Changes in plasma levels of reproductive hormones during first sexual maturation in European male sea bass (Dicentrarchus labrax L.) under bartificial day lengths

Photoperiod has been considered one of the most important factors triggerin g puberty, as well as reproduction, in several fish species, including sea bass (Dicentrarchus labrax L.). In the present work, the effect of expanded (EX) and compressed (CO) photoperiods on plasma levels of reproductive hor mones (gonadotropin-2 (GTH-2), testosterone (T) and 11-ketotestosterone (11 -KT)) and gonadal maturation (spermiation time and gonadosomatic index (GSI )) were investigated in male sea bass during the first sexual maturation (O ctober to April). Spermiation in controls was apparent from December to Feb ruary-March. In EX and CO groups, spermiation was advanced by 2 months, alt hough the CO group displayed a bimodal pattern, and testicular growth in bo th experimental groups was significantly reduced with respect to the contro ls. Plasma GTH-2 levels in controls showed the highest value (similar to 30 ng/ml) in the middle of the spermiation period, while EX group displayed t he maximum level 4 months earlier (35 +/- 2.7 ng/ml) than controls. The CO group presented two peaks, the first of which (15.16 +/- 5.20 ng/ml) was ad vanced by 3 months with respect to the control peak. T and 11-KT levels in the control displayed the highest values during the spermiation period. The EX group showed lower T levels than controls, but both peaked at the same time. However, 11-KT levels remained low and unchanged. The CO group displa yed two significant increases in T levels accordingly with the spermiation pattern, while the 11-KT profile only exhibited a significant increase 2 mo nths earlier than in controls. Results obtained indicated an involvement of GTH-2 in gonadal maturation. In addition, T is suggested to be involved in the activation of the brain-pituitary-gonad (BPG) axis during pubertal dev elopment, while 11-KT may act by stimulating spermatogenesis and/or spermia tion in juvenile male sea bass. Furthermore, the profiles of these reproduc tive hormones were altered by both expanded and compressed photoperiods, an d first sexual maturation was advanced by at least 2 months. (C) 2001 Elsev ier Science B.V. All rights reserved.