Reproductive biology of bream Abramis brama (L.) in the lower reaches of the Irtysh River, China

The reproductive biology of bream Abramis brama (L.) was studied from 546 fish collected from the lower reaches of the Irtysh River in Xinjiang, north-west China, from March to November 2013. The overall sex ratio (M/F) was 1.06:1, and was not significantly different from the theoretical 1:1 ratio (P>0.5). However, there was a dominance of males during the pre-spawning season (1.33 in March and 1.56 in April, P<0.5), while females were dominant during the peak spawning season (0.88 in June, P<0.5). The monthly variation in gonadosomatic index (GSI) and proportion of gonads at each macroscopic maturity stage, indicated that A. brama spawned once a year with peak spawning occurring from late May to June. The unimodal distribution of oocyte diameter each month indicated that A. brama is a single spawner, with a high degree of spawning synchronicity. The standard lengths (SL50) and ages (A50) at first maturity for males and females, were 178 and 204 mm, and 5.6 and 6.8 years, respectively. The mean absolute fecundity (AF) was 77 311 eggs per fish, and mean relative fecundity (RF) was 162 eggs per gram of body weight (BW). The AF of A. brama increased linearly with increasing of gonad weight (GW), eviscerated weight (EW) and standard length (SL), but was not significantly correlated with age. As, A. brama in the lower reaches of the Irtysh River reaches reproductive maturity relatively late in their life span, is mature for a short period and spawns in aggregations, this fish is vulnerable to overexploitation.