EVALUATION OF FECUNDITY OF ERIOCHEIR SINENSIS PARENT CRABS RELEASED INTO THE YANGTZE RIVER ESTUARY

FENG Guang-Peng ， ZHANG Hang-Li ， WANG Hai-Hua， ， XU Jing-Jing ， JI Hui and ZHUANG Ping

(1. East China Sea Fisheries Research Institute， Chinese Academy of Fishery Sciences， Shanghai 200090， China; 2. College of Fisheries and Life Science， Shanghai Ocean University， Shanghai 201306， China; 3. Jiangxi Institute for Fisheries Sciences，Poyang Lake Fisheries Research Centre of Jiangxi Province， Nanchang 330039， China)

Abstract: During the reproductive migration of Chinese mitten crab in the Yangtze River estuary， the relationship between fecundity and carapace width of released and wild Chinese mitten crab populations was analyzed， and then the differences between the wild and released populations were compared. The results showed that， with the increase of carapace width (CW)， the fecundity (F) of released and wild population crabs increased significantly (P＜0.05). In the same range of carapace width， there was no significant differences of the fecundity between the wild and released populations (P＞0.05). Regression analysis showed that the relationship between the F and CW of the released population belongs to exponential function， which is F=3.979CW6.208 (R2=0.822). In addition， the relationship between F and CW of the wild population is F=1.696CW6.636 (R2=0.673). Covariance analysis showed that the curves of the wild and released crab populations fit well in significant 0.05. It could be concluded that the fecundity of the wild and released populations was at the same level. There was no significant difference between the wild and released crab population on the relationship of F and CW. The released population could reproduce well in the natural water of the Yangtze River estuary.

Key words: Yangtze River estuary; Eriocheir sinensis; Wild population crab; Released population crab;Fecundity

Eriocheir sinensis， commonly known as river crab， is an important crustacean based on its delicious and fresh taste and high economic value. It is distributed in China's Bohai Sea， Yellow Sea， and the east coast of several provinces. High-quality germplasm resources ofE. sinensishave been found in the Yangtze River estuary， where the largest spawning grounds are located[1，2]. Due to excessive fishing and the construction of hydraulic projects in the Yangtze River estuary， the yield ofE. sinensishas significantly decreased[3]. To protect and restore the stock ofE. sinensis， breeding and released activities have been sporadically conducted since the 1970s by relevant departments[4]. Large-scale parent crab stock released has been performed since 2004 in the Yangtze River estuary.

Crab reproduction biology research can assess crab resources and the outcome of breeding and released. Fecundity (F) is the inherent ability of crabs that is determined by various traits associated with its reproduction， including carapace length (CL)， carapace width (CW)， body weight (W)， andF[5]. Therefore， the accurate evaluation of its reproductive capacity requires comprehensive evaluation of multiple indicators ofF， instead of a single indicator[6，7]. Studies on crab breeding biology have mainly focused onF. Foreign scholars have performed some studies on crab spawning， including theFofParaalithodescamtschaticusin Alaska[8，9]and the effects of food on the number， quality， and size of eggs[10]. Research on crabFin China have mainly focused onScylla serrata Forskal， including the effects of temperature and salinity[11]， as well as the effects of different sources of lipids onFofE. sinensis[12]. While the comparative study on theFof the released population and the wild population ofE. sinensisin the Yangtze River estuary was not be studied. This paper studied theFof two populations in the Yangtze River estuary in order to evaluate the reproductive ability of the released parent crabs and to provide basis for evaluating breeding and released efficiencies.

1 Materials and Methods

1.1 Source of the experimental crab

Berried crabs were obtained from monitoring ships and divided into the released and wild groups.The released group was labeled with chelate cuffs，which can be distinguished from the wild populations.In addition significant morphological differences were observed between the released and wild populations. All released berried crabs obtained from the different sections of the Yangtze River estuary from 29 December， 2011 to 22 May， 2012. And they were released on 10 December， 2011 at Shanghai Bao Yang Port (31°26.450′ N， 121°28.870′ E). The wild berried crabs were obtained near the Xinmin Port (31°14.124′ N，121°59.766′ E) of Hengsha Island from 25 November，2011 to 7 March， 2012.

1.2 Determination of the fecundity parameters of berried crabs

The experimental berried crabs were transported to the laboratory and theirCL，CW， andWwere measured. The number of eggs per brood was measured based on weight as described by Hamasaki[13]using an electronic balance with an accuracy of 0.0001 g.Measurements were performed in triplicate for each crab， and the mean value was adopted in the analysis.

1.3 Calculation of fecundity

Fecundity includes absoluteFwhich is the number of eggs per brood of an individual， and the relativeF， which is the ratio of absoluteFto body weight(F/W). Research previously indicated that there is no significant difference in the number of eggs per brood between crabs from various seasons[13]. In the present study， only the number of eggs produced at the first spawning was analyzed. Analyzing all the data of the number of eggs per brood of berried crabs helps to identify of the two populations， as well as to compareFof the released and the wild group of theE. sinensisin the Yangtze River estuary. The berried crabs ofE. sinensiswere divided into four groups according to theCW: 4.6—5.5 cm， 5.6—6.0 cm， 6.1—6.5 cm， and 6.6—7.3 cm.

