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Background  Male European seabass, already predominant (~ 70%) in cultured stocks, show a high incidence (20–30%) of precocious sexual maturation under current aquaculture practices, leading to important economic losses for the industry. In view of the known modulation of reproductive development by swimming exercise in other teleost species, we aimed at investigating the effects of sustained swimming on reproductive development in seabass males during the first year of life in order to determine if swimming could potentially reduce precocious sexual maturation.    Methods  Pre-pubertal seabass (3.91 ± 0.22 g of body weight (BW)) were subjected to a 10 week swimming regime at their optimal swimming speed (U opt ) in an oval-shaped Brett-type flume or kept at rest during this period. Using Blazka-type swim tunnels, U opt  was determined three times during the course of the experiment: 0.66 m s − 1  at 19 ± 1 g BW, 10.2 ± 0.2 cm of standard length (SL) (week 1); 0.69 m s − 1  at 38 ± 3 g BW, 12.7 ± 0.3 cm SL (week 5), and also 0.69 m s − 1  at 77 ± 7 g BW, 15.7 ± 0.5 cm SL (week 9). Every 2 weeks, size and gonadal weight were monitored in the exercised ( N  = 15) and non-exercised fish ( N  = 15). After 10 weeks, exercised and non-exercised males were sampled to determine plasma 11-ketotestosterone levels, testicular mRNA expression levels of genes involved in steroidogenesis and gametogenesis by qPCR, as well as the relative abundance of germ cells representing the different spermatogenic stages by histological examination.    Results  Our results indicate that sustained swimming exercise at U opt  delays testicular development in male European seabass as evidenced by decreased gonado-somatic index, slower progression of testicular development and by reduced mRNA expression levels of follicle stimulating hormone receptor  (fshR) , 3-beta-hydroxysteroid dehydrogenase ( 3βhsd) , 11-beta hydroxysteroid dehydrogenase  (11βhsd),  estrogen receptor-beta  (erβ2) , anti-mullerian hormone  (amh),  structural maintenance of chromosomes protein 1B  (smc1β) , inhibin beta A ( inhba)  and gonado-somal derived factor 1  (gsdf1)  in exercised males as compared with the non-exercised males.    Conclusions  Swimming exercise may represent a natural and non-invasive tool to reduce the incidence of sexually precocious males in seabass aquaculture.

Swimming exercise to control precocious maturation in male seabass (Dicentrarchus labrax)