Spawning migrations, sexual maturity and sex steroid levels in female roach Rutilus rutilus friom the River Meuse

Relationships between sexual maturity, steroid hormone levels and spawning migrations of female roach Rutilus rutilus were investigated in the River Meuse (Belgium). Levels of plasma testosterone (T), estradiol-17 β (E2) and 17,20 β-dihydroxy-4-pregnen-3-one (17,20 βP) were measured in three groups of fish: the roach swimming up the fish pass, those entering the backwaters and those caught in the main channel. Gonadosomatic index (GSI) and relation between stages of maturity and hormone concentrations were also determined. The migration can be divided into a prespawning migration from early February to late March limited to an intense moving of roach to the backwaters and a spawning migration in April and May. In the former period, all fish in exogenous vitellogenesis presented high levels of E2 (1.30 ± 0.64 ng/ml early March) and low levels of other steroid hormones. During the spawning period, roach migrating to the backwaters had higher GSI (18 % in the females caught in mid-May) than those migrating upstream through the fish pass (about 15 % in April). High levels of T (maximum 2.71 ng/ml and 1.06 ng/ml, in backwater and fish pass samples respectively) and 17,20 βP (maximum 19.2 ng/ml and 12.7 ng/ml in backwater and fish pass samples, respectively) were measured during the spawning season. However, T and E2 levels (up to 5.5 ng/ml for both hormones) were significantly higher in fish caught in the main channel of the river at about 2 kms from the migration sites (fish pass and backwater inlet).