Age, Growth And Reproductive Biology Of Invasive Fish Hemiculter Leucisculus (Basilewski,1855) In Erhai Lake

Hemiculter leucisculus (Basilewski,1855) belongs to genus Hemiculter, subfamily Cultrinae, family Cyprinidae, order Cypriniformes. The native range of H. leucisculus is East Asia:from Far East Russia and Mongolia in the north, through to China (including Hong Kong and Taiwan) and West Korea, to North Vietnam in the south. With worldwide introduction of aquaculture Chinese carp, it had invaded into many other countries, include Iran, Iraq, Kazakhstan, Afghanistan, Uzbekistan, Turkmenistan, Japan and became the dominant species in some waterbodies such as in Iran. H. leucisculus was first found in2004and now becomes the dominant species and one of the important commercial fishing species of Erhai Lake. This research will analyze how H. leucisculus adapted to new habitat and became the dominant species of Erhai Lake in such a short time based on its characteristics of age, growth, mortality and reproduction. The main results are as follows:The age, growth and reproduction of invasive H. leucisculus in the Erhai Lake were studied in4539collected during Jul.2009to Jun.2011. The standard length ranged4.3-19.1cm for females and4.6-12.3cm for males. The length-weight relationships of female and male were significantly different and were described as W=0.0076SL3.1901, and W=0.0084SL3.1901, respectively. It was demonstrated that otolith was a high-quality material for age determination because a single annulus was formed per year on otolith based on marginal increment analysis and the total mean CV for age estimate between two readings was3.55%. The age structure was simple, for male97.58%was1to2years old with maximum of3years old and93.14%was1to3years old for female with maximum of6years old. females had a much greater length than males in all age groups, the von Bertalanffy growth curves by the observed length-at-age data were expressed as Lt=25.6(1-e-0.176(t+1.347)) for females and Lt=16.4(1-e-0.354(t+0.819) for males. The parameters of growth evaluated from otolith and length-frequency analysis were nearly identical for females, whereas males were significant difference. The instantaneous rates of mortality were much higher for males (4.22year-1) than females (1.17year-1).The present study describes the reproductive strategy of H. leucisculus, based on monthly samples in Erhai Lake, from July2009to June2011. H. leucisculus is a multiple spawner with indeterminate fecundity by oocyte size-frequency distributions and gonadal histological sections analysis of female gonads. The estimated sizes at first sexual maturity (L50) were5.5cm and5.6cm standard length for males and females, respectively, which is much smaller than its native ecosystems. The observed minimum length at sexual maturity was5.1cm standard length for males and4.6cm for females. The spawning period for the population extends from April to September by monthly changes of gonadosomatic indices and gonad stages analysis for male and female. When the spawning time is over, the gonad returned into stage Ⅱ, and maintained this stage till to March. Gonads began to develop and mature with the temperature increases in April. The female-male ratio was significant difference and females were predominant in the peaking spawning period (June to August), but in the other spawning period (April、May and September), there was no significant difference. Females dominated in sizes larger than10cm, all the specimens larger than12.3cm were females.Previous fecundity estimates for H. leucisculus assumed a determinate spawning pattern. But H. leucisculus are multiple spawners with indeterminate fecundity by macro-(oocyte size-frequency distributions) and microscopic (gonadal histological sections) analysis of female gonads. So the previous approach of fecundity estimation based on the standing stock of yolked oocytes could underestimate fecundity in fishes that actually unyolked oocytes continue to mature and to be spawned during the reproductive season for the multiple spawners. In this study, the hydrated oocytes method was used to assess batch fecundity, and the average daily spawning fraction of mature females showing postovulatory follicles (POF) was used to estimate spawning frequency (batch interval). Histological sections of gonads were used to identify the hydrated oocytes and the postovulatory follicles. Spawning frequency, determined from the percent of females with postovulatory follicles12-36h old during reproductive period, was16.05%; which means that the average interval between spawning was6.2days during the entire sampling period (27May to8Aug) and total spawning batches was no less than16during whole reproductive period. Some females showed histological evidence of spawning on two consecutive days. The average batch fecundity (with±SD) in40females (standard length:9.1～14.4cm) estimated based on hydrated oocytes was11934±5921oocytes per female, and the relative batch fecundity was560±137eggs per gram wet weight of female. Consequently, the total potential annual fecundity was about190944oocytes over the whole spawning season, which is much higher than that estimated from the standing stock of yolked oocytes of36females (body length:9.5～16.2cm) at the beginning of the spawning season in April (31585oocytes).By comprehensive understanding of the age, growth, mortality and reproduction of H. leucisculus, we think two reasons could be attributed to its rapid establishment and growth of opulation in Erhai Lake. One is its early maturation with rapid development and short generation time. The other is its high fecundity with batch spawning pattern and extended spawning season.