Comparative study of reproductive biology in single and multiple‐spawner cyprinid fish. II. Sex steroid and plasma protein phosphorus concentrations

Abstract

The dynamics and regulation of oogenesis in single and multiple‐spawner cyprinid fish showing group‐synchronous oocyte development, was investigated in three species from the River Meuse (Belgium): the roach Rutilus rutilus as a single spawner, and the bleak Alburnus alburnus and the white bream Blicca bjoerkna as multiple spawners. This paper compares the seasonal profiles of sex steroids (oestradiol‐17β, testosterone and 17,20β‐dihydroxy‐4‐pregnen‐3‐one) and plasma alkali‐labile protein phosphorus. Different patterns of plasma oestradiol‐17β (E2) and plasma protein phosphorus (PPP) have been observed not only between the single and the multiple spawner fish, but also among the two multiple spawners. In roach, two increases of E2 levels were observed. The first occurred in September after a short gonadal quiescent period, and coincided with the increase of the PPP at the onset of exogenous vitellogenesis. The second took place in spring before the spawning season. The low PPP recorded during that period probably reflected its rapid incorporation by the oocytes. In both multiple spawners, highest values of PPP were recorded just before the spawning season. In white bream, the PPP declined progressively once the differentiation of exogenous vitellogenic oocytes was completed before the onset of the spawning season. In bleak, PPP levels remained high throughout the spawning season and corresponded to a sustained oocyte recruitment during the whole of this period. Regardless of the pattern of oocyte growth recruitment, the E2 concentrations were high in both multiple spawner species during the breeding season. In the three species, testosterone levels remained low regardless of the maturation stage (ranging from 0.6 to 13.4ng ml−1). Except for relatively high concentrations of 17,20βP in roach during final maturation and postspawning stages (20 and 28 ng ml−1, respectively), low levels of this steroid were measured in these cyprinids, and especially in the multiple spawner fish. The role of this progestogen as the maturation inducing Steroid is discussed.