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

The reproductive biology of bream Ahramis 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 （SLs0） and ages （As0） 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 ofA. 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.

Dynamics of the content of H_(2)O,Na,K,Ca and Mg in the eggs of bream,Abramis brama L.in natural conditions and under stress

Objective:To study the dynamics of H_(2)O,Na,K,Ca and Mg content in the oocytes of bream,Abramis brama observed in prespawning period on breeding ground and in reply to catching,transportation and subsequent remaining of spawners in a cage.Methods:For research on the dynamics of H_(2)O and cations content in the oocytes of bream females,Abramis brama in the course of eggs transition from maturity Stage IV to Stage V,fishes were caught from breeding ground in the coastal zone of the Volga Reach of the Rybinsk reservoir at the Vereteya Station.For studying the influence of stress,the bream(76 individuals)were caught from breeding ground by hauling the seine during 15 min.Capture,sorting and transportation for 3 h to the ponds were the stress factors.Samples of oocytes from 6–8 fishes were taken immediately after capturing,then two more were taken during transportation.Later fishes were removed from the cage in certain time intervals.Concentration of Na and K in the dissolved samples of oocytes was measured by the spectrometer(Flapho-4,Carl Zeiss,Iena,Germany)and content of Ca and Mg was measured by atomic-absorption spectrometer-1(the same producer).Results:In natural conditions before spawning in the course of maturation of oocytes from maturity Stage IV to V,H_(2)O content in the ovicells of bream has increased by 3.3%and concentration of Na,K,Ca and Mg has decreased by 24.9%,38.1%,56.2%and 65.7%,accordingly.Stress caused by capturing,transportation and the subsequent remaining of bream spawners in a cage did not change parameters of water-salt exchange of the oocytes.Conclusions:In natural conditions before spawning,the maturation of oocytes of bream from maturity Stage IV to V take place.Stress caused by capturing,transportation and the subsequent remaining of bream spawners in a cage prevents the transition of eggs from maturity Stage IV to V.It is suggested that in order to develop optimal technique stimulating oocytes maturation and the process of ovulation in industrial conditions,study of these reactions of spawners of one or another species in natural environment is preferentially conducted.Conditions of incubation and combination of hormonal preparations selected on the basis of these data should invoke similar effects as the ones spawners experience in natural habitat.