|Other Abstract||Neosalanx taihuensis Chen is a freshwater resident icefish mainly inhabiting the middle and lower reaches of the Yangtze River, including its tributaries, and is recognized as the most dominant fish in some lakes affiliated to it. Because of its high commercial value and popularity as a food fish, N. taihuensis has been introduced into lakes and reservoirs in over 20 provinces of China, and has become the most commercially exploited icefish in the country. Populations of N. taihuensis in some waters often showed considerable fluctuation, i.e., rapidly expanding in some years and drastically declining in others. Furthermore, icefish introduction has been observed to induce endangerment (and even extinction) of some native fishes, most notably in some Yunnan lakes. N. taihuensis was one of the most abundant fish species in the Tian-e-zhou Oxbow (TEO), but annual commercial catch of N. taihuensis in the oxbow has declined dramatically since 2003. Abundance of this species has significantly increased following impoundment in 2003 and became a major commercial species in the Three-Gorges Reservoir (TGR). In this study, we investigated the annual reproductive cycle of female N. taihuensis in the TEO and in the TGR. We also compared the reproductive characteristics of the spring mature female between 2006 and 2007 in the TEO, and between TEO population and TGR population. The growth of the spring spawning fish larvae were compared between two populations based on the otolith analyses. The main objective was to reveal the life history characteristics of N. taihuensis, and to provide critical information to understand the biological mechanisms underlying population fluctuations. Essential information for population forecasts, helping in the effective management of this fish are also given. The main results and conclusions were listed as following:
1. Both in the TEO and TGR, two separate spawning seasons were identified based on the annual reproductive cycles of the females, designated as the spring spawning season from January to May and the autumn spawning season from September to November. Life span of the offspring of the spring-spawning fish was 1 year, with them dying shortly after the subsequent spring spawning season. Autumn-spawned females seem to be the offspring of the spring-spawning fish, based on monthly changes in the length-frequency distributions, but this needs to be confirmed with otolith analysis. Fate of the offspring of the autumn spawning fish also needs further investigation.
2. Reproductive characteristics of N. taihuensis varied significantly between the spring spawning stock and the autumn spawning stock in the TGR and also in the TEO. Larger body size, higher fecundity and higher GSI of the spring spawners indicated higher individual reproductive investment of the spring spawning stock than the autumn spawning stock. With larger egg size, early survival rate of the offspring of the spring spawning stock may also be higher than the autumn spawning stock. Thus, the individual contribution of the spring spawning stock to the recruitment of the population should be higher than the autumn spawning stock. Also, with higher abundance of the spring spawning stock when compared to the autumn spawning stock, we suggest the spring spawning stock as the major contributor to the recruitment of the N. taihuensis population in the TGR and in the TEO, and monitoring and fisheries management on N. taihuensis in the TGR and in the TEO should mainly consider the spring spawning stock.
3. The reproductive characteristics of mature spring female varied significantly between 2006 and 2007 in the TEO. They had higher values for absolute fecundity, gonadosomatic index, and relative condition factor in 2007 than in 2006. Relative condition factor of the immature female offspring of spring-spawning fish was also higher in 2007 than in 2006, portending a further increase in reproductive investment during the spring spawning of 2008. The increasing reproductive investment suggests that the population of N. taihuensis in the Tian-e-zhou Oxbow is recovering from its recent decline.
4. Reproductive characteristics of the spring spawning stock of N. taihuensis varied significantly between the populations in the TGR and in the TEO. Larger body size, higher condition, higher fecundity and larger oocyte diameter of the spawning stock in the TGR indicated faster individual growth and higher reproductive investment of the TGR population than the TEO population. With higher population abundance associated with higher reproductive investment of N. taihuensis in the TGR than in the TEO population, we suggest that reproductive investment is an important factor regulating resource fluctuation of N. taihuensis populations. The impacts of increasing N. taihuensis population on other fish species and on the fish community succession in the TGR needs to be further investigated.
5. The sagittal otoliths of larval fishes were analysed to understand early growth characteristics of N. taihuensis in the TGR and in the TEO. The primordium and nucleus were round in shape. Fourty clear increments were evident around the nucleus but there was no deposition at the otolith margin. Standard length and otolith radius were linearly correlated. The nuclear radius of N. taihuensis in the TGR was significantly larger than in the TEO. Having a larger oocyte diameter, and larger nuclear radius, the early survival rate of the offspring of the spring spawning stock in the TGR may also be higher than in the TEO. Starting from nineteenth to thirty-fifth increment, the larval population in the TGR was significantly more abundant than in the TEO. The results indicated that the larval growth rates of N. taihuensis in the TGR were higher than in the TEO.|