1.4 Statistical analysis

The results were expressed as the mean±standard deviation (mean±SD). Significance analysis was performed with Excel (Ver. 2007) and SPSS (Ver.17.0). The significance test of the mean value was performed using the one-way ANOVA， and the significant difference is set as 0.05. Covariance analysis was used to test the regression equation of fecundity andCWof the released and the wild populations[14].

2 Results

2.1 The fecundity of the released and wild population

For the released population， the averageCWandWwere (5.59±0.57) cm and (80.96±25.32) g， respectively， and the average absoluteFwas (206508±10616)pcs (Tab. 1). TheCWandCLof the smallest berried crab were 4.6 cm and 4.0 cm， respectively， with the correspondingWof 41.9 g and number of eggs per brood of 78900 pcs. In addition， theCWandCLfor the largest berried crabs were 6.5 cm and 5.7 cm， respectively， with the correspondingWof 113.9 g and number of eggs per brood of 489100 pcs. For the wild population，CWandWwere (5.85±0.56) cm and(91.60±31.89) g， respectively， with an absoluteFof(263995±23677) pcs. TheCWandCLof the minimum berried crab were 4.9 cm and 4.2 cm， respectively， with the correspondingWof 44.9 g and number of eggs per brood of 41800 pcs. TheCWandCLof maximum berried crabs were 7.3 cm and 6.6 cm，respectively， with the correspondingWof 178.5 g and number of eggs per brood of 1037200 pcs. A significant difference in absoluteFwas observed between the released and the wild populations (P＜0.05). However，for relativeF， no significant difference was observed between the two populations (P＞0.05).

Tab. 1 Morphological parameters and fecundity of Chinese mitten crab (Eriocheir sinensis)

2.2 The relationship between F and biological indicators of the released populations

TheFof berried crabs increased with theCWfor released populations (Fig. 1). Due to the limited population size of the released crabs， no significant advantage inCWwas observed for different size groups.The number of eggs ranged from 50000 pcs to 500000 pcs for released crabs. The relationship between theFand theCWof the released population ofE. sinensisin the Yangtze River estuary can be presented using the exponential function:F=3.979CW 6.208(R2=0.822).

2.3 The relationship between F and biological indicators of the wild populations

The statistical data ofFandCWof the wild population are shown in Fig. 2. TheFof berried crabs increased withCW. The advantageousCWof the berried crabs was 5.0—6.0 cm， andFranged from 100000 pcs to 400000 pcs. The relationship betweenFandCWof the wild population ofE. sinensisin the Yangtze River estuary can be presented as an exponential function:F=1.696CW6.636(R2=0.673).

2.4 Comparison of F between the released and wild populations

The absoluteFof the wild population ofE. sinensisin the Yangtze River estuary was significantly higher than that of the released population (P＜0.05)，whereas no significant difference was observed for the relativeF(P＞0.05) (Tab. 1).

TheFof both the released and the wild populations ofE. sinensisin the Yangtze River estuary increased withCW(Tab. 2). A significant difference inFwas observed among crabs of different sizes from the released population (P＜0.05). TheFof the wild population withCWranging 5.6—6.0 cm was slightly higher than that of crabs withCWwithin the range of 4.6—5.5 cm (P＞0.05)， and theFwas significantly different among crabs of the other different sizes(P＜0.05). For crabs withCWs within the range of 5.6—6.0 cm， theFof the released population was higher than the wild population; the opposite trend was also observed among crabs of the other CWs， yet the difference was not statistically significant (P＞0.05).

Fig. 1 The relationship between carapace width and fecundity of the released population of Eriocheir sinensis

Fig. 2 The relationship between carapace width and fecundity of the wild population of Eriocheir sinensis

The logarithm of theF-CWcurves of the released and the wild populations can be expressed as follows: lnF=a+blnCW. The two curves are analyzed by fitting with goodness variance analysis and regression significance test (Tab. 3):F1＜F0.05=161，F2＞F0.05=18.51. Therefore， it can be seen that the regression equation between the two curves showing the relationship betweenFand theCWwas significant and can be well fitted.

3 Discussion

3.1 F of E. sinensis in the Yangtze River estuary

Tab. 2 The comparison of fecundity between released and wild populations of Eriocheir sinensis

The present study determined that a small wild population of berriedE. sinensisstill possesses mature ovaries. Berried crabs are likely to undergo a second ovigerous stage after hatching. Therefore， theFofE. sinensisis generally greater than what can be measured at the first spawning[15]. For a given year，theFofE. sinensisdepends on the eggs produced during both the first and second spawning.

