifferentiated gonad at sufficient dosage. This hormone has been widely accepted as a safe and effective treatment for sex-reversal in many fish species, and it is administered to larval tilapia (3 – 12 days old) for approximately 28 consecutive days to produce populations of >90% males. This study assessed the safety of 17MT-treated feed when administered to larval Nile tilapia Oreochromis nilotica at 1-, 3- and 5-times the proposed dosage of 9 mg 17MT kg fish d-1 for 28 consecutive days. Despite elevated ammonia levels measured during the last 5 d of the study, environmental conditions were acceptable for rearing tilapia. Fish fed aggressively regardless of the concentration of 17MT in the feed, behavior was considered normal with no dose-related differences detected and no mortality was observed in the 3 x group. Fish that were treated with 5-times the proposed therapeutic dosage had significant pathological changes. Based on the results of this study, the 17MT margin of safety extends to at least 3 x (27 mg 17MT kg fish-1 d-1) the proposed dosage of 9 mg 17MT kg fish-1 d-1when administered in feed for 28 d to tilapia.