1 Introduction
Coilia nasus, also called Japanese grenadier anchovy, are widely distributed in the northwest Pacific, including the Yellow Sea and East Sea as well as penetrating over 1400 km (middle reaches) up the Yangtze River. Matured fish make productive migration in succession from near ocean to river when reproductive season comes. Guo et al. (2014) found the rise of water temperature off Yangtze River Estuary may be an important environmental factor, which can induce C. nasus to start migration．They also found the peak fishing period usually started from late March to middle April with the high yield in each year based on the catch analysis. The junction of the Poyang Lake with the Yangtze River was at Hukou. Poyang Lake is China's largest seasonal fresh water lake, is rich in fishing resources. But the shallow depth of the lake, the short water cycle, and water area of high water period, low water period, normal water period is large differences, causing this system stability and anti- amming capability poor, fish resource security risk. With increasing commercial demand for its delicacy, C. nasus has become an important fishery resource of Poyang Lake in recent years. Especially in the background of actively promote the Poyang Lake water conservancy project construction, high perfor-mance scientific assessment of fish resources will provide strong support for the ecological environment protection in Poyang Lake.

Through the consult literature and identify, to 2014, the accumulated number of 134 fish species was recorded in Poyang lake, belonging to 26 families, 12 orders. The largest of families was the Cyprinidae (71 species), contributing 53.0% to the total. The mainly fish in Poyang lake showed a trend of low age miniaturization, and low-quality, one-year old fishes. According to the main fishery resources exploitation degree, Poyang lake natural fishery now was in the over exploitation period. The function of its natural fishery was recession. C. nasus is a kind of mid small-sized fish with resident and migration ecotypes. The anadromous population has long been recognized as very important commercial aquatic products in the lower reaches of the Yangtze River．However, habitat loss, overfishing, and the C. nasus have diminished. Considering the importance of demography and population regulation to the theory of sustainable exploitation (Freckleton et al., 2003), it is surprising how little we know about the population dynamics of C. nasus in Poyang Lake. C. nasus play a major role in food webs and community structure. They constitute an important component of the freshwater ecosystems in Poyang Lake.

The objectives of this study were (1) to assess four important parameters namely growth, natural and fishing mortality and recruitment of C. nasus; and (2) to compare our results with those obtained in other studies carried out on C. nasus, in order to observe possible differences effectuated by geographic parameters.

2 Materials and methods
The study was carried out at the Poyang Lake through the Yangtze River Waterway (116.11-116.20 N and 29.51-29.74 E) for four month, from April to July( C. nasus Only can be fished in reproductive season in the Poyang Lake through the Yangtze River Waterway, and our investigation including the whole breeding season ) three years in a row, 2012-2014. Monthly random samples of C. nasus were caught using trammel net and barrier net. The annual average temperature is 16-18℃ with 240-330 frost-free days. Precipitation (effectively: rainfall) is around 1340-1780 mm (Leeuw, 2010). This lake is shallow (maximum depth: 21 m) and is connected to five main rivers, namely the rivers Ganjiang, Fuhe, Raohe, Xinjiang, and Xiushui. Waters from these rivers run through Poyang Lake and discharge in the Yangtze River through a narrow passage at Hukou. In addition to the excellent geographical conditions, clear water and abundant aquatic plants also provide a good habitat for C. nasus survival. Accordingly, Poyang Lake is seen as one of the major sites suitable for C. nasus reproduction in China.

The body length (BL, from the tip of the rostrum to the end of the tail section) measured along the dorsal mid-line to the nearest 0.1 cm. The length frequency data of C. nasus analyzed using FiSAT II (FAO- CLARM Stock Assessment Tools-Version 1.2.2). The FiSAT routines were followed thoroughly base on the user’s manual (Gayanilo et al., 1996) and reference manual (Gayanilo and Pauly, 1997). Bhattacharya’s method (BM), implemented from the package FiSAT II (Gayanilo et al., 1996), was used to identify the modes in the polymodal body length-requency distributions of the C. nasus, and to simulate the Von Bertalanffy equation: Lt = L∞* [1 − exp(−K*(t − t 0 ))] to calculate the asymptotic length L∞ and the growth parameter K, where, Lt is the length at age t, L∞ is gth), K is the curvature parameter, and t0 is the initial condition parameter. K and L∞ were calculated using the program Electronic Length Frequency Analysis (ELEFAN). his method is based on the assumption that the observed distribution in size classes results from the overlap of various normal distributions. The process converts normal distributions into lines that simplify the procedure, linearization being performed by computing the natural logarithms of frequencies. Intercepts and slopes of the regression lines were used to estimate the parameters of each normal distribution. Given a distribution in size classes, Bhattacharya’s (1967) method allows for the iterative computation of regression lines up to the total decomposition of the overall size-frequency distribution.

3 Results and discussion
A total of 576 C. nasus were used in this study. The length frequency data of C. nasus was analyzed in FiSAT software using various methods to estimate the growth, mortality and recruitment. For many fishery resources, growth parameters (L∞ and K) have been estimated because these population parameters are important to describe the species and inputs in several fishery production models (Hilborn and Walters, 1992). L∞ is the largest theoretical mean length that a species could attain in its habitat whereas k is the speed it grows towards their final size.

3.1 Bhattacharya’s plot
By using the Bhattacharya’s method in FiSAT, C. nasus produced one group or cohort at modal length 25.00 ± 1.32 cm.

