Study On The Biology And Population Dynamics Of Abramis Brama(L.) In The Irtysh River

Bream Abramis brama?L.?belongs to the genus of Abramis,the subfamily of Leuciscinae,the family of Cyprinidae and the order of Cypriniformes.The A.brama's home range is Europe north of the Alps and Pyrenees,as well as the Balkans.It was introduced into Lake Zaysan from Russia in the 1950 s and gradually spreading into Xinjiang along the Irtysh River,and became one of the most important commercial fish of the downstream section of Irtysh River.The biological characteristics and population dynamics of bream A.brama were studied based on the examination of individuals collected from the downstream section of Irtysh River in China from April to October,2013,and May 2014.1.The standard length?SL?of A.brama ranged 102mm³33mm for males,105mm³44mm for females,and 29mm¹95mm for unsexed.Estimated ages ranged 3-13 year for males,3-15 year for females,and 1-8 year for unsexed,respectively.The SL-BW relationship was described as BW=1.556×10^-5 SL^3.092 for the combined sexes.Relationships between otolith dimensions and fish growth were described,and otolith weight?OW?was the only variable which linearly correlated with fish length and age.The mean length-at-age had no significant difference between sexes,whereas the females catch a larger size than males,and the mean SL of females?228.1±50.76mm?were significantly larger than males?213.2±40.78mm?.The von Bertalanffy growth function,which was used to describe A.brama's growth pattern were expressed as Lt = 348.6(1-e^{-0.123?t+0.205?})and Lt = 338.5(1-e^{-0.139?t+0.473?}),respectively based on back-calculated and observed length-at-age.The age inflexion point for A.brama was 9.0 years.2.There were pearl organs on rostral side,orbit vicinity and finrays of matured males in spawning season.Gonad maturity stages of both sexes were described according to macroscopic and histological scales.The overall sex ratio?M/F?was 1.06:1,and not significantly different from theoretical 1:1.A.brama spawn once each year,peak spawning occurred from late May to early June and had a high degree of spawning synchronicity.The mean standard length?SL50?and age?A50?at first maturity for males and females were 177.7 mm and 203.5 mm,and 5.6 year and 6.8 year,respectively.The mean absolute fecundity?AF?was 77311±5461 eggs per fish,and mean relative fecundity?RF?was 162±7 eggs per gram of body weight?BW?.The absolute fecundity increased with standard length?SL?,eviscerated weight?EW?,and gonad weight?GW?,but had nosignificant correlations with ages.3.According to N%,alga was dominated?99.82%?in the diet composition of A.brama with the highest percentage of Bacillariophyta?73.09%?.While base on IRI%?W% and IP%,aquatic insectlarvae?especially Chironomidae larvae?was the most important prey of A.brama.The Amundsen figure indicated that A.brama was a generalized feeder with a bias toward to aquatic insect larvae,and there a difference in food composition between different individuals.The structure of digestive organs was fit to the feeding habits of A.brama.4.Chapman-Robson method produced an estimate of total mortality rate?Z?of 0.65 year-1.The natural mortality rates?M?were estimated as 0.13 year-1 and 0.26 year-1,resulting in corresponding estimates of current fishing mortality rate?Fcur?being 0.52 year-1 and 0.39 year-1,respectively.The uncertainty of growth and maturity parameters had limited impacts on the estimates of BRPs,but had large effect on the YPR and SSB/R outputs.Our findings indicate that A.brama may be at higher risk of growth overfishing than recruitment overfishing.In addition,increasing the tc to 9 years old might ensure that the SPRs were above the management targets under all ranges of F with comparatively smallimpacts on YPR regardless of M.