A positive correlation betweenFandCWwas observed inE. sinensis， which is similar to that in Portunidae crabs， includingOvalipes punctatus[16，17]andArenaeus cribrarius[18]. Furthermore， the individualFwas lower when compared withO. punctatusof the correspondingCW[17]， whereas it was higher thanE. hepuensis， which has the closest relationship withE. sinensis[19]. Because the size ofScylla serratawas significantly larger thanE. sinensis， the latter species has significantly lowerFthan the former[20]. TheFof both wild and released populations increased withCW(Tab. 2). However， within a certain rage ofCW， a relatively significant increase inFof up to 40000 eggs (50%) was observed in the wild population (CW=6.5 cm， Fig. 2). Tuset[21]reported that the fluctuation inFin the American deep-sea red crab (Chaceon affinis) was within the range of 45%—65%. This difference is due to the loss of eggs during spawning. Egg loss during crab incubation may be due to disease， fungal infection， or natural loss during egg development[22]. The relatively long spawning period of female parent crabs may also be another reason[23]. The spawning period ofE. sinensismay last for 5 to 6 months[24]， during which large amount of eggs may be lost. Kuris[16]indicated that the egg loss rate in crustaceans is within the range of 11%—71%， which may be due to egg retention， prey of other crabs， parasitic diseases， and spawning failure， which may all contribute to fluctuations inF.The femaleE. sinensisburies itself in the sediment of a river after laying its eggs. Therefore， sediment quality also affects theFofE. sinensis. Soft sediment， in which crabs can easily bury their eggs， contributes to higher spawning. On the contrary， crabs in harder beds often move forward and backward with the water， thereby making it difficult for the eggs to attach to the bristle and thus are easily shed off[25]. The Yangtze River estuary is composed of alluvial sediments， with the bottom mostly covered with soil and sand， which limits the effect of sediments on fluctuations inF.

Crabs of smaller sizes may also present with high absoluteF(Tab. 2). The size of the egg is determined by its inherent energy. Hines[26]and Nakaoka[27]previously reported that the hepatopancreas provide a certain amount of energy to female crabs，thereby balancing egg size and yield. Increase in egg yield with decrease in egg size was observed. Temperature is often closely related to this result: egg size decreases with increasing temperature but increases with lowering temperature[13]. Therefore， the results of this study could explain the fluctuations in the amount and size of crustacean eggs due to change in seasons and regions. Size of eggs ofE. japonicaalso changes with breeding season[28]. Sampling studies should also be conducted in different breeding seasons to study whether similar results can be achieved withE. sinensis. Due to the long sampling period (from December 2011 to May 2012)， the observed size of the eggs may be affected by seasonal factors considering that crab eggs generated in the spring tend to be smaller in size when the temperature is relatively high.

3.2 Comparison of F of the released and wild population in the Yangtze River estuary

Tab. 3 Estimated parameters with 95% confidence intervals between parentheses for the linear regression model， lnF=lna+blnCW， between carapace width of females (CW) and fecundity of Eriocheir sinensis

TheFof the released and wild population ofE.sinensisin the Yangtze River estuary both increased withCW(P＜0.05) (Tab. 2). For crabs withCWwithin the range of 4.6—5.5 cm and 6.1—6.5 cm，Fof the wild population was higher than the released population， but without significant difference (P＞0.05).Upon reaching brackish water， part of the energy of crabs is consumed for osmotic pressure regulation.Therefore， in addition to being transferred to the ovary， the energy generated by the hepatopancreas is utilized for other purposes， thereby resulting in a lowerFin the releasedE. sinensispopulation compared to the wild population. The observedFvalues of the released and wild populations were comparable to crabs withCWwithin the range of 5.6—6.0 cm，and this may be explained by inter-individual differences among the collected crabs. The absoluteFof the released population was lower than that of the wild population， and the relativeF(F/W) of the released population was also lower than that of the wild population. This indicated that the wild population ofE. sinensishas higher overall quality than the released population. However， covariance analysis showed that (Tab. 3) with significance level set at 0.05， theF-CWcurves of the released and wild populations can be relatively satisfactory fitted. This result suggested that there was no significant difference between the regressionF-CWequations. The released population experienced similar spawning conditions after adjusting osmotic pressure regulation.

TheFof crabs is evaluated by multiple factors such as individual size， species， and external factors，including temperature， salinity， sediment， and food[7].The individual size of male crabs can affect theFof female crabs. In natural conditions， one large and strong male crabs can fertilize with three female crabs[29]. Food affects the spawning frequency of female crabs， as well as the number of eggs that are laid[30，31]; therefore， sufficient amount of food is an important aspect of increasing theFof female crabs.Previous studies have shown that changes in crab population structure decrease crab yield andF[9].Therefore， the population structure in the releasing area should be considered before releasing crab eggs to maximize its potential population size. Sexual precocity and hybrid species with smaller body size and lowFmay also influence theFofE. sinensisin the Yangtze River estuary[32]. To protect the reproductive ability ofE. sinensisin the Yangtze River estuary， it is necessary to perform long-term studies and involving various parameters.

4 Conclutions

The released population was able to adapt to the environment of the natural water in the Yangtze River estuary. The fecundity of the wild and released populations was at the same level.