3.2 Growth parameters
A total of 576 were examined and their length-masst relationships were computed as body mass = 0.002× body length^3.192(R=0.958；p＜0.05；n= 576), are presented in fig.1. The parameters of the Von Bertalanffy growth equation (VBGF) L∞ and K were estimated by running the program ELEFAN included in the FiSAT package. The monthly length-frequency distributions fitted with growth curves by the program ELEFAN of FiSAT II, are presented in fig. 2. This routine gave the L∞ = 40.95 cm and k = 0.25 year^-1. This value found to be the best combination of K and L∞ with the Rn at 0.532. This value further used to obtain the graph of von Bertalanffy Growth Function (VBGF). The VBGF of C. nasus illustrated in Fig. 2 indicated that the origin of the growth curve starting in May for the group of C. nasus. On annual basis, the growth of C. nasus was described by the following Von Bertalanffy growth equations: L= 40.95 ( 1 - e^{- 0.250( t +1.025)})；W = 280.12( 1 - e^{- 0.250( t +1.025)})³.

Pauly and Munro (1984) have indicated a method to compare the growth performance of various fish stocks was by computing Growth performance index(Ø′)= log K + 2log L∞. Generally, Growth performance index(Ø′) are species specific parameters, means that their values are usually similar within related taxa and have narrow normal distributions. We found Ø′=2.62 for C. nasus. Sparre and Venema (1992) stated that the value of K= 1.0 is fast growth, K= 0.5 is medium growth and k= 0.2 is slow growth. Hence, k=0.25, for C. nasus obtained from this study considered as an slow growth.

3.3 Mortality coefficients
Mortality means the death of fish from the stock due to fishing mortality or natural mortality includes predation, disease and old age. Fishing mortality assumed to be associated with physical injury or physiological stress from being captured in the gear used during capture. Natural mortality (M) and fishing mortality (F) were additive instantaneous rates that sum up to total mortality (Z). The total mortality coefficient, Z= M + F (Gulland, 1971). When comparing mortality rates to the total births or recruits to the population, we can determine if a population is increasing or decreasing.

The length-converted catch curve was used to determine the value of natural mortality (M), fishing mortality (F) and exploitation rate (E). The Z, M and F of C. nasus were estimated as 1.58 year^-1, 0.54 year^-1 and 1.04 year^-1, respectively. C. nasus in the Poyang Lake through the Yangtze River Waterway showed high mortality rates which related to fishing mortality and natural mortality. The exploitation rate estimated to be 0.66 year^-1. This value higher than 0.5 year^-1, indicated an overexploitation of C. nasus in the Poyang Lake through the Yangtze River Waterway. Estimating natural mortality (M) is one of the most difficult and critical elements of a stock assessment(Hewitt et al. 1985). The Pauly’s Model by using growth parameters is an indirect way of estimating natural mortality. It assumes that there is a relationship between size and natural mortality. Pauly’s original method was based on the correlation of M with von Bertalanffy growth parameters (K and L∞) and temperature.
 
3.4 Length at first capture (Lc)
The length at first capture, Lc of C. nasus was estimated at 22.84 cm (Fig. 3). The Lc was the length at which 50% of the fish are vulnerable to be captured by fishermen. This is the average size of fish vulnerable to be fishing or enter the fishing ground, in the Poyang Lake through the Yangtze River Waterway.

3.5 Recruitment pattern
The recruitment patterns of C. nasus suggested that there was one main pulse of annual recruitment, in agreement with the group separation using Bh -tacharya’s Plot. The major ulse appeared in July (Fig. 4), Compared with historical data, the miniaturization of the fish was evident and the community age structure tended to be one fold and low-aged, at the same time, the resource of C. nasus had decreased significantly.

3.6 Relative yield-per recruit (Y′/R) and relative biomass-per recruit (B′ /R)
The plots of relative yield-per recruit against exploitation rate (E) fig. 6) indicated that the present exploitation rates (E_present) < the maximum exploitation rate(E_max) that could be applied for a sustainable exploitation of fisheries. For C. nasus, the plots of relative Biomass-per recruit against exploitation rate associated with the relative yield-per recruit against exploitation rate plots showed that E_present > E_0.5 (the exploitation rate at which 50% of biomass of the recruit is fished), indicating that, at the current rate of exploitation, there was the threat of over fishing as >50% of biomass-per recruit was fished.

4 Conclusion
Present study discovered that C. nasus composed of one cohort in a year with supported by the recruitment patterns of one main pulses annually. The growth parameters of asymptotic length 40.95 cm, growth coefficient 0.25 year^-1 and growth performance index 2.62. C. nasus from study area had high total mortality 1.58 year^-1 and fishing mortality 1.04 year^-1. The exploitation rates 0.66 year^-1 indicated over exploitation. Thus, all of this information would be the valuable sources for comparison in future, especially when the conservation and management of this fish stock is to be made.

Author's contributions
Wu Bin, Fang Chunlin, and Fu Huiyun designed and conducted the experiments. Fu Peifeng, Zhou Huiming, Zhang Yanping, He Gang, Wang Sheng collected the samples and did the meaurement. All authors read and approved the final manuscript.

Acknowledgements
The authors wish to thank Hubei Environmental Monitoring Central Station, Zhang Yuan, for the creation
of a sampling map to conduct this study